chorionicity

Chorionicity is the number of chorions surrounding fetuses in a multiple pregnancy, most commonly twins. The chorion is the outer fetal membrane contributing to the placenta, while the amnion lies inside it around the developing fetus. In twins, chorionicity is an important determinant of risk and management.

The three main patterns are: dichorionic diamniotic (two chorions and two amnions), monochorionic diamniotic (one chorion and two amnions), and monochorionic monoamniotic (one chorion and one amnion). These patterns reflect the zygosity and the timing of embryonic cleavage after fertilization. Cleavage within the first about 0–4 days before formation of the chorion yields dichorionic diamniotic twins. Cleavage after chorion formation but before amnion formation (roughly days 4–8) yields monochorionic diamniotic twins. Cleavage after the amnion has formed (roughly days 8–12) yields monochorionic monoamniotic twins.

Chorionicity is typically determined by first-trimester ultrasound. In dichorionic pregnancies, two placental masses are usually seen, often with a lambda sign indicating two placentas. Monochorionic pregnancies show a single placental mass without the lambda sign; the membranes may appear as a single shared interface, and a T-sign can be described in some imaging conventions. While ultrasound provides the usual assessment, clinical management relies on chorionicity, not simply zygosity.

Clinical significance rests on risk differences. Monochorionic pregnancies carry higher risks for twin-twin transfusion syndrome, selective growth restriction, and other complications requiring closer surveillance and, in some cases, intrauterine interventions. Dichorionic pregnancies have comparatively lower risk, but standard twin-pregnancy monitoring remains essential.