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First- and Second-Trimester Screening for Fetal Aneuploidy and Neural Tube Defects
Abstract
Prenatal screening for Down syndrome and other aneuploidies, such as trisomies 13 and 18, has advanced significantly since its advent in the 1980s. Historically, women 35 years or older were offered prenatal genetic counseling and the option of a diagnostic test such as chorionic villus sampling or amniocentesis. With this screening approach, only 20% of the fetal Down syndrome population are detected antenatally. Sonographic and biochemical markers are now employed to screen for aneuploidies and neural tube defects in the first and second trimester. Maternal serum screening for Down syndrome in the second trimester started in the mid-1980s, with low levels of the analyte α-fetoprotein associated with an increased risk of fetal Down syndrome. Today, an increased nuchal translucency in the first trimester and/or a thickened nuchal fold in the second trimester are highly specific sonographic markers in trisomy 21 fetuses. Prenatal diagnosis should be made available, if requested, after appropriate counseling, including risks and benefits, to all pregnant women, regardless of maternal age.
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To provide estimates and confidence intervals for the performance (detection and false-positive rates) of screening for Down's syndrome using repeated measures of biochemical markers from first and second trimester maternal serum samples taken from the same woman.
Stored serum on Down's syndrome cases and controls was used to provide independent test data for the assessment of screening performance of published risk algorithms and for the development and testing of new risk assessment algorithms.
15 screening centres across the USA, and at the North York General Hospital, Toronto, Canada.
78 women with pregnancy affected by Down's syndrome and 390 matched unaffected controls, with maternal blood samples obtained at 11-13 and 15-18 weeks' gestation, and women who received integrated prenatal screening at North York General Hospital at two time intervals: between 1 December 1999 and 31 October 2003, and between 1 October 2006 and 23 November 2007.
Repeated measurements (first and second trimester) of maternal serum levels of human chorionic gonadotrophin (hCG), unconjugated estriol (uE3) and pregnancy-associated plasma protein A (PAPP-A) together with alpha-fetoprotein (AFP) in the second trimester.
Detection and false-positive rates for screening with a threshold risk of 1 in 200 at term, and the detection rate achieved for a false-positive rate of 2%.
Published distributional models for Down's syndrome were inconsistent with the test data. When these test data were classified using these models, screening performance deteriorated substantially through the addition of repeated measures. This contradicts the very optimistic results obtained from predictive modelling of performance. Simplified distributional assumptions showed some evidence of benefit from the use of repeated measures of PAPP-A but not for repeated measures of uE3 or hCG. Each of the two test data sets was used to create new parameter estimates against which screening test performance was assessed using the other data set. The results were equivocal but there was evidence suggesting improvement in screening performance through the use of repeated measures of PAPP-A when the first trimester sample was collected before 13 weeks' gestation. A Bayesian analysis of the combined data from the two test data sets showed that adding a second trimester repeated measurement of PAPP-A to the base test increased detection rates and reduced false-positive rates. The benefit decreased with increasing gestational age at the time of the first sample. There was no evidence of any benefit from repeated measures of hCG or uE3.
If realised, a reduction of 1% in false-positive rate with no loss in detection rate would give important benefits in terms of health service provision and the large number of invasive tests avoided. The Bayesian analysis, which shows evidence of benefit, is based on strong distributional assumptions and should not be regarded as confirmatory. The evidence of potential benefit suggests the need for a prospective study of repeated measurements of PAPP-A with samples from early in the first trimester. A formal clinical effectiveness and cost-effectiveness analysis should be undertaken. This study has shown that the established modelling methodology for assessing screening performance may be optimistically biased and should be interpreted with caution.
It is now clear that the vast majority of major fetal abnormalities can be diagnosed prenatally by ultrasound, that most of these abnormalities can be detected in the first trimester of pregnancy and that women want first- trimester rather than later diagnosis. It is also clear that effective diagnosis of fetal abnormal- ities often necessitates the identification of easily recogniz- able markers which direct the attention of the sonographer to the specific abnormality. Good examples of such mark- ers are the scalloping of the frontal bones (the 'lemon' sign) and caudal displacement of the cerebellum (the 'banana' sign), observed in the second trimester in most fetuses with open spina bifida, and increased nuchal translucency thickness (NT) which identifies in the first trimester the majority of fetuses with major aneuploidies, lethal skeletal dysplasias and a high proportion of major cardiac defects. It is now widely accepted that increased NT at 11-13 weeks is the single most effective marker of tri- somy 21 and all other major aneuploidies. First-trimester screening by a combination of maternal age, fetal NT, nasal bone, Doppler assessment of blood flow in the duc- tus venosus and across the tricuspid valve together with maternal serum free β-hCG and PAPP-A can identify more than 95% of all major aneuploidies for a screen-positive rate of less than 3%. A major remaining challenge in first-trimester ultra- sonography has been the diagnosis of open spina bifida. This challenge, however, may now have been resolved by the realization that open spina bifida can be suspected by an easily detectable marker within the brain in the same mid-sagittal plane of the fetal face as for measurement of NT and assessment of the nasal bone. In normal fetuses the fourth cerebral ventricle presents as an intracranial translucency (IT) parallel to the NT, while in fetuses with open spina bifida there may be absence of the IT 1 .
