Dichorionic

Dichorionic refers to a placentation pattern in which two chorions are present. In twin pregnancies, this typically corresponds to two placentas and two amniotic sacs, a configuration known as dichorionic diamniotic (DCDA). The term is used to describe the presence of two separate fetal placental membranes, each supporting its own fetus.

Most dichorionic twins are dizygotic, arising from two separate zygotes. In monozygotic twinning, dichorionicity can occur if the fertilized egg splits within the first four days after fertilization, before the formation of the chorion. This early split results in two chorions and usually two amniotic sacs.

Diagnosis is usually made by ultrasound in the first trimester. Chorionicity assessment looks for two placental discs and membranes, and a lambda (λ) sign supports dichorionicity, while a single placenta with a T-sign suggests monochorionicity. By mid-pregnancy, the presence of two placentas and typically two amniotic sacs confirms a DCDA configuration.

Clinical significance includes a lower risk of twin-to-twin transfusion syndrome compared with monochorionic pregnancies, due to separate placental circuits. However, dichorionic twin pregnancies remain high risk relative to singleton pregnancies, with risks such as preterm birth, growth discordance, and congenital anomalies. Management involves regular obstetric surveillance, including serial ultrasound assessment of fetal growth and amniotic fluid status, and delivery planning based on maternal and fetal condition, typically in the late preterm to term period unless complications arise.