Why is it important to diagnose chorionicity and how do we do it?

Because the monochorionic (MC) placenta is designed for a singleton fetus, and might not provide adequate physiological support for twins, obstetric problems are more frequent in MC than dichorionic (DC) twins. Problems arise because asymmetric cord insertions cause growth discordance as a result of unequal sharing of placental tissue. Approximately 95% of MC twin placentas contain interfetal vascular connections of some kind, sometimes in several combinations. Such connections can cause twin–twin transfusion syndrome and twin reversed arterial perfusion. The survivor can also suffer damage if the co-twin dies spontaneously or from inappropriate methods of selective termination. These complications are progressive and often advanced by 18 weeks gestation. Monoamniotic twins carry greater risks than diamniotic twins, especially entangled cords. MC twins are often discordant for congenital anomalies. Diagnosis of MC twinning is optimal in the first trimester. Optimal management of these MC twin disorders is not yet established; long-term follow-up studies are unsatisfactory. In clinical practice, chorionicity is not always determined in the first trimester.