Background:
In 1968 the first antenatal diagnosis of Down's syndrome was made and screening on the basis of selecting women of advanced maternal age for amniocentesis was gradually introduced into medical practice. In 1983 it was shown that low maternal serum alpha fetoprotein (AFP) was associated with Down's syndrome. Later, raised maternal serum human chorionic gonadotrophin (hCG), and low unconjugated oestriol (uE3) were found to be markers of Down's syndrome. In 1988 the three biochemical markers were used together with maternal age as a method of screening, and this has been widely adopted. PRINCIPLES OF ANTENATAL SCREENING FOR DOWN'S SYNDROME: Methods of screening need to be fully evaluated before being introduced into routine clinical practice. This included choosing markers for which there is sufficient scientific evidence of efficacy, quantifying performance in terms of detection and false positive rates, and establishing methods of monitoring performance. Screening needs to be provided as an integrated service, coordinating and managing the separate aspects of the screening process. SERUM MARKERS AT 15-22 WEEKS OF PREGNANCY: A large number of serum markers have been found to be associated with Down's syndrome between 15 and 22 weeks of pregnancy. The principal markers are AFP, hCG or its individual subunits (free alpha- and free beta-hCG), uE3, and inhibin A. Screening performance varies according to the choice of markers used and whether ultrasound is used to estimate gestational age (table 1). When an ultrasound scan is used to estimate gestational age the detection rate for a 5% false positive rate is estimated to be 59% using the double test (AFP and hCG), 69% using the triple test (AFP, hCG, uE3), and 76% using the quadruple test (AFP, hCG, uE3, inhibin A), all in combination with maternal age. Other factors that can usefully be taken into account in screening are maternal weight, the presence of insulin dependent diabetes mellitus, multiple pregnancy, ethnic origin, previous Down's syndrome pregnancy, and whether the test is the first one in a pregnancy or a repeat. Factors such as parity and smoking are associated with one or more of the serum markers, but the effect is too small to justify adjusting for these factors in interpreting a screening test.
Urinary markers and fetal cells in maternal blood:
Urinary beta-core hCG has been investigated in a number of studies and shown to be raised in pregnancies with Down's syndrome. This area is currently the subject of active research and the use of urine in future screening programmes may be a practical possibility. Other urinary markers, such as total oestriol and free beta-hCG may also be of value. Fetal cells can be identified in the maternal circulation and techniques such as fluorescent in situ hybridisation can be used to identify aneuploidies, including Down's syndrome and trisomy 18. This approach may, in the future, be of value in screening or diagnosis. Currently, the techniques available do not have the performance, simplicity, or economy needed to replace existing methods.
Demonstration projects:
Demonstration projects are valuable in determining the feasibility of screening and in refining the practical application of screening. They are of less value in determining the performance of different screening methods. Several demonstration projects have been conducted using the triple and double tests. In general, the uptake of screening was about 80%. The screen positive rates were about 5-6%. About 80% of women with positive screening results had an invasive diagnostic test, and of those found to have a pregnancy with Down's syndrome, about 90% chose to have a termination of pregnancy. ULTRASOUND MARKERS AT 15-22 WEEKS OF PREGNANCY: There are a number of ultrasound markers of Down's syndrome at 15-22 weeks, including nuchal fold thickness, cardiac abnormalities, duodenal atresia, femur length, humerus length, pyelectasis, and hyperechogenic bowel. (ABSTRA
To assess the possible role of Doppler ultrasound assessment of ductus venosus blood flow in screening for chromosomal abnormalities at 10-14 weeks of gestation.
Ductus venosus flow velocity waveforms were obtained immediately before fetal karyotyping in 486 consecutive singleton pregnancies at 10-14 weeks of gestation. All cases were screened for chromosomal defects by a combination of maternal age and fetal nuchal translucency thickness. The peak systolic and diastolic velocities, the velocity during atrial contraction and the pulsatility index were measured.
There were 63 chromosomal defects (38 cases of trisomy 21, 12 cases of trisomy 18, seven cases of trisomy 13, three cases of Turner's syndrome and three cases of triploidy). In 57 (90.5%) cases there was reverse or absent flow during atrial contraction. Abnormal ductus venosus flow was also observed in 13 (3.1%) of the 423 chromosomally normal fetuses. In the chromosomally abnormal group, compared to the normal group, the median heights of the S and D waves were significantly lower and the pulsatility index was significantly higher. However, multivariate regression analysis demonstrated that only the height of the A wave provided a significant independent contribution in distinguishing between the chromosomally normal and abnormal groups.
These preliminary results suggest that assessment of ductus venous blood flow in pregnancies considered to be at high risk for chromosomal defects may result in a major reduction in the need for invasive testing, with only a small decrease in sensitivity.
Screening for aneuploid pregnancies is routinely performed after 15 weeks of gestation and has a sensitivity of approximately 65 percent, with a false positive rate of 5 percent. First-trimester markers of aneuploidy have been developed, but their use in combination has not been adequately evaluated in clinical practice.
We conducted a multicenter study of screening for trisomies 21 and 18 among patients with pregnancies between 74 and 97 days of gestation, based on maternal age, maternal levels of free beta human chorionic gonadotropin and pregnancy-associated plasma protein A, and ultrasonographic measurement of fetal nuchal translucency. A screening result was considered to be positive for trisomy 21 if the calculated risk was at least 1 in 270 pregnancies and positive for trisomy 18 if the risk was at least 1 in 150.
Screening was completed in 8514 patients with singleton pregnancies. This approach to screening identified 85.2 percent of the 61 cases of Down's syndrome (95 percent confidence interval, 73.8 to 93.0), with a false positive rate of 9.4 percent (95 percent confidence interval, 8.8 to 10.1). At a false positive rate of 5 percent, the detection rate was 78.7 percent (95 percent confidence interval, 66.3 to 88.1). Screening identified 90.9 percent of the 11 cases of trisomy 18 (95 percent confidence interval, 58.7 to 99.8), with a 2 percent false positive rate. Among women 35 years of age or older, screening identified 89.8 percent of fetuses with trisomy 21, with a false positive rate of 15.2 percent, and 100 percent of fetuses with trisomy 18.
First-trimester screening for trisomies 21 and 18 on the basis of maternal age, maternal levels of free beta human chorionic gonadotropin and pregnancy-associated plasma protein A, and measurement of fetal nuchal translucency has good sensitivity at an acceptable false positive rate.
It is uncertain how best to screen pregnant women for the presence of fetal Down's syndrome: to perform first-trimester screening, to perform second-trimester screening, or to use strategies incorporating measurements in both trimesters.
Women with singleton pregnancies underwent first-trimester combined screening (measurement of nuchal translucency, pregnancy-associated plasma protein A [PAPP-A], and the free beta subunit of human chorionic gonadotropin at 10 weeks 3 days through 13 weeks 6 days of gestation) and second-trimester quadruple screening (measurement of alpha-fetoprotein, total human chorionic gonadotropin, unconjugated estriol, and inhibin A at 15 through 18 weeks of gestation). We compared the results of stepwise sequential screening (risk results provided after each test), fully integrated screening (single risk result provided), and serum integrated screening (identical to fully integrated screening, but without nuchal translucency).
