In contrast, monochorionic pregnancies have a dividing membrane that is so thin (generally <2 mm) that it may not be seen until the second trimester. The relationship between the membranes and placenta without apparent extension of placenta between the dividing membranes is called the T sign (Fig. 45-5). Evaluation of the dividing membrane can establish chorionicity in more than 99 percent of pregnancies in the first trimester (Miller, 2012). Lack of a dividing membrane signals a monochorionic monoamnionic gestation.
Last, twins with difering gender indicates a dichorionic (and dizygotic) gestation (Emery, 2015). A rare exception to this scenario would be a heterokaryotypic monochorionic gestation, described earlier (p. 867). If both twins are the same gender, additional measures are necessary.
A carefully performed visual examination of the placenta and membranes ater delivery serves to establish zygosity and chorionicity promptly in approximately two thirds of cases. The following systematic examination is recommended. As the irst neonate is delivered, one damp is placed on a portion of its
FIGURE 45-6 Dichorionic diamnionic twin placenta. The membrane partition that separated twin fetuses is elevated and consists of chorion (c) between two amnions (0).
cord. Cord blood is generally not collected until after deliv ery of the other twin. As the second neonate is delivered, two damps are placed on that cord, and so on as necessary. lter natively, in higher-order deliveries, color-tagged damps can be simpler. Until the delivery of the last fetus, each cord segment must remain damped to prevent fetal hypovolemia and anemia caused by blood leaving the placenta via anastomoses and then through an undamped cord. At this time, evidence is insuicient in multifetal gestations (American College of Obstetricians and
Gynecologists, 2017a). At Parkland Hospital, we currently do not perform delayed cord damping in these pregnancies.
The placenta is carefully delivered to preserve the attachment of the amnion and chorion. With one common amnionic sac or with juxtaposed amnions not separated by chorion, the fetuses are monozygotic (see Fig. 45-1). If adjacent amnions are separated by chorion, the fetuses could be either dizygotic or monozygotic, but dizygosity is more common (Fig. 45-6). If the neonates are of the same sex, blood typing of cord blood samples may be helpful. Diferent blood types conirm dizygosity, although demonstrating the same blood type in each fetus does not conirm monozygosity. For deinitive diagnosis, more complicated techniques such as DNA ingerprinting can be used. However, these tests are generally not performed at birth unless medical indications dictate a need.
During physical examination, accurate fundal height measurement, described in Chapter 9 (p. 164), is essential. With multifetal pregnancies, uterine size is typically larger during the second trimester than expected for a singleton. Rouse and associates (1993) reported fundal heights in 336 well-dated twin pregnancies. Between 20 and 30 weeks' gestation, fundal heights averaged approximately 5 cm greater than expected for singletons of the same fetal age.
FIGURE 45-7 Sonograms of first-trimester twins. A. Dichorionic diamnionic twin pregnancy at 6 weeks' gestation. Note the thick dividing chorion (yellow arrow). One of the yolk sacs is indicated (blue arrow). B. Monochorionic diamnionic twin pregnancy at 8 weeks' gestation. Note the thin amnion encircling each embryo, resulting in a thin dividing membrane (blue arrow).