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Variation in placental weight may cause significant differences in the birth weight of dichorionic twins. Dichorionic twins have two distinct chorions, the outer extraembryonic membrane. The lighter twin may also have a larger number of placental lesions that the heavier twin. Researchers examined the placentas of 91 dichorionic twin pairs and 40 monochorionic twin pairs. Monochorionic twins share the same chorion. Among the dichorionic twin pairs, the lighter twin was more likely to belong to the lighter placenta than the heavier twin. The lighter dichorionic twin was more likely to have the largest number of placental lesions than the heavier twin. No association between placental abnormalities and differences in birth weight was found in the monochorionic twin pairs.