First-trimester screening was performed in 38,167 patients; 117 had a fetus with Down's syndrome. At a 5 percent false positive rate, the rates of detection of Down's syndrome were as follows: with first-trimester combined screening, 87 percent, 85 percent, and 82 percent for measurements performed at 11, 12, and 13 weeks, respectively; with second-trimester quadruple screening, 81 percent; with stepwise sequential screening, 95 percent; with serum integrated screening, 88 percent; and with fully integrated screening with first-trimester measurements performed at 11 weeks, 96 percent. Paired comparisons found significant differences between the tests, except for the comparison between serum integrated screening and combined screening.
First-trimester combined screening at 11 weeks of gestation is better than second-trimester quadruple screening but at 13 weeks has results similar to second-trimester quadruple screening. Both stepwise sequential screening and fully integrated screening have high rates of detection of Down's syndrome, with low false positive rates.
Background
Prenatal diagnosis of trisomy 21 currently relies on assessment of risk followed by invasive testing in the 5% of pregnancies at the highest estimated risk. Selection of the high-risk group by a combination of maternal age and second-trimester maternal serum biochemistry gives a detection rate of about 60%. We investigated assessment of risk by a combination of maternal age and fetal nuchal-translucency thickness, measured by ultrasonography at 10–14 weeks of gestation.
Methods
The risk of trisomy 21 was estimated for 96127 women of median age 31 years (range 14–49) with singleton pregnancies. Ultrasonography was done by 306 appropriately trained sonographers in 22 centres. Risk of trisomy 21 was calculated from the maternal age and gestational-age-related prevalence, multiplied by a likelihood ratio depending on the deviation from normal in nuchal-translucency thickness for crown-rump length. The distribution of risks was investigated and the sensitivity of a cut-off risk of 1 in 300 was calculated. Phenotype was assessed by fetal karyotyping or clinical examination of liveborn infants.
Findings
The estimated trisomy-21 risk, from maternal age and fetal nuchal-translucency thickness, was 1 in 300 or higher in 7907 (8·3%) of 95 476 normal pregnancies, 268 (82·2%) of 326 with trisomy 21, and 253 (77·9%) of 325 with other chromosomal defects. The 5% of the study population with the highest estimated risk included 77% of trisomy-21 cases.
Interpretation
Selection of the high-risk group for invasive testing by this method allows the detection of about 80% of affected pregnancies. However, even this method of risk assessment requires about 30 invasive tests for identification of one affected fetus.
To estimate the effectiveness of second-trimester genetic sonography in modifying Down syndrome screening test results.
The First and Second Trimester Evaluation of Risk (FASTER) aneuploidy screening trial participants were studied from 13 centers where a 15- to 23-week genetic sonogram was performed in the same center. Midtrimester Down syndrome risks were estimated for five screening test policies: first-trimester combined, second-trimester quadruple, and testing sequentially by integrated, stepwise, or contingent protocols. The maternal age-specific risk and the screening test risk were modified using likelihood ratios derived from the ultrasound findings. Separate likelihood ratios were obtained for the presence or absence of at least one major fetal structural malformation and for each "soft" sonographic marker statistically significant at the P<.005 level. Detection and false-positive rate were calculated for the genetic sonogram alone and for each test before and after risk modification.
A total of 7,842 pregnancies were studied, including 59 with Down syndrome. Major malformations and 8 of the 18 soft markers evaluated were highly significant. The detection rate for a 5% false-positive rate for the genetic sonogram alone was 69%; the detection rate increased from 81% to 90% with the combined test, from 81% to 90% with the quadruple test, from 93% to 98% with the integrated test, from 97% to 98% with the stepwise test, and from 95% to 97% with the contingent test. The stepwise and contingent use of the genetic sonogram after first-trimester screening both yielded a 90% detection rate.
Genetic sonography can increase detection rates substantially for combined and quadruple tests and more modestly for sequential protocols. Substituting sonography for quadruple markers in sequential screening was not useful.
II.
Maternal serum screening for neural tube defects and fetal aneuploidy in the second trimester has been incorporated into obstetrical practice over the past two decades. Now, as a result of several multicenter trials, first trimester screening between 11 and 14 weeks has been shown to be an effective and reliable screening test for Down syndrome and trisomy 18. This policy updates the American College of Medical Genetics policy statement entitled Second Trimester Maternal Serum Screening for Fetal Open Neural Tube Defects and Aneuploidy (2004), incorporates First trimester diagnosis and screening for fetal aneuploidy (2008) and complements the sections of American College of Medical Genetic's Standards and Guidelines for Clinical Genetics Laboratories entitled Prenatal Screening for Down syndrome (2005) and Prenatal Screening for Open Neural Tube Defects (2005).
Ultrasound records were examined retrospectively in the cases of 70 fetuses with open spina bifida diagnosed by ultrasonography at 16-23 weeks' gestation. Biparietal diameter was below the 5th centile for gestation in 61% and head circumference measurements in 26%. The anterior horn of the lateral cerebral ventricle to hemisphere ratio was above the 95th centile in 77%, and ventriculomegaly of the posterior horn of the lateral venricle was observed in 86%. In the 54 fetuses for which pictures were taken at the level of the biparietal diameter a scalloping of the frontal bones (the lemon sign) was seen. In 12 of 21 fetuses for which a suboccipital bregmatic view of the cranium had been obtained, the cerebellar hemispheres were curved anteriorly with simultaneous obliteration of the cisterna magna (the banana sign); in a further 8 cases the cerebellum was not displayed. None of these changes was seen in 100 patients presenting consecutively for routine ultrasound examination in the second trimester.
To determine the prevalence of increased fetal nuchal translucency thickness in twin pregnancies and to evaluate screening for trisomy 21 by a combination of translucency thickness and maternal age.
Prospective screening study at 10 to 14 weeks of gestation.
Fetal Medicine Centre.
22,518 self-selected pregnant women at 10 to 14 weeks of gestation, including 21,477 singleton and 448 twin pregnancies with live fetuses.
