[Specific aspects of monochorionic pregnancies].

Monochorionic pregnancies are at high risk of developing foetal and neonatal complications due to the presence of placental vascular anastomoses. These vascular anastomoses circulate on the chorionic plate and connect directly or indirectly the two foetal circulations. They may cause acute and chronic hemodynamic imbalance between the two foetuses and may cause significant morbidity, especially in the survivor at the time or nearly after the intrauterine death of its co-twin. In addition, some monochorionic pregnancies are complicated by twin-to-twin transfusion syndrome or twin-reversed-arterial-perfusion sequence. The late embryo division (>8 days following the fertilization) originates in monoamniotic monochorionic pregnancy and conjoined twins and is also at risk of developing specific complications related to these conditions. Ultrasound represents the angular stone in the management of these pregnancies, firstly for the diagnosis of monochorionicity in the first trimester, and second by screening and monitoring of their specific complications.