Monokorionisissa

Monokorionisissa is a Finnish obstetric term describing monochorionic twin pregnancies, in which two fetuses share one chorion and typically one placenta. This occurs when a single fertilized egg splits after conception. Depending on the timing of division, monochorionic twins may be diamniotic (two amniotic sacs) or monoamniotic (one amniotic sac). Division after the first week increases the risk of monoamniotic and, very late division can result in conjoined twins. Monochorionic pregnancies are contrasted with dichorionic pregnancies, where two chorions (and usually two placentas) are present.

They carry higher perinatal risk due to placental vascular connections. The most notable complication is twin-twin transfusion syndrome (TTTS), arising from uneven blood flow between twins through placental vessels. Other risks include selective fetal growth restriction, twin anemia-polycythemia sequence, and, in monoamniotic pregnancies, umbilical cord entanglement. Management relies on regular fetal ultrasound surveillance, fetal echocardiography, and Doppler studies. If TTTS or other complications occur, treatments may include fetal laser surgery to coagulate placental anastomoses or amnioreduction. Delivery is planned based on the twin type and complications, often around 34-37 weeks for monochorionic diamniotic pregnancies, earlier if problems develop.