Fetal nuchal translucency thickness was measured by ultrasound examination at 10-14 weeks. Sensitivity and false positive rates of screening for trisomy 21 by a combination of fetal nuchal translucency thickness and maternal age were calculated.
Prevalence of increased nuchal translucency thickness and detection of trisomy 21.
In the 448 twin pregnancies the nuchal translucency thickness was above the 95th centile of the normal range (for crown-rump length in singletons) in 65/896 fetuses (7.3%), including 7/8 (88%) with trisomy 21. Increased translucency was also present in four fetuses with other chromosomal abnormalities. In the chromosomally normal twin pregnancies the prevalence of increased nuchal translucency was higher in fetuses from monochorionic (8.4%; 16/190) than in those with dichorionic pregnancies (5.4%; 37/688). The minimum estimated risk for trisomy 21, based on maternal age and fetal nuchal translucency thickness, was 1 in 300 in 19.5% (175/896) of the twins including all eight of those with trisomy 21.
In twin pregnancies the sensitivity of fetal nuchal translucency thickness in screening for trisomy 21 is similar to that in singleton pregnancies, but the specificity is lower because translucency is also increased in chromosomally normal monochorionic twin pregnancies.
To describe and test a method of individual risk assessment for fetal Down's syndrome based on maternal age and second-trimester ultrasound findings.
A case–control study of 142 fetuses with Down's syndrome was compared with 930 control fetuses with normal karyotype. All patients underwent second-trimester ultrasound at a single institution with a standardized ultrasound protocol without knowledge of fetal karyotype. Age-adjusted ultrasound risk assessment (AAURA) for Down's syndrome was performed by multiplying the a priori risk, based on maternal age, with likelihood ratios resulting from the presence or absence of specific ultrasound findings for each patient. Individual ultrasound findings were assigned likelihood ratios (LR) as follows: structural abnormality (LR 25), nuchal thickening (LR 18.6), echogenic bowel (LR 5.5), shortened humerus (LR 2.5), shortened femur (LR 2.2), echogenic intracardiac focus (LR 2), and renal pyelectasis (LR 1.6). A normal ultrasound was assigned a LR of 0.4.
One or more ultrasound markers were identified in 68.3% (97) of fetuses with Down's syndrome compared to 12.5% of fetuses with normal karyotype. Among fetuses with positive ultrasound, 31% of those with Down's syndrome and 80% of those with normal karyotype showed a single non-structural finding. Using AAURA and a threshold of 1 : 200, 74% (105 of 142) of fetuses with Down's syndrome were identified, including 61.5% (24 of 39) from women aged less than 35 years, 67.2% (45 of 67) from women aged 35–39 years inclusively, and 100% (36 of 36) from women aged 40 years or older. AAURA of 930 fetuses with normal karyotype showed an overall false-positive rate of 14.7%, including 4% (21 of 519) from women aged less than 35 years, 12.5% (42 of 337) from women aged 35–39 years inclusively, and 100% from women aged 40 years or older.
AAURA permits improved individual counselling regarding the risk of fetal Down's syndrome following a second-trimester sonogram. For low-risk women under age 35 years, ultrasound assessment can identify over half of the affected fetuses with Down's syndrome with an acceptable false-positive rate (4%). For women aged 35–39 years, a normal ultrasound can substantially reduce the risk of unnecessary amniocentesis (12.5% from 100%) but will also miss approximately one-third of affected fetuses. Biochemical screening of maternal serum is also suggested for this group. Based on their high a priori risk, women aged 40 years or more should consider genetic amniocentesis regardless of a normal ultrasound. Copyright © 1998 International Society of Ultrasound in Obstetrics and Gynecology
This extended series of 303 monochorionic twin pregnancies examined at 10–14 weeks gestation explores the possible association
of increased fetal nuchal translucency thickness (NT) in the early prediction of severe twin-to-twin transfusion syndrome
(TTS). Of 303 pregnancies, there were 16 in which at least one fetus was structurally or chromosomally abnormal and in the
remaining 287 ongoing pregnancies there were 43 (15%) which developed severe TTS. The median fetal NT was 1.0 multiples of
the median (MOM) and NT was >95th centile in 47 (8.2%) fetuses and in at least one fetus in 37 (12.9%) pregnancies. The prevalence
of increased NT in the pregnancies that developed TTS [17.4% (n = 15) of fetuses and 28% (n = 12) of pregnancies] was significantly higher than in the non-TTS group [6.6% (n = 32) and 10.2% (n = 25) respectively; Z = –3.4, P < 0.001 and Z = 3.2, P < 0.001 respectively], likelihood ratio of increased fetal NT for prediction of TTS = 3.5 [95% confidence interval (CI) 1.9–6.2].
In 153 of the pregnancies, an ultrasound examination was also performed at 15–17 weeks gestation and intertwin membrane folding
was seen in 49 (32%) cases; 21 of these (43%) subsequently developed TTS compared to two (1.9%) of the 104 pregnancies without
membrane folding (Z = 6.6, P < 0.001), likelihood ratio of membrane folding for prediction of TTS = 4.2 (95% CI 3.0–6.0).
To determine whether sonographic "markers" are associated with fetal Down syndrome during the second trimester and to estimate the degree of risk of individual markers using likelihood ratios.
Second-trimester (14-20 weeks) sonographic findings in 186 fetuses with trisomy 21 were compared with a control group of 8728 consecutive control fetuses. Six markers were evaluated: nuchal thickening, hyperechoic bowel, shortened femur, shortened humerus, echogenic intracardiac focus, and renal pyelectasis.
Major or structural abnormalities were observed in 31 fetuses with trisomy 21 (16.7%) and 53 control fetuses (0.6%) (P< .001). Some type of sonographic finding (major abnormality, minor marker, or both) was observed in 68.8% of fetuses with trisomy 21 compared with 13.6% of control fetuses (P < .001). An isolated minor or "soft" marker was the only sonographic finding in 42 (22.6%) of 186 fetuses with trisomy 21 compared with 987 (11.3%) of 8728 control fetuses (P < .001). Nuchal thickening (P < .001; likelihood ratio, 11) and hyperechoic bowel (P < .001; likelihood ratio, 6.7) showed the strongest association with trisomy 21 as isolated markers, followed by shortened humerus (likelihood ratio, 5.1), echogenic intracardiac focus (likelihood ratio, 1.8), shortened femur (likelihood ratio, 1.5), and pyelectasis (likelihood ratio, 1.5). Echogenic intracardiac focus was the single most common isolated marker in both affected fetuses (7.1%) and control fetuses (3.9%) but carried a low risk (P= .046; likelihood ratio, 1.8).
