3D ultrasound of triplets at 8th week of gestation: 3 embryos in three gestational sacs (TCTA-trichorionic triamniotic triplets). Slika 6. Prikaz trojaka u 8. gestacijskom tjednu 3D ultrazvukom. Vide se tri zametka u tri gestacijske vre}ice (TC/TA trikorijalni triamnijalni trojci). 

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... in early pregnancy confirms that the num- ber of conceived embryos in multiple pregnancies ex- ceeds the multiple birth rate. Spontaneous embryo- nic/fetal loss in the first trimester of multiple pregnancies is known as the »vanishing twin« phenomenon 6 ( Figure 5.). Pregnancy number before the 6 th week is determined by counting the number of gestational sacs. Late appear- ance of twins is recognized on the basis of discrepancy between two sonograms: initial sonogram, usually ob- tained at 5.0 to 5.7 GW, and a subsequent sonogram at more than 6 GW demonstrates more embryos or fetuses than initially counted number of gestational sacs. Ac- cording to Manzur et al. 7 , the outcome of successful pregnancy after finding 3 chorionic sacs was as follows: for triplets 47.4%, for twins 31.6%, for singletons 18.4%, while miscarriage rate was 2.6%. There are two types of undercounting: polyzygotic and monozygotic. Recent meta-analysis showed that more than 50% of pregnancies with 3 or more gestational sacs have spon- taneous reduction before 12 th GW. 8 The surviving fe- tuses weigh less and are born earlier than unreduced pregnancies with the same initial number of fetuses. 8 Polyzygotic undercounting is a result of the limitations of 2D TVS. The anatomy of the female reproductive tract as well as contemporary design of ultrasonic probes limits the number of examination planes to the sagittal and transversal. Because the uterus can be ex- amined only in these two planes, the failure to visualize the total number of gestational sacs on a single screen is quite probable. Stated in other way, one or more gesta- tional sacs may be overlooked. This problem is en- hanced in high-order multiple pregnancies in which is impossible to visualize all gestational sacs on a single screen. Therefore, the risk of undercounting is corre- lated to the number of embryos. Undercounting of the number of gestational sacs is the most common pitfall during the first trimester. This pitfall can be avoided ei- ther by conventional or by 3D US. 3,4,8 Whenever spontaneous reduction is suspected in high-order multiple pregnancies on conventional sono- graphic exam, the additional use of surface rendering mode is recommended. If one uses surface rendering mode, distinguishing between the spontaneous reduc- tion and normal pregnancy in high-order pregnancy can be easily done. Using 3D US the frontal (coronal) plane enables examination of the uterine cavity in sections which are unobtainable with conventional 2D sono- graphy. Furthermore, 3D sonography enables the appro- priate counting of gestational sacs without any risk of undercounting even for less experienced ultrasono- graphers ( Figure 6). Therefore, inter observer variabil- ity in detecting the number of gestational sacs is signifi- cantly lower. Even quadruplet is recognizable without any difficulties (Figure 7). This advantage strongly sug- gests that 3D sonography should become the new stan- dard in the early diagnosis of high order multiple preg- nancies. 9 Before introduction of 3D ultrasound, 11% of dichorionic twins were initially undercounted as single- tons, and 16% of high order multiple gestations were also undercounted. ...