Monochorionische

Monochorionic refers to a twin pregnancy in which the fetuses share a single chorion, the outer fetal membrane that contributes to the placenta. This occurs when a single zygote splits after fertilization and after the chorion begins to form, typically in the first week of development. Monochorionic pregnancies are subdivided by the amniotic membranes: monochorionic diamniotic (MCDA), where twins share one chorion but have two amniotic sacs, and monochorionic monoamniotic (MCMA), where both the chorion and amnion are shared. In contrast, dichorionic pregnancies (usually from two zygotes) have two placentas and two amniotic sacs.

Chorionicity can be determined by early ultrasound. The presence of a lambda or twin-peak sign suggests dichorionic placentation, whereas a single placental mass with no lambda sign suggests monochorionic placentation. The arrangement of placental membranes and vessels within the shared placenta also informs risk.

Monochorionic twin pregnancies carry distinctive risks due to the shared placenta. Twin-to-twin transfusion syndrome (TTTS) arises from imbalanced blood flow between twins via placental vessels and is a major cause of morbidity and mortality. Other risks include fetal growth restriction, twin anemia-polycythemia sequence (TAPS), and, in MCMA pregnancies, cord entanglement with potential sudden fetal demise. Management involves regular high-risk obstetric surveillance, with treatment options such as laser ablation of placental anastomoses for TTTS and planned delivery strategies to balance prematurity against stillbirth risk. Delivery is commonly planned in the late third trimester, earlier in MCMA pregnancies.

Prognosis depends on chorionicity and the presence and severity of complications; with appropriate monitoring and intervention, outcomes have improved significantly.