A single soft marker is commonly encountered during the second trimester among fetuses with trisomy 21. The risk of fetal Down syndrome, reflected by likelihood ratios, was determined for 6 individual markers. This information can be combined with the a priori risk to estimate the individual patient risk for fetal Down syndrome.
Prenatal diagnosis of trisomy 21 requires an invasive test in women regarded as being at high risk after screening. At present there are four screening tests, and for a 5% false-positive rate, the sensitivities are about 30% for maternal age alone, 60-70% for maternal age and second-trimester maternal serum biochemical testing, 75% for maternal age and first-trimester fetal nuchal translucency scanning, and 85% for maternal age with fetal nuchal translucency and maternal serum biochemistry at 11-14 weeks. In this study, we examined the possible improvement in screening for trisomy 21 by examining the fetal nasal bone with ultrasound at 11-14 weeks of gestation.
We did an ultrasound examination of the fetal profile in 701 fetuses at 11-14 weeks' gestation immediately before karyotyping for a possible chromosomal abnormality detected by maternal age and fetal nuchal translucency screening. The presence or absence of a nasal bone was noted.
The fetal profile was successfully examined in all cases. The nasal bone was absent in 43 of 59 (73%) trisomy 21 fetuses and in three of 603 (0.5%) chromosomally normal fetuses. The likelihood ratio for trisomy 21 was 146 (95% CI 50-434) for absent nasal bone and 0.27 (0.18-0.40) for present nasal bone. In screening for trisomy 21, by a combination of maternal age and fetal nuchal translucency, we estimated that inclusion of examination of the fetal profile for the presence or absence of nasal bone could increase the sensitivity to 85% and decrease the false-positive rate to about 1%.
In screening for trisomy 21, examination of the fetal nasal bone could result in major reduction in the need for invasive testing and a substantial increase in sensitivity.
To evaluate the performance of screening for fetal trisomy 21 in the first trimester of twin pregnancies by a combination of maternal serum biochemistry and ultrasonography.
Three year retrospective review of screening performance.
District General Hospital maternity unit.
All women booked to receive routine antenatal care at Harold Wood Hospital between 1 June 1998 and 30th September 2001. The population included 13,940 women of all ages presenting with pregnancies between 10 weeks 3 days and 13 weeks 6 days gestation. Of these, 230 had a twin pregnancy.
Women booked into the clinic were offered screening using a combination of maternal serum free beta-hCG and pregnancy-associated plasma protein-A (PAPP-A) and fetal nuchal translucency thickness. Women at increased risk of carrying a fetus with trisomy 21 or trisomy 13/18 (>/=1 in 300 at sampling) were offered counselling and an invasive diagnostic procedure. Follow up of the outcome of all pregnancies was carried out. For women who on examination were at 14 weeks of gestation or greater, or for women presenting as late bookers beyond 14 weeks, screening was performed in the same time frame using only maternal serum free beta-hCG and alpha-fetoprotein.
The first trimester detection rate for trisomy 21 and all aneuploides, false positive rate, uptake of screening, uptake of invasive testing in women identified at increased risk and fetal loss rates after invasive testing.
Overall, 97.4% of the women with twins (224/230) accepted first trimester screening. The rate of detection of trisomy 21 was 75% (3/4). Fetal death at presentation was found in 3.4% of fetuses (16/460). Of women who accepted screening, 4.3% (10/230) presented too late for fetal nuchal translucency measurement and 10.0% of women (23/230) presented too early. A risk for trisomy 21 was calculated for each fetus based on the individual fetal nuchal translucency thickness and the maternal biochemistry. The false positive rate among those eligible for first trimester screening was 9.0% (19/206) of pregnancies and 6.9% of fetuses (28/412). Uptake of invasive testing was 59% (10/17) with chorionic villus sampling in eight cases and amniocentesis in two. No fetal loss occurred within 28 days of chorionic villus sampling and no loss occurred after amniocentesis. One case of trisomy 21 was identified for every three invasive procedures.
First trimester screening for trisomy 21 in twin pregnancies is both theoretically possible and practically achievable using a combination of nuchal translucency thickness and maternal serum biochemistry. However, dilemmas for the mother and health professionals when both nuchal translucency thickness measurements are normal might suggest that greater reliance be placed on the nuchal translucency thickness risk alone when counselling women about invasive testing.
Screening for Down syndrome is an important part of routine antenatal care. The most common screening method in the United States involves the assessment of a combination of factors: maternal age, multiple second-trimester serum markers, and second-trimester ultrasonography (as a so-called "genetic sonogram"). More recently, however, there has been significant interest in first-trimester methods of screening, including screening for first-trimester serum markers and the sonographic measurement of fetal nuchal translucency. Multiple studies have demonstrated that fetal nuchal translucency has the potential of being a very powerful predictor of fetal aneuploidy. However, for clinicians a large void remains between this knowledge and the practical issues that must be addressed prior to endorsing this form of screening for widespread use. This article provides an objective assessment of the literature describing nuchal translucency, as well as some adjunct first-trimester sonographic techniques, such as ductus venosus flow and nasal bone studies. Additionally, a detailed description of practical problems that might limit the implementation of this form of screening is presented.
