BACKGROUND: Pregnant women are placed on the highest priority group for receiving influenza vaccination, but may be reluctant to receive the vaccination due to concerns about the influence of vaccination on the fetuses. METHODS: This prospective cohort study of 10,330 pregnant women examined the safety of influenza vaccination in terms of adverse birth outcomes. Influenza vaccination during pregnancy was determined from questionnaires before and after the 2013/2014 influenza season. All subjects were followed until the end of pregnancy. Adverse birth outcomes, including miscarriage, stillbirth, preterm birth, low birth weight, and malformation, was assessed by obstetrician's reports. RESULTS: Adverse birth outcomes were reported from 641 (10%) among 6,387 unvaccinated pregnant women and 356 (9%) among 3,943 vaccinated pregnant women. Even after adjusting for potential confounders, vaccination during pregnancy showed no association with the risk of adverse birth outcomes (OR = 0.90, 95%CI = 0.76-1.07). Vaccination during the first or second trimester displayed no association with adverse birth outcomes, whereas vaccination during the third trimester was associated with decreased risk of adverse birth outcomes (OR = 0.70, 95%CI = 0.51-0.98). CONCLUSIONS: Influenza vaccination during pregnancy did not increase the risk of adverse birth outcomes, regardless of the trimester in which vaccination was performed, compared to unvaccinated pregnant women.