OBJECTIVE: To identify factors associated with post-term birth and factors associated with adverse outcomes in post-term births. METHODS: The sample was a 10-year cohort (1978-1987) of term (n = 379,445) and post-term (n = 65,796) births from the Medical Birth Registry of Norway. Gestational age was based on mothers' recall of the last menstrual period. Multivariate analyses included proportional hazards regression and binomial logistic regression. RESULTS: After controlling for covariates, there was only a slightly increased risk of perinatal mortality in post-term as compared with term births (adjusted relative risk [RR] 1.11; 95% confidence interval [CI] 0.97, 1.27). For post-term births, risk factors for perinatal mortality were small size for gestational age (SGA) (adjusted RR 5.68; 95% CI 4.37, 7.38) and maternal age 35 years or older (adjusted RR 1.88; 95% CI 1.22, 2.89), whereas large size for gestational age (LGA) was a protective factor (adjusted RR 0.51; 95% CI 0.26, 1.00). Similar risk factor RRs were found for perinatal mortality in term births. Fetal distress was associated with both SGA and post-term birth; labor dysfunction and obstetric trauma were associated with both LGA and post-term birth; shoulder dystocia and maternal hemorrhage were associated with LGA only. CONCLUSIONS: Among post-term births, maternal complications were generally associated with larger fetal size, and fetal complications were associated with smaller fetal size. The evidence for an adverse impact on perinatal mortality of post-term birth is weak once other factors are taken into account.