Objectives: To identify the most effective, safe and cost-effective method of antenatal screening for Down's syndrome using nuchal translucency (NT), maternal serum and urine markers in the first and second trimesters of pregnancy, and maternal age in various combinations. Design: A prospective study of women who booked for their antenatal care at about 8-14 weeks of gestation, with follow-up to identify pregnancies with Down's syndrome ascertained through second trimester screening or at birth. Setting: Twenty-five maternity units (24 in the UK and one in Austria) offering second trimester Down's syndrome serum screening that agreed to collect observational data in the first trimester. Participants: The results were based on 47,053 singleton pregnancies, including 101 pregnancies with Down's syndrome. Measurements and tests: NT measurements were included if obtained between 9 and 13 weeks of pregnancy; serum and urine samples were also taken and stored. Another pair of serum and urine samples was collected in the second trimester and included if obtained between 14 and 20 weeks. Urine and serum samples from each affected pregnancy and five matched controls were tested for: Serum: alphafetoprotein (AFP) total human chorionic gonadotrophin (hCG) unconjugated oestriol (uE3) pregnancy associated plasma protein A (PAPP-A) free β-hCG dimeric inhibin-A. Urine: invasive trophoblast antigen (ITA) β-core fragment total hCG free β-hCG. The matching criteria were gestation (using an ultrasound crown-rump length or biparietal diameter measurement), duration of storage, and centre. Screening performance of the individual markers and combinations of markers together with maternal age was assessed using standard methods. In addition pairs of first and second trimester serum samples from 600 controls were tested to secure a larger set in which screening performance could be determined using distribution parameters based on dates (time since first day of the last menstrual period). Main outcome measures: The following were determined for different combinations of markers: efficacy (by assessing screening performance, focusing on the false-positive rate (FPR) for an 85% detection rate (DR)) safety (focusing on the number of fetal losses due to amniocentesis (or chorionic villus sampling) in 100,000 women screened) cost-effectiveness (focusing on the cost of screening 100,000 women and the cost per Down's syndrome pregnancy diagnosed). Results: Efficacy (screening performance): The false-positive rates for an 85% detection rate for the main screening tests are shown in the above table, in decreasing order of screening performance: With the serum integrated test, 10 weeks is the preferred time in pregnancy for the PAPP-A measurement. For the integrated test and the combined test, the timing of the measurement of the first trimester markers is less critical. Safety: The lower false-positive rate with the integrated test compared with other tests means that at an 85% detection rate there would be nine diagnostic procedure-related unaffected fetal losses per 100,000 women screened compared with 44 using the combined test or 45 with the quadruple test. Cost-effectiveness: Screening using the integrated test is less costly than might be expected because the extra screening costs tend to be offset by savings in the cost of diagnosis arising from the low false-positive rate. It was estimated that to achieve an 85% detection rate the cost to the UK NHS would be £15,300 per Down's syndrome pregnancy detected. The corresponding cost using the second trimester quadruple test would be £16,800 and using the first trimester combined test it would be £19,000. Conclusions: Implications for healthcare: The results showed that screening performance in the first trimester of pregnancy was virtually the same as that in the second trimester, and in either it was much less effective than integrating screening measurements from both trimesters into a single test. In applying these results to screening practice several conclusions can be drawn. The following tests offer the most effective and safe method of screening: overall: the integrated test if an NT measurement is not available: the serum integrated test for women who do not attend for antenatal care until the second trimester of pregnancy: the quadruple test for women who choose to have a screening test in the first trimester: the combined test. At a constant detection rate, the cost-effectiveness of these four tests is broadly similar, any extra screening costs tending to be offset by fewer diagnostic costs. The evidence presented in this report does not support retaining the double test, the triple test, or NT measurements on their own (with or without maternal age) because each would lead to many more women having invasive diagnostic tests, without increasing the proportion of Down's syndrome pregnancies detected.
To update the likelihood ratio for trisomy 21 in fetuses with absent nasal bone at the 11-14-week scan.
Ultrasound examination of the fetal profile was carried out and the presence or absence of the nasal bone was noted immediately before karyotyping in 5918 fetuses at 11 to 13+6 weeks. Logistic regression analysis was used to examine the effect of maternal ethnic origin and fetal crown-rump length (CRL) and nuchal translucency (NT) on the incidence of absent nasal bone in the chromosomally normal and trisomy 21 fetuses.
The fetal profile was successfully examined in 5851 (98.9%) cases. In 5223/5851 cases the fetal karyotype was normal and in 628 cases it was abnormal. In the chromosomally normal group the incidence of absent nasal bone was related first to the ethnic origin of the mother, being 2.2% for Caucasians, 9.0% for Afro-Caribbeans and 5.0% for Asians; second to fetal CRL, being 4.7% for CRL of 45-54 mm, 3.4% for CRL of 55-64 mm, 1.4% for CRL of 65-74 mm and 1% for CRL of 75-84 mm; and third to NT, being 1.6% for NT < or = 95th centile, 2.7% for NT > 95th centile-3.4 mm, 5.4% for NT 3.5-4.4 mm, 6% for NT 4.5-5.4 mm and 15% for NT > or = 5.5 mm. In the chromosomally abnormal group there was absent nasal bone in 229/333 (68.8%) cases with trisomy 21 and in 95/295 (32.2%) cases with other chromosomal defects. Logistic regression analysis demonstrated that in the chromosomally normal fetuses significant independent prediction of the likelihood of absent nasal bone was provided by CRL, NT and Afro-Caribbean ethnic group, and in the trisomy 21 fetuses by CRL and NT. The likelihood ratio for trisomy 21 for absent nasal bone was derived by dividing the likelihood in trisomy 21 by that in normal fetuses.
At the 11-14-week scan the incidence of absent nasal bone is related to the presence or absence of chromosomal defects, CRL, NT and ethnic origin.
To evaluate the performance and use of second-trimester multiple-marker maternal serum screening for trisomy 21 by women who had previously undergone first-trimester combined screening (nuchal translucency, pregnancy-associated plasma protein A, and free beta-hCG), with disclosure of risk estimates.
In a multicenter, first-trimester screening study sponsored by the National Institute of Child Health and Human Development, multiple-marker maternal serum screening with alpha-fetoprotein, unconjugated estriol, and total hCG was performed in 4,145 (7 with trisomy 21) of 7,392 (9 with trisomy 21) women who were first-trimester screen-negative and 180 (7 with trisomy 21) of 813 (52 with trisomy 21) who were first-trimester screen-positive. Second-trimester risks were calculated using multiples of the median and a standardized risk algorithm with a cutoff risk of 1:270.
Among the first-trimester screen-negative cohort, 6 of 7 (86%) trisomy 21 cases were detected by second-trimester multiple-marker maternal serum screening with a false-positive rate of 8.9%. Among the first-trimester screen-positive cohort, all 7 trisomy 21 cases were also detected in the second trimester, albeit with a 38.7% false-positive rate.
