OBJECTIVE: To estimate the risks for medically attended events occurring within 42 days of receiving trivalent inactivated influenza vaccine and to evaluate specific risks of first-trimester vaccination. METHODS: This retrospective observational cohort study compared rates of medically attended adverse events in trivalent inactivated influenza-vaccinated and unvaccinated pregnant women in the Vaccine Safety Datalink. Using a Poisson distribution and log link, we calculated maternal adjusted incident rate ratios for composite safety outcomes for the full cohort and the subset vaccinated during the first trimester. RESULTS: The cohort included 75,906 vaccinated (28.4% in the first trimester) and 147,992 unvaccinated women matched by age, site, and pregnancy start date. In the first 3 days after vaccination, trivalent inactivated influenza vaccine was not associated with increased risk of specified medically attended events, including allergic reactions, cellulitis, and seizures (full cohort adjusted incident rate ratio 1.12, 95% confidence interval [CI] 0.81-1.55; P=.48; first-trimester adjusted incident rate ratio .97, 95% CI 0.53-1.78; P=.93). In the first 42 days, no incident cases of Guillain-Barré syndrome, optic neuritis, transverse myelitis, or Bells palsy were identified. Trivalent inactivated influenza vaccine was not associated with thrombocytopenia (full cohort adjusted incident rate ratio 0.90, 95% CI 0.68-1.19; P=.45; first-trimester adjusted incident rate ratio 0.56, 95% CI 0.22-1.39; P=.21) or an acute neurologic event (full cohort adjusted incident rate ratio 0.92, 95% CI 0.54-1.6; P=.75; first-trimester adjusted incident rate ratio 1.05, 95% CI 0.46-2.38; P=.91). CONCLUSIONS: Receipt of trivalent inactivated influenza vaccine during pregnancy was not associated with increased risk of adverse events in the 42 days after vaccination, supporting its safety for the mother.