Preventing influenza during pregnancy is an essential element of prenatal care, and the most effective strategy for preventing influenza is annual immunization. The Centers for Disease Control and Prevention's Advisory Committee on Immunization Practice recommends influenza vaccination for all women who will be pregnant through the influenza season (October through May in the United States). The American College of Obstetricians and Gynecologists' Committee on Obstetric Practice supports this recommendation. No study to date has shown an adverse consequence of inactivated influenza vaccine in pregnant women or their offspring. Vaccination early in the season and regardless of gestational age is optimal, but unvaccinated pregnant women should be immunized at any time during influenza season as long as the vaccine supply lasts.