dichorion

Dichorion is a term used in obstetrics to describe a twin pregnancy in which each fetus has its own chorion, the outer fetal membrane that contributes to the placenta. In practical terms, dichorionic pregnancies most often correspond to dichorionic-diamniotic twins, where there are two chorions and typically two amniotic sacs with separate placental circulations, though placental tissue can be fused or overlapped in some cases.

Most dichorionic twins are dizygotic, arising from two separate fertilized eggs. Monozygotic twins can also be dichorionic if the zygote splits very early—before or around the formation of the chorion—resulting in two chorions. The distinction between chorions is clinically important because it influences placentation and potential pregnancy risks.

Diagnosis is usually made by ultrasound in the first or early second trimester. A classic indicator of dichorionic placentation is the presence of two separate placental masses and, in many cases, a visible intertwin membrane separating the sacs. The lambda sign may be observed at 10–14 weeks and supports a dichorionic pregnancy, whereas a T sign on ultrasound suggests monochorionic placentation.

Clinical significance includes a generally lower risk of twin-twin transfusion syndrome compared with monochorionic twins, but there remains elevated risk for preterm birth, growth discordance, and other obstetric complications relative to singleton pregnancies. Management involves routine care for multiple gestations with closer monitoring of growth, amniotic fluid, and placental function, typically with serial imaging and obstetric follow-up until delivery.