- Objective: To establish whether postterm pregnancies are associated with adverse perinatal outcomes in a linear fashion. Study design A retrospective cohort study investigating perinatal outcomes in singleton term (≥37 weeks gestation) and postterm pregnancies was conducted. Deliveries occurred between the years 1988 and 2010. Parturients were classified into three groups according to their gestational age: 37-39+6, 40-41+6, and over 42 weeks' gestation (postterm). Statistical analysis included multiple logistic regression model to control for possible confounders. Results Out of 202,462 deliveries, meeting the inclusion criteria, during the study period, 47.7%occurred at 37-39+6 weeks, 47.4% occurred at 40-41+6 weeks and 4.9% were postterm. Fertility treatments, diabetes mellitus, hypertensive disorders, intrauterine growth restriction (IUGR) and premature rupture of membranes (PROM) were less likely to be associated with postterm pregnancies. However, postterm was found as a significant risk factor for labor complications and adverse perinatal outcome including perinatal mortality. Using a multivariable logistic regression model, controlling for confounders such as maternal age and macrosomia, postterm was found to be an independent risk factor for perinatal mortality (adjusted OR=1.6; 95% CI 1.20-1.99; P<0.001). Conclusion Although postterm pregnancies were less likely to be associated with obstetrical risk factors and complications such as fertility treatments, diabetes mellitus, hypertensive disorders, IUGR, and PROM, postterm is an independent risk factor for perinatal mortality.