An association of chorionicity with preterm twin birth.

To assess the risk factors for preterm birth in twin pregnancies, particularly monochorionicity. A cohort study of 767 sets of twins, each twin weighing more than 500 g, born between January 1, 1992, and December 31, 2001, at St. Joseph's Health Care in London, Ontario. Statistical analysis was performed using forward stepwise logistic regression models, with gestational age at birth less than 28 or 32 weeks as the outcome. Polyhydramnios and chorioamnionitis were significant risk factors for preterm birth prior to 28 or 32 weeks' gestation. Monochorionicity was a risk factor for preterm birth prior to 32 weeks' gestation. Past term birth and maternal age over 30 years were associated with reduced risk for preterm birth. Monochorionic placentation is a significant risk factor for preterm twin birth.