Our data demonstrate that a sequential screening program that provides patients with first-trimester results and offers the option for early invasive testing or additional serum screening in the second trimester can detect 98% of trisomy 21-affected pregnancies. However, such an approach will result in 17% of patients being considered at risk and, hence, potentially having an invasive test.
II-2
To evaluate the role of fetal nasal bone imaging at 10 3/7 to 13 6/7 weeks as a screening tool for aneuploidy, in a prospective multicenter trial.
Unselected patients from the general population with viable singleton pregnancies at 10 3/7 to 13 6/7 weeks were recruited at 15 U.S. centers. All had screening with nuchal translucency (NT) ultrasound by specially trained sonographers. In the last 8 months of this trial, first trimester nasal bone evaluation was added to the screening protocol. Nasal bones were described as present, absent, or unable to determine.
A total of 38,189 patients completed first trimester NT screening, of whom 6,324 also underwent nasal bone sonography. An acceptable nasal image was obtained in 4,801 cases (76%), with nasal bones described as present in 4,779 (99.5%), and absent in 22 (0.5%). There were 11 identified cases of trisomy-21 in the population of 6,324 patients. In 9 of the 11 cases (82%) the nasal bones were described as present, and 2 cases were described as unable to determine. The only other aneuploidies were 2 cases of trisomy-18, in 1 of which the nasal bones were described as absent, and in 1 present. Absence of nasal bones had sensitivity for aneuploidy of 7.7%, false-positive rate 0.3%, and positive predictive value 4.5%.
First-trimester nasal bone evaluation was not a useful test for population screening for trisomy-21 and added little to first-trimester NT screening. The difficulty in performing first-trimester nasal bone sonography consistently, in the general population setting, will significantly limit the usefulness of this aneuploidy screening technique.
To determine the likelihood ratio for trisomy 21 in fetuses with tricuspid regurgitation at the 11 to 13 + 6-week scan.
Fetal echocardiography was carried out by specialist pediatric cardiologists in 742 singleton pregnancies at 11 to 13 + 6 weeks' gestation and pulsed wave Doppler was used to ascertain the presence or absence of tricuspid regurgitation. To avoid confusion with other adjacent signals, a strict definition of tricuspid regurgitation was used, in that it had to occupy at least half of systole and reach a velocity of over 80 cm/s. The fetal crown–rump length (CRL) and the nuchal translucency (NT) thickness were measured and the presence of any congenital heart abnormality noted. Follow-up of the pregnancy was carried out to determine the presence of chromosomal abnormalities. The likelihood ratio for trisomy 21 in fetuses with and without tricuspid regurgitation was determined.
The tricuspid valve was successfully examined in 718 (96.8%) cases. Tricuspid regurgitation was present in 39 (8.5%) of the 458 chromosomally normal fetuses, in 82 (65.1%) of the 126 with trisomy 21, in 44 (53.0%) of the 83 with trisomy 18 or 13, and in 11 (21.6%) of the 51 with other chromosomal defects. The prevalence of tricuspid regurgitation was also associated with fetal CRL, delta NT and the presence of cardiac defects. Logistic regression analysis, irrespective of cardiac defects, demonstrated that in the chromosomally normal fetuses significant independent prediction of the likelihood of tricuspid regurgitation was provided by fetal delta NT (odds ratio (OR), 1.26; 95% CI, 1.34–1.41; P < 0.0001), while in trisomy 21 fetuses prediction was provided by CRL (OR, 0.94; 95% CI, 0.89–0.99; P = 0.021). The likelihood ratio for trisomy 21 for tricuspid regurgitation was derived by dividing the likelihood in trisomy 21 by that in normal fetuses. In the chromosomally normal fetuses, the prevalence of tricuspid regurgitation in those with cardiac defects was 46.9% and 5.6% in those without cardiac defects, and the likelihood ratio of tricuspid regurgitation for cardiac defects was 8.4.
At 11 to 13 + 6 weeks' gestation, there is a high association between tricuspid regurgitation and trisomy 21, as well as other chromosomal defects. The prevalence of tricuspid regurgitation increases with fetal NT thickness and is substantially higher in those with, than those without, a cardiac defect. Copyright © 2005 ISUOG. Published by John Wiley & Sons, Ltd.
To estimate prevalence, natural history, and outcome of septated cystic hygroma in the first trimester in the general obstetric population, and to differentiate this finding from simple increased nuchal translucency.
Patients at 10.3-13.6 weeks of gestation underwent nuchal translucency sonography as part of a multicenter clinical trial. Septated cystic hygroma cases were offered chorionic villi sampling for karyotype, and targeted fetal anatomical and cardiac evaluations. Survivors were followed up for fetal and long-term pediatric outcome (median 25 months, range 12-50 months). Cases of septated cystic hygroma were also compared with cases of simple increased nuchal translucency.
There were 134 cases of cystic hygroma (2 lost to follow-up) among 38,167 screened patients (1 in 285). Chromosomal abnormalities were diagnosed in 67 (51%), including 25 trisomy-21, 19 Turner syndrome, 13 trisomy-18, and 10 others. Major structural fetal malformations (primarily cardiac and skeletal) were diagnosed in 22 of the remaining 65 cases (34%). There were 5 cases (8%) of fetal death and 15 cases of elective pregnancy termination without evidence of abnormality. One of 23 (4%) normal survivors was diagnosed with cerebral palsy and developmental delay. Overall, survival with normal pediatric outcome was confirmed in 17% of cases (22 of 132). Compared with simple increased nuchal translucency, cystic hygroma has 5-fold, 12-fold, and 6-fold increased risk of aneuploidy, cardiac malformation, and perinatal death, respectively.
First-trimester cystic hygroma was a frequent finding in a general obstetric screening program. It has the strongest prenatal association with aneuploidy described to date, with significantly worse outcome compared with simple increased nuchal translucency. Most pregnancies with normal evaluation at the completion of the second trimester resulted in a healthy infant with a normal pediatric outcome.
