OBJECTIVE To estimate whether first-trimester exposure to nitrofurantoin is associated with increased malformation rates and, secondarily, to estimate whether exposure to nitrofurantoin is associated with increased rates of negative pregnancy outcomes and perinatal complications. METHODS We conducted a population-based cohort study using the Norwegian Prescription Database linked to data on all live births, stillbirths, and induced abortions after 12 weeks of gestation from The Medical Birth Registry of Norway. The study population consisted of 180,120 pregnancies in 2004-2008. The pregnancy outcomes of women who were dispensed nitrofurantoin during pregnancy were compared with the outcomes of women who were dispensed pivmecillinam (disease comparison group) and unexposed women. RESULTS In all, 5,794 (3.2%) filled prescriptions for nitrofurantoin during pregnancy, 1,334 women (0.7%) in the first trimester and 979 women (0.5%) in the last 4 weeks of pregnancy. Dispensing nitrofurantoin during the first trimester was not associated with increased risk of major malformations (31 of 1,334 [2.3%]) compared with disease controls (162 of 5,800 [2.8%], odds ratio [OR] 0.79, 95% confidence interval [CI] 0.51-1.23). No increased risk for secondary adverse pregnancy outcomes was observed when compared with the disease comparison group. Dispensing nitrofurantoin the last 30 days before delivery was associated with increased risk of neonatal jaundice (103 of 959 [10.8%]) compared with unexposed women (10,336 of 127,507 [8.1%], OR 1.31, 95% CI 1.02-1.70). CONCLUSIONS The results from this large, population-based study suggest that the teratogenic potential of nitrofurantoin is low. METHODS II.