BACKGROUND: 5-Aminosalicylic acid (5-ASA) preparations are the firstline drugs in the treatment of inflammatory bowel disease. Data on the safety of these drugs in pregnancy are sparse. AIMS: To examine the risk of adverse birth outcome in women who were prescribed 5-ASA drugs during pregnancy. PATIENTS: Women were included in the study if they were prescribed 5-ASA drugs immediately before or during pregnancy. To examine the risk of malformations, we included 60 pregnancies exposed to 5-ASA drugs 30 days before pregnancy or in the first trimester. To examine stillbirths, preterm births, and low birth weight, we included 88 pregnancies exposed during the entire pregnancy. Outcomes were compared with those of 19 418 pregnancies in which no drugs were prescribed for mothers during the study period. METHODS: We conducted a Danish cohort study based on data from a population based prescription registry, the Danish Birth Registry, and the Hospital Discharge Registry in North Jutland County. RESULTS: Odds ratios for malformations, stillbirth, preterm birth, and low birth weight in women who received prescriptions for 5-ASA drugs were 1.9 (95% confidence interval 0.7-5.4), 6.4 (1.7-24.9), 1.9 (0.9-3.9), and 1.2 (0.4-3.3), respectively. The increased risk of stillbirth and preterm birth were found only in patients with ulcerative colitis. CONCLUSIONS: We found an increased risk of stillbirth and preterm birth in women who had been prescribed 5-ASA drugs during pregnancy but no substantial increased risk of malformations. It was difficult to distinguish the specific effects of disease activity and 5-ASA drugs.