Pathology of twinning

Perinatal mortality and morbidity is three to seven times higher in twin pregnancies compared to singletons due to a higher incidence of antepartum complications, preterm birth and uteroplacental insufficiency. In comparison to dichorionic twins, monochorionic twins are at increased risk for perinatal mortality and serious morbidity. In both types of twins, growth discordance can occur. Discordant growth of dichorionic twins could be due to differences in placental mass or differences in placental parenchymal lesions, whereas birth weight discordancy in monochorionic twins can also be caused by the type of placental vascular anastomoses. In this chapter the different types of complications specifically associated with the formation of twins are discussed (e.g. acardiac twins, acute and chronic twin-twin transfusion syndrome). Special attention will be given to different combinations of vascular anastomoses in monochorionic twins. Monochorionic twin pregnancies are a high risk pregnancy with a high chance of mortality and/or morbidity; placental and vascular characteristics are a major contributor to adverse outcome in these pregnancies. If mortality occurs in a monochorionic twin placenta, pregnancies with double mortality usually have an arterio-arterial anastomosis and if pregnancies are complicated by one death, a veno-venous anastomosis is more likely to be present. While this chapter focusses on twins, the same considerations usually apply to higher multiples of gestation. Multiple gestations have become more common with the increased use of assisted reproduction.