Diagnosis and Characterization of Multiple Gestations
23.1 Fetal Number
Description and Clinical Features
Twins can arise from a single fertilized ovum (zygote) that splits into two separate embryos within the first few days after conception. Twins can also occur as a result of fertilization of two ova. The first of these mechanisms yields identical (monozygotic) twins, and the second results in fraternal (dizygotic) twins. Approximately two-thirds of naturally occurring twins are dizygotic and one-third are monozygotic. Higher-order multiples can be of various types of zygosity. For example, triplets may arise from three zygotes (trizygotic triplets); two zygotes, one of which splits after conception, resulting in an identical pair together with a third nonidentical fetus; or, rarely, from one zygote that splits into three identical embryos.
Several factors influence the frequency of dizygotic twins. Specifically, dizygotic twinning occurs more frequently in:
Pregnancies achieved through fertility treatments (e.g., in vitro fertilization) that use follicular-stimulating hormones
Mothers with a family history of dizygotic twins
Mothers older than age 35
Mothers of particular ethnic or racial backgrounds: Africans more often than Europeans, Europeans more often than Asians
The rate of monozygotic twinning, on the other hand, is fairly constant over all populations.
In pregnancies that begin as multiple gestations, the pregnancy number may subsequently decrease. For example, if a pregnancy begins with twins and one of the developing fetuses dies, a singleton pregnancy will result. Partial or complete pregnancy loss can occur at any time, but is most frequent in the early first trimester.
Sonography
Before 6 weeks, when the embryo and its heartbeat are not consistently visible on sonography, pregnancy number can be estimated by counting the number of gestational sacs (Figure 23.1.1) and yolk sacs (Figure 23.1.2). Each gestational sac will, in most cases, give rise to as many embryos as the number of yolk sacs contained within it. From 6 weeks gestational age onward, fetal number is determined by counting the number of embryos (or fetuses) (Figures 23.1.3 and 23.1.4).
 Figure 23.1.1 Dichorionic twins at 5 weeks diagnosed by counting gestational sacs. A: Sagittal view of the uterus demonstrates two intrauterine gestational sacs (arrows). B: 3D view of the uterus in another patient reveals two gestational sacs (arrows). In both cases, no yolk sac or embryo is identifiable within either sac, and follow-up sonograms revealed twins with heartbeats. |
 Figure 23.1.2 Monochorionic twins at 5. 5 weeks diagnosed by counting yolk sacs. A: Magnified image of the uterine cavity shows a single gestational sac (arrow) with two yolk sacs (arrowheads). B: Follow-up scan at 10 weeks demonstrates two fetuses (arrowheads). Each twin has its own amnion (arrows), and the two amnions have not yet fused into a single dividing membrane. |
 Figure 23.1.3 Monochorionic twins at 11 weeks, diagnosed by counting fetuses. 3D sonogram demonstrates two fetuses (arrows). |
 Figure 23.1.4 High-order multiple gestations diagnosed by counting fetuses. A: 3D image at 11 weeks demonstrates three fetuses (arrows), each in its own gestational sac. B: Transverse view of the uterus in another patient at 10 weeks gestation demonstrates four fetuses (arrowheads), each in its own gestational sac. |
 Figure 23.1.5 “Vanishing” twin. A: At 8 weeks, there are two fetuses (arrows), each in its own gestational sac. Heartbeats were seen in both. B: At 10 weeks, one of the fetuses (short arrow) was small and had no heartbeat, while the other (long arrow) had grown normally and demonstrated cardiac activity. C: Several weeks later, the scan showed a singleton pregnancy with no evidence of the prior twin. |
Pregnancy number determined before 6 weeks, however, may prove to be an over- or undercount of the number found on a follow-up sonogram. For example, an early sonogram may overcount the subsequent pregnancy number if it demonstrates two gestational sacs but only one embryo develops, after which the other gestational sac resorbs completely (“vanishing twin”) (Figure 23.1.5). On the other hand, the early sonogram can undercount the pregnancy number if it demonstrates what appears to be a singleton pregnancy and then two fetuses are seen on follow-up ultrasound (“appearing twin”) (Figures 23.1.6 and 23.1.7). One situation in which undercounting occurs is when monochorionic twins are scanned at 5 weeks gestation (Figure 23.1.6). In such cases, the sonogram will demonstrate a single gestational sac without yolk sacs, thus providing no evidence of the existence of a multiple gestation.
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