Congenital anomaly rates in monoamnionic twins reach 18 to 28 percent (Post, 2015). Since concordance of anomalies is found in only approximately one quarter of cases, the finding of normal anatomy in one twin does not negate the need for a thorough evaluation in the second. Also, because of the higher risk of cardiac anomalies, fetal echo cardiography is indicated in these pregnancies. Of note, monoamnionic twins are by definition monozygotic and thus presumed to be genetically identical. Consequently, either both or none of the fetuses have chromosomal abnormalities except in rare cases of discordance (Zwijnenburg, 2010). Indeed, the risk for Down syndrome in each fetus of the monozygotic pair is similar to or lower than the risk in maternal age-matched singletons (Sparks, 2016). he standard methods for Down syndrome screening in these pregnancies can be applied (Chap. 14, p. 281).
The rate of twin-twin transfusion syndrome in monoamnionic twins is lower than the rate reported in monochorionic diamnionic pregnancies. This may be due to the near universal presence in monoamnionic twins of arterioarterial anastomoses, which are presumed to be protective (Hack, 2009b; Post, 2015). Nonetheless, twin-twin transfusion syndrome surveillance is recommended and described on page 879.
Umbilical cords frequently entangle (Fig.
lorbid cord entanglement appears to occur early, and monoamnionic pregnancies that have successfully reached 30 to 32 weeks' gestation are at reduced risk. In one Dutch series, the incidence of intrauterine demise dropped from 15 percent after 20 weeks to 4 percent at gestational ages >32 weeks (Hack, 2009a). Although color-low Doppler sonography is used to diagnose entanglement (Fig. 45-12), factors that lead to pathological umbilical vessel constnctton are unknown. A consequence is that fetal death from cord entanglement is unpredictable. Unfortunately, monitoring for this is relatively inefective. In one study, after analysis of more than 10,000 hours of fetal tracing from 17 sets of monoamnionic twins, Quinn and colleagues (2011) concluded that monitoring was physically
FIGURE 45-1 1 Monozygotic twins in a single amnionic sac. The smaller fetus apparently died first, and the second subsequently succumbed when umbilical cords entwined.
FIGURE 45-12 Monochorionic monoamnionic cord entanglement.
A. Despite marked knotting of the cords, vigorous twins were delivered by cesarean. B. Preoperative sonogram of this pregnancy shows entwined cords. C. This finding is accentuated with application of color Doppler. (Used with permission from Dr. Julie Lo.)
FIGURE 45-13 Possible outcomes of monozygotic twinning. The asymmetrical category contains twinning types in which one twin complement is substantially smaller and incompletely formed.
possible in only 50 percent of cases. An abnormal fetal heart rate tracing prompted delivery in only six cases.