Monochorionic diamniotic twin pregnancies pregnancy outcome, risk stratification and lessons learnt from placental examination.

Monochorionic diamniotic twin pregnancies have a more hazardous intrauterine stay than their dichorionic counterparts because of the vascular anastomoses that connect the two fetal circulations. The survival of monochorionic twins diagnosed in the first trimester is 89%. Twin-to-twin transfusion syndrome (TTTS) occurs in 9% and is the most important cause of death. Risk assessment by ultrasound scan in the first and early second trimester identifies a subgroup of monochorionic twins with a more than 70% risk of a complicated outcome and a survival rate of only 69%. For complicated monochorionic twin pregnancies, umbilical cord coagulation for selective feticide has a survival rate of 83% with a normal development in 92%. Umbilical cord coagulation also results in a good outcome for the healthy co-twin of a heterokaryotypic monochorionic pair. Unequally shared placentas have a more elaborate blood exchange, which reduces the birthweight discordance. In these cases, the anastomoses fulfill a beneficial role by increasing the availability of oxygen and nutrients to the twin on the smaller placental share. Pairs with early onset discordant growth have a higher mortality and a more unequally shared placenta than pairs with late onset discordant growth. Unequal placental sharing therefore appears to be the cause of early onset discordant growth, whereas a late intertwin transfusion imbalance may be involved in some cases with late onset discordant growth. Finally, placental examination after laser treatment for TTTS demonstrated that successful coagulation of all visible anastomoses cures TTTS. However, anastomoses can be missed and lead to a complicated pregnancy outcome.