Background: The objective was to study pregnancy outcomes between groups of Danish women, with pregnancy ending between 1998 and 2009, according to their exposure to montelukast. Methods: Cross-sectional observational study in Danish women, selecting live births and stillbirths (Birth Registry) and spontaneous abortions and induced terminations (Patient Registry). Montelukast exposure was obtained from the Prescription Registry (ATC code R03DC03). Exposure period was from 3 months before the last menstrual period until the end of the first trimester. Four groups were studied: (1) women with prescription for montelukast, (2) women with prescription for montelukast and other anti-asthmatic medications, (3) women with prescription for other anti-asthmatic medications, (4) women without prescription for any anti-asthmatic medications. Results: A total of 754,300 singleton pregnancies (>12 weeks) were identified: 401 pregnancies in group 1, 426 pregnancies in group 2, 24878 in group 3 and 728,595 in group 4. Risk of preterm birth, maternal preeclampsia and gestational diabetes was increased for pregnancies exposed to montelukast. No significant differences were found for the risk of major congenital anomalies (CA). Adjusted odds ratio for CA was 1.4 (95% CI 0.9-2.3) for the group 1 and 1.0 (95% CI 0.6-1.8) for group 2. Conclusion: Pregnant women with prescriptions for montelukast had a higher risk of preterm birth and maternal complications. These risks are known to be associated with maternal asthma, no increased risk of CA was found. Further analysis including more exposed pregnancies will be needed to determine the risk of specific CA. (C) 2017 Wiley Periodicals, Inc.