Monochorionic twin pregnancies

Abstract

Following widespread application of assisted reproductive technology modalities and the increased age of motherhood, the incidence of twin gestations has increased markedly. Twins are either monozygotic or dizygotic. Dizygotic (i.e. fraternal) twins result from the fertilization of two different eggs and are per definition dichorionic (DC). Chorionicity of monozygotic twins (i.e. identical twins) depends on timing of splitting of the fertilized egg. If splitting occurs within three days after fertilization, the monozygotic twin will be DC. Splitting after the third day results in monochorionic (MC) twinning. Twins have an increased risk of perinatal mortality and neonatal morbidity compared to singletons. MC twin pregnancies, which occur in about 1 of 400 pregnancies, carry a higher risk of perinatal complications compared to DC twins. This is mainly attributed to the unique architecture of the MC placenta. The aim of this thesis was to study perinatal mortality and (long-term) morbidity in MC and DC twin pregnancies. We report on perinatal outcome in 1407 twin pregnancies from two Dutch centers in which we found an almost 9-fold higher risk of intrauterine fetal death in MC twins compared to DC twins. In most of these cases, no antenatal signs of impaired fetal condition had been present. To further elucidate the risk of unexpected fetal death in MC twin pregnancies, a cohort from ten Dutch perinatal centers was established which is the largest cohort of MC twin pregnancies to date (n=639). In this series, the prospective risk of fetal death after 32 weeks of gestation was 0.5%. This risk, albeit increased, was not as high as previously estimated. Compared to a cohort of twin pregnancies in the early 1900s, when fetal surveillance and intervention options were limited, perinatal mortality has decreased considerably in DC pregnancies but only to a lesser extend in MC twins. A study on the role of Doppler velocimetry findings in MC twin pregnancies revealed that current antenatal surveillance sometimes fails to predict and prevent adverse outcome in MC twin gestations. This has led to the discussion as to how to manage the (uncomplicated) MC twin pregnancy. The relatively low prospective risk of death after 32 weeks of gestation does not justify elective preterm delivery. However, mortality at term was still 3 times higher in MC twins as compared to singletons and term DC twins and did occur in pregnancies in which spontaneous onset of labour was awaitened (?37 weeks). We therefore suggest planned delivery of the MC diamniotic twin around 37 weeks of gestation. Fortunately, long-term outcome of MC twins was favorable in the absence of twin-to-twin transfusion syndrome (TTTS) or co-twin death and we found no differences in outcome between MC and DC twins. Perinatal outcome was also studied in 98 monoamniotic (MA) twin pregnancies, a rare subgroup of MC twins. In our large series, the risk of perinatal mortality was still high (17%) and mortality occurred throughout pregnancy. This elevated risk was not associated with the vascular architecture of the MA placenta. Intensive monitoring and early delivery seemed to reduce perinatal mortality after 32 weeks of gestation.