monochorionic

Monochorionic refers to a pregnancy in which two or more fetuses share a single chorion, and usually a single placenta, as a result of a single fertilized egg that has split early in development. In twin pregnancies, monochorionicity is typically paired with either two amniotic sacs (monochorionic diamniotic) or one amniotic sac (monochorionic monoamniotic). The presence of one chorion indicates that the fetuses are closely connected via placental vessels.

Diagnosis is usually made by ultrasound, which assesses placental structure and the number of amniotic sacs. Understanding chorionicity is important for risk assessment and management. Monochorionic pregnancies carry higher risks than dichorionic pregnancies because shared placental circulation can lead to vascular complications between the twins.

Common complications associated with monochorionic pregnancies include twin-to-twin transfusion syndrome (TTTS), where unequal blood flow through shared placental vessels can cause discordant fetal growth and fluid imbalances; selective intrauterine growth restriction; and, in monochorionic monoamniotic pregnancies, umbilical cord entanglement and sudden fetal compromise. Less frequent risks include twin anemia-polycythemia sequence (TAPS) and/TRAP sequence.

Management requires specialized fetal surveillance and obstetric care. Regular ultrasound examinations monitor growth and amniotic fluid, and assessments of the placental vascular connections guide treatment decisions. TTTS may be treated with fetoscopic laser photocoagulation to seal abnormal placental vessels. Delivery plans are individualized but often occur earlier than uncomplicated singleton pregnancies, with MCMA pregnancies typically delivered closer to 34–37 weeks and MCDA pregnancies closer to 37–38 weeks, depending on progression and complications. Prognosis varies with timely diagnosis and quality of prenatal care, but many monochorionic twins reach term with good outcomes.