diamnionicity

Diamnionicity, also called diamnioticity, is the condition in pregnancy in which the developing fetuses are enclosed within two amniotic sacs, each separated by an amnion. In twin or higher-order pregnancies, diamnionicity denotes two distinct amniotic membranes around the fetuses. Diamnionicity may occur with two placentas (dichorionic diamniotic) or with a single shared placenta (monochorionic diamniotic).

In monozygotic twinning, the timing of zygote division determines amnionicity and chorionicity: division within about the first four days yields two placentas and two amniotic sacs (dichorionic diamniotic); division between days 4 and 8 yields one placenta with two amniotic sacs (monochorionic diamniotic); division after day 8 can produce a single shared amnion (monoamniotic). Thus diamnionicity refers to two amniotic sacs regardless of placentation.

Diagnosis and assessment are primarily by prenatal ultrasound. Ultrasound can reveal two amniotic sacs separated by a membrane; the number of placentas helps determine chorionicity. The lambda sign on ultrasound suggests dichorionic diamniotic twins, while the T sign indicates monochorionic diamniotic twins, where a single chorion surrounds both amniotic sacs.

Clinical significance centers on risk stratification and management. Dichorionic diamniotic pregnancies generally have lower risk of twin-to-twin transfusion syndrome (TTTS) than monochorionic pregnancies. Monochorionic diamniotic pregnancies carry TTTS, TAPS, and growth-discordance risks and require closer surveillance and specialized care. Knowledge of diamnionicity is essential for guiding monitoring, prognosis, and potential interventions in twin gestations.