ABSTRACT

Twin and higher-order multiple pregnancy (HOMP) placentas should be examined routinely because:

(1) Chorionicity is not always determined correctly by prenatal ultrasound, and is relevant to any adverse pregnancy outcomes;

(2) If placentation is proved to be monochorionic (MC) by pathology, twins are monozygotic (MZ) (with very rare exceptions);

(3) Explanations of growth discordance, fetal demise, neurologic injury, extent of chorioamnionitis/fetal inflammation depend on chorionicity;

(4) The usual ‘rules’ about twin pathology are regularly broken, so that only familiarity with norms allows sophisticated exceptions to be recognized, e.g. ‘hybrid’ MC/dichorionic (DC) twin placentas, succenturiate lobes in MC placentas masquerading as ‘DC’ by ultrasound.