The Northwestern twin chorionicity study: testing the 'placental crowding' hypothesis.

Objective: To evaluate the relation between placental proximity and frequency of birth weight discordance and small-for-gestational age (SGA) infants.

Methods: Retrospective three-tier chorionicity analysis of 1155 twin placentas comparing birth weight characteristics of the twins in different placental types.

Results: Dichorionic-separate, but not dichorionic-fused twins, are heavier than monochorionic-diamniotic and monoamniotic twins (2376+/-721 vs. 2274+/-770, P < 0.006, and 2376+/-721 vs. 2166+/-782, P < 0.04). SGA twins are less frequent among dichorionic twins (OR 0.4; 95% CI 0.3, 0.6). Fewer sets with two SGA infants are present among dichorionic-separate compared to monochorionic-diamniotic pairs (OR 0.3; 95% CI 0.1, 0.8). The same trends are found when comparing all dichorionic to all monochorionic twins. Twins of all placental types have similar gestational ages and discordance values.

Conclusions: Dichorionic-separate placentas are least likely to experience 'placental crowding' and thus are associated with heavier twins and fewer sets with one or two SGA infants.