- Objective: To investigate whether postterm pregnancy (≥42 0/7 weeks’ gestation) increases the risk for adverse perinatal outcome. Study design: In this population based cohort study, all singleton deliveries occurring between 1991 and 2014 in a tertiary medical center were included. Pregnancy and perinatal outcomes were compared between postterm and term deliveries (37 0/7 to 41 6/7 weeks’ gestation). Preterm deliveries, unknown gestational age, congenital malformations, and multiple gestations, were excluded. The association between postterm and adverse perinatal outcomes was evaluated using a general estimation equation (GEE) multivariable analyses. Results: During the study period, 226,918 deliveries were included in the analysis. Of them, 95.9% (n = 217,544) were term and 4.1% (n = 9374) were postterm. Post-term pregnancies were more likely to be complicated with oligohydramnios, macrosomia, meconium stained amniotic fluid, shoulder dystocia, low Apgar scores, and hysterectomy (p < .05 in all). Perinatal mortality rates were significantly higher at postterm as well. Using the GEE model, the association between postterm and total perinatal mortality persisted (OR = 1.73, 95%CI 1.2–2.4), as well as specifically intrauterine fetal death (OR = 1.76, 95%CI 1.1–2.7) and intrapartum death (OR = 3.71, 95%CI 1.3–10.4). Conclusions: Post-term delivery involves higher rates of adverse perinatal outcomes and is independently associated with significant perinatal mortality.