MonochorionischDiamniot

MonochorionischDiamniot, also known as monochorionic diamniotic (MCDA), describes a monozygotic twin pregnancy in which the twins share a single chorion and placenta but have two separate amniotic sacs. This arrangement results when the fertilized egg splits after formation of the chorion but before the amnion forms, typically between days 4 and 8 after fertilization. MCDA pregnancies account for the majority of monochorionic twin pregnancies.

The shared placental circulation in MCDA twins creates vascular connections between the twins. While the amniotic sacs are separate, these placental vascular anastomoses can lead to complications such as twin-twin transfusion syndrome (TTTS), where one fetus receives more blood than the other, and other discordant conditions like twin anemia polycythemia sequence (TAPS) or selective fetal growth restriction. The overall risk profile is higher than in dichorionic pregnancies, and MCDA pregnancies require careful, specialized monitoring.

Diagnosis is typically made by obstetric ultrasound early in pregnancy with assessment of chorionicity and amnionicity. Serial ultrasound examinations and Doppler studies are used to monitor growth, amniotic fluid levels, and placental perfusion. Expectant management is common in uncomplicated MCDA pregnancies, while TTTS or other complications may necessitate interventions such as amnioreduction or fetoscopic laser photocoagulation of placental vessels. Delivery is often planned in the late third trimester or earlier if complications arise, balancing fetal maturity with risk of events.

Prognosis depends on prompt recognition and management of complications. With modern surveillance and treatment, many MCDA pregnancies have favorable outcomes, but they remain higher risk than dichorionic pregnancies and require ongoing obstetric care.