Monoamniotic

Monoamniotic refers to a type of twin pregnancy in which the two fetuses share a single amniotic sac. In obstetric terminology, monoamniotic twins are part of monochorionic monoamniotic (MCMA) pregnancies, meaning they share both the chorion and the amnion. This is in contrast to monochorionic diamniotic pregnancies, where twins share a chorion but have separate amniotic sacs, and to dichorionic pregnancies, where each fetus has its own placental and amniotic compartments.

Monoamniotic pregnancies arise from monozygotic twinning with division of the embryo occurring after the formation of the chorion and amnion, typically around day 8 or later. Sharing a single amniotic sac increases the risk of umbilical cord entanglement and compression, which can lead to sudden fetal deterioration. They also have higher risks of fetal growth restriction and perinatal complications compared with other twin pregnancies.

Diagnosis is made by ultrasound, which shows two fetuses in a single amniotic cavity with one chorionic sac and no separating amniotic membrane. Continuous or frequent fetal surveillance is standard in known MCMA pregnancies, often with inpatient monitoring beginning in the late second or early third trimester. Surveillance may include regular ultrasounds for growth and amniotic fluid assessment and nonstress tests or fetal heart rate monitoring.

Management typically involves planned delivery by cesarean section in the late second or early third trimester, commonly around 32 to 34 weeks gestation, to reduce the risk of stillbirth. Some centers may extend surveillance and delivery timing based on fetal status and maternal factors. Despite advances in care, MCMA pregnancies remain high risk, requiring specialized obstetric management and neonatal preparedness.