monochorion

Monochorion is a term used in biology and medicine to describe a condition in which a single chorion surrounds the developing embryo or embryos. The chorion is the outer fetal membrane formed from the trophoblast and contributes to the formation of the placenta. The noun monochorion is sometimes used to denote this single-chorion arrangement, while the more common adjective in medical literature is monochorionic.

In humans, monochorionic pregnancies most often arise from monozygotic (identical) twinning when the fertilized egg splits after the chorion has begun to form. Because there is only one chorionic sac, the placentation is shared to varying degrees between the fetuses. The amnion can be two membranes (monochorionic diamniotic) or one membrane (monochorionic monoamniotic). The placenta is typically shared, and there can be vascular connections between the fetuses.

Clinical significance and risks: Monochorionic twinning carries higher perinatal risks than dichorionic twinning, largely due to inter-fetal placental blood flow connections. Complications can include twin-to-twin transfusion syndrome (TTTS), twin anemia-polycythemia sequence (TAPS), and growth discordance. Because of these risks, monochorionic pregnancies are usually monitored closely with serial ultrasound examinations and Doppler studies. Treatments, when needed, may involve fetal therapy such as laser coagulation of placental vessels or amnioreduction, and delivery planning is guided by gestational age and fetal condition.

See also: dichorion; chorion; monozygotic twinning; twin-twin transfusion syndrome.