OBJECTIVE: Uterine synechiae have generally been considered benign findings in pregnancy. We used a large perinatal database to test the hypothesis that uterine synechiae are associated with pregnancy complications. METHODS: We performed a retrospective cohort study of women with singleton pregnancies presenting for routine ultrasonographic examinations at 17-22 weeks from 1990-2009. Pregnancies with multiple fetuses, amniotic bands, and congenital uterine anomalies were excluded. We compared pregnancy outcomes between women with and without uterine synechiae. Multivariable logistic regression was used to adjust for confounders. RESULTS: Of 65,518 pregnancies meeting inclusion criteria, 296 (0.45%) were diagnosed with uterine synechiae. Women with uterine synechiae were significantly more likely to have placental abruption (2.1% compared with 0.6%, adjusted odds ratio [OR] 3.25, 95% confidence interval [CI] 1.43-7.36), preterm premature rupture of membranes (PROM) (5.5% compared with 2.3%, adjusted OR 2.51, 95% CI 1.51-4.18), and cesarean delivery for malpresentation (5.1% compared with 3.0%, adjusted OR 1.75, 95% CI 1.04-2.95). The risks of placenta previa, fetal growth restriction, stillbirth, and preterm delivery were not significantly different. CONCLUSION: Uterine synechiae are associated with significant increase in the risk of preterm PROM, placental abruption, and cesarean delivery for malpresentation. The notion of uterine synechiae as benign findings in pregnancy should be re-evaluated. LEVEL OF EVIDENCE: II.