Nine pregnancies were thought to be monochorionic diamniotic and only recognized as monoamniotic at delivery. Median gestational age at delivery was 36 1/7 weeks (range 26 4/7 to 38 0/7 weeks). Four of these pregnancies were delivered before 34 weeks of gestation, all with spontaneous onset of preterm labor. Five pregnancies were delivered after 34 weeks of gestation; three women had an induction of labor due to monochorionicity, and two delivered after spontaneous onset of labor. One neonate died (born at 27 5/7 weeks) due to necrotizing enterocolitis. Monoamniotic twin pregnancies that turned out to be monochorionic diamniotic (either later in pregnancy or at time of delivery) were not recorded.
Data were obtained from the 10 level-3 perinatal centers and include approximately 40% of all monoamniotic twins delivered in the Netherlands in that time period. It is therefore not a population-based study and outcome in the other 60% of cases may have been different.
There was a high incidence of perinatal mortality and neonatal morbidity in nonanomalous monoamniotic infants. The incidence of twin–twin transfusion syndrome in this cohort was 6%. Congenital heart anomalies and severe cerebral injury occurred in 4% and 5% of monoamniotic twin infants, respectively.
Entanglement and knots of the umbilical cords are the major cause of fetal death in monoamniotic twins.8 It may be detected by prenatal ultrasonography already in the first trimester.8,18,19 It is hypothesized that initiation of cord entanglement is a phenomenon of early pregnancy, when amniotic fluid volume in relation to the fetal mass is greater,20 and that location of the cord entanglement may influence the risk of antepartum death. (Arabin B, HackKEA Does the location of cord entanglement matter for ante partum death in monoamniotic twins? Ultrasound Obstet Gynecol. In press).
Congenital malformations are found in 15% to 20% of monozygotic twin pregnancies.21 Acardiac twinning, anencephaly, and congenital heart defects are typically related to monoamniotic twins. A proportion of these defects are acquired as a result of the altered hemodynamics in the recipient twin associated with twin–twin transfusion syndrome. Karatza et al22 reported a 2% incidence of congenital cardiac abnormalities in 89 monochorionic twin pregnancies unaffected by twin–twin transfusion syndrome. In another study of 165 monochorionic twin pregnancies,23 the overall risk of at least one fetus in a monochorionic twin pair having a structural congenital cardiac anomaly was 9%. In a small subgroup of 7 monoamniotic twins, five children had a congenital heart malformation (36%). Ventricular septal defect was the most common lesion diagnosed. In our study, the incidence of congenital heart malformations was 4%. Since the incidence of heart anomalies and cerebral ultrasound abnormalities is high among monoamniotic twins, all monoamniotic infants (especially after death of the cotwin) should be examined postnatally, including a cerebral and cardiac ultrasound scan. In our study, three of the single fetal deaths were followed by neonatal death of the second twin due to cerebral artery infarction. This knowledge could alter counseling of parents with single fetal death with regard to the possibility of severe neuromorbidity, either leading to neonatal death or (severe) handicap of the second twin.
The incidence of twin–twin transfusion syndrome in this cohort of monoamniotic twins was low (6%), which is in agreement with other reports (incidence ranging from 3% to 10%15, 24–26). In contrast, the incidence of twin–twin transfusion syndrome in monochorionic diamniotic twins is higher (10% to 15%).27,28 The difference in the rate of twin–twin transfusion syndrome is partly due to a different anastomotic pattern between monoamniotic placentas and monochorionic diamniotic placentas. Almost all monoamniotic placentas have arterio-arterial anastomoses.29 The presence of these arterio-arterial anastomoses protects against hemodynamic disequilibrium by allowing intertwin blood flow and hence the development of twin–twin transfusion syndrome.30 Moreover, in monoamniotic pregnancies, twin–twin transfusion syndrome cannot be diagnosed using the standard criteria, since it is impossible to diagnose the occurrence of the hydramnios-oligohydramnios sequence in monoamniotic pregnancies. Delayed or lack of identification of clinical manifestations of twin–twin transfusion syndrome may also account for the reduced rate of twin–twin transfusion syndrome in monoamniotic twins.31 There is an urgent need for clear diagnostic criteria of twin–twin transfusion syndrome in monoamniotic twins. More studies are needed to determine standardized criteria.
In our series, 40% of infants were born vaginally, since it was not routine in all hospitals to deliver monoamniotic twins by cesarean. The risks of a vaginal delivery relate primarily to cord entanglement and cord compression during labor and especially after delivery of the first infant. Therefore most authors favor cesarean delivery in monoamniotic twins, but vaginal delivery has been reported in several case reports.10,36 In our series, in two cases, a cesarean delivery of the second neonate was done due to problems after the vaginal birth of the first neonate, but none of the neonates died due to labor related complications. However, our data set was relatively small and does not allow definitive answers. At present, we would favor a cesarean delivery in all monoamniotic twins. In summary, the incidence of perinatal mortality in monoamniotic twins has decreased over the years, but remains high (15% to 20%) and occurs throughout pregnancy. There is no consensus about optimal antenatal management and timing of delivery. We assume that intensive monitoring (ie, more frequently) and earlier delivery has contributed to this decrease in perinatal mortality.
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