The number of chorionic membranes present on direct examination of the placenta and membranes following delivery of twins. Chorionicity can be determined antenatally by ultrasound, ideally between 10 and 14 weeks gestation. Where two placental masses are identified, the pregnancy is dichorionic. More frequently a single placental mass exists and chorionicity can only be determined by evaluation of the intertwin membrane; the pregnancy is monochorionic when the fetuses share a chorion (see illustration; See also lambda sign, T-sign). Chorionicity rather than zygosity is a better determinant of perinatal outcome in twins. The perinatal mortality rate of monochorionic pregnancies may be up to four times that of dichorionic.
Subjects: Medicine and Health.