Perinatal outcome, placental pathology, and severity of discordance in monochorionic and dichorionic twins.

OBJECTIVE: To evaluate differences in pregnancy outcomes and placental findings among severely discordant monochorionic and dichorionic twins.
METHODS: We studied retrospectively a cohort of 382 twin pregnancies with gestational ages that ranged from 24 to 40 weeks. Pregnancies were classified as dichorionic or monochorionic by histologic examination of placentas. Infants were subdivided into concordant (less than 5% difference in birth weight), mildly discordant (5-25% difference), and severely discordant groups (more than 25% difference), and their clinical characteristics and findings at placental examination were analyzed and compared.
RESULTS: Severe discordance occurred significantly more often in monochorionic than in dichorionic twins and was associated with significantly more deliveries before 36 weeks and more newborns remaining more than 10 days in the neonatal intensive care unit. Severely discordant monochorionic and dichorionic twins had significantly worse perinatal mortality and morbidity than mildly discordant and concordant twins. The weight of the placenta of the smaller fetus in severely discordant dichorionic twins with separate placentas and the total placental weight in severely discordant monochorionic twins were significantly smaller than the weights of the placentas in their concordant and mildly discordant counterparts. The umbilical cords of the smaller fetuses in both dichorionic and monochorionic pregnancies exhibited significantly more velamentous insertions and single umbilical arteries than in concordant or mildly discordant twins of similar chorionicity.
CONCLUSION: Severe discordance is more frequent and has greater morbidity in monochorionic than dichorionic twins. The most frequent findings in the placentas of severely discordant twins were small placental weight and umbilical cord abnormalities.