To examine whether in pregnancies with fetal trisomy 21 the level of maternal serum free beta-human chorionic gonadotropin (beta-hCG) and pregnancy-associated plasma protein-A (PAPP-A) at 11 + 0 to 13 + 6 weeks' gestation is independent of the presence or absence of tricuspid regurgitation and to estimate the performance of a screening test that combines tricuspid regurgitation with fetal nuchal translucency (NT) thickness and serum free beta-hCG and PAPP-A.
The study population comprised 77 trisomy 21 and 232 chromosomally normal fetuses from singleton pregnancies at 11 + 0 to 13 + 6 weeks of gestation. In all cases the fetal karyotype was determined by chorionic villus sampling (CVS), which was carried out at the request of the parents after first-trimester screening for trisomy 21 by fetal NT and maternal serum free beta-hCG and PAPP-A. Immediately before chorionic villus sampling, fetal echocardiography was performed and the presence or absence of tricuspid regurgitation was determined by pulsed wave Doppler ultrasonography. The distribution of fetal NT, maternal serum free beta-hCG and PAPP-A in trisomy 21 fetuses with absent and present tricuspid regurgitation was examined. We examined two screening strategies: first, integrated first-trimester screening in all patients and second, first-stage screening of all patients using fetal NT and maternal serum free beta-hCG and PAPP-A followed by second-stage assessment of tricuspid regurgitation only in those with an intermediate risk of 1 in 101 to 1 in 1000 after the first stage.
Tricuspid regurgitation was observed in 57 (74.0%) of the trisomy 21 fetuses and in 16 (6.9%) of the chromosomally normal fetuses. There were no significant differences in median maternal age, median gestational age, free beta-hCG multiples of the median (MoM) and PAPP-A MoM in trisomy 21 fetuses with and without tricuspid regurgitation. The modeled detection rates of trisomy 21 for fixed false positive rates of 1%, 2% and 5% in screening by maternal age, fetal NT thickness and maternal serum free beta-hCG and PAPP-A and assessment of tricuspid flow in all cases were 87%, 90% and 95%. In the two-stage approach, the estimated detection rate was 91% and the false positive rate was 2.6%.
There is no relationship between tricuspid regurgitation and the levels of maternal serum free beta-hCG and PAPP-A in cases with trisomy 21. An integrated sonographic and biochemical test at 11 + 0 to 13 + 6 weeks can potentially identify about 90% of trisomy 21 fetuses for a false-positive rate of 2-3%.
Comparison of contingent, step-wise and integrated screening policies.
Mid-trimester Down syndrome risks were retrospectively calculated from FaSTER trial data. For contingent screening, initial risk was calculated from ultrasound measurement of nuchal translucency (NT), maternal serum pregnancy-associated plasma protein (PAPP)-A and free beta-human chorionic gonadotrophin (hCG) at 11-13 weeks, and classified positive (>1 in 30), borderline (1 in 30-1500) or negative. Borderline risks were recalculated using alpha-fetoprotein, hCG, unconjugated estriol (uE3) and inhibin at 15-18 weeks, and reclassified as positive (>1 in 270) or negative. For step-wise screening, initial negative risks were also recalculated. For integrated screening, a single risk was calculated from NT, PAPP-A and the second trimester markers.
There were 86 Down syndrome and 32,269 unaffected pregancies. The detection rate for contingent screening was 91% and false-positive rate was 4.5%; initial detection rate was 60%, initial false-positive rate was 1.2% and borderline risk was 23%. Step-wise screening had 92% detection rate and 5.1% false-positive rate; integrated screening had 88% and 4.9% respectively.
As predicted by modelling, the contingent screening detection rate for a fixed false-positive rate is comparable with step-wise and integrated screening, but substantially reduces the number needing to return for second trimester testing.
Trisomy 21 is associated with a flat face, which can now be quantified by measurement of the frontomaxillary facial (FMF) angle. The aim of this study was to examine whether in trisomy 21 fetuses fetal nuchal translucency (NT) thickness and maternal serum free ss-human chorionic gonadotropin (ss-hCG) and pregnancy-associated plasma protein-A (PAPP-A) are independent of the FMF angle, and to estimate the performance of a first-trimester screening test for trisomy 21 that includes measurement of the FMF angle.
This was a prospective study in singleton pregnancies at 11 + 0 to 13 + 6 weeks of gestation in which three-dimensional volumes of the fetal head were obtained and measurement of the FMF angle performed immediately before fetal karyotyping by chorionic villus sampling (CVS). The women chose to have CVS after risk assessment by a combination of maternal age, fetal NT thickness and maternal serum free ss-hCG and PAPP-A. Regression analysis was used to examine the significance of the association within the euploid and within the trisomy 21 fetuses between the deviation from the normal median in FMF angle and the deviation in NT, free ss-hCG and PAPP-A. We estimated the detection rate (DR) and false positive rate (FPR) of first-trimester screening for trisomy 21 by measuring the FMF angle in all cases and of an alternative policy in which first-stage screening is by fetal NT and maternal serum biochemistry in all patients, followed by second-stage assessment of FMF angle only in those with an intermediate risk (1 in 51 to 1 in 1000) after the first stage.
The FMF angle was measured in 782 euploid and 108 trisomy 21 fetuses. In the euploid fetuses the mean FMF angle decreased linearly with CRL from 83.5 degrees at a crown-rump length (CRL) of 45 mm to 76.4 degrees at a CRL of 84 mm. In the euploid fetuses the mean delta FMF angle was 0.0 (SD, 4.264) degrees and the respective values in the trisomy 21 fetuses were 7.172 (SD, 4.092) degrees . Incorporating the FMF angle in first-trimester combined screening increased the estimated DR from 90 to 94% at an FPR of 5% and from 85 to 92% at an FPR of 3%. In two-stage screening it would be necessary to measure the FMF angle in 12% of cases and the DRs would be 93 and 91% at FPRs of 5 and 3%, respectively.
Measurement of the FMF angle improves the performance of first-trimester screening for trisomy 21.
First trimester nasal bone evaluation for aneuploidy in the general population: results from the FASTER Trial
- Malone FD
- Ball RH
- Nyberg DA
A comparison of first trimester screening, second trimester screening, and the combination of both for evaluation of risk for Down syndrome
- Malone