dichorionicity

Dichorionicity is a term used in obstetrics to describe a twin pregnancy in which two chorions are present, indicating that each fetus has its own chorionic membrane and typically its own placenta. The condition most commonly arises in dizygotic (fraternal) twinning, where two separate zygotes implant. It can also occur in monozygotic (identical) twinning if the zygote splits within the first four days after fertilization, before the chorion forms, producing two placental units.

Diagnosis is usually made by ultrasound in the first trimester. Dichorionicity is suggested by two separate placental masses and a thick dividing membrane separating two amniotic sacs. The lambda or twin-peak sign may be seen, reflecting two placentas. If a single placental mass with a thin dividing membrane is identified, monochorionicity is more likely. Accurate chorionicity assessment early in pregnancy guides subsequent prenatal care.

Clinical significance involves a different risk profile than monochorionic twin pregnancies. Dichorionic twins have a lower risk of twin-to-twin transfusion syndrome and intertwin anemia because they typically do not share placental circulations. However, they remain at higher risk than singleton pregnancies for twin-pregnancy complications such as preterm birth, growth discordance, and perinatal morbidity associated with multiple gestations. Discordant growth can occur even with separate placentas.

Management centers on confirming chorionicity early, followed by appropriate obstetric surveillance. This includes regular fetal growth assessments and planning for potential preterm delivery, with involvement of maternal-fetal medicine specialists as indicated. Overall prognosis for dichorionic twin pregnancies is generally favorable with proper monitoring.