All women should be vaccinated with the combined tetanus, diphtheria, and pertussis (Tdap) vaccine during each pregnancy, regardless of whether they have been previously vaccinated, recommends the US Centers for Disease Control and Prevention's (CDC) Advisory Committee for Immunization Practices (ACIP).
Previously, the ACIP recommended Tdap vaccination only for pregnant women who had not previously received the vaccine. Vaccinating during each pregnancy offers an advantage in that maternal pertussis antibodies may be transferred to the fetus, potentially providing some protection for the newborn against pertussis until the infant is old enough to receive a first dose of the diphtheria, tetanus, and pertussis (DTaP) vaccine at 2 months of age. The ACIP recommends that children receive 5 doses of DTaP by age 4 years; Tdap is typically used for older age groups. Vaccination during pregnancy may also prevent mothers from being infected around the time of delivery, which may decrease the chance of pertussis being transmitted from mother to newborn.
Kuehn BM. ACIP: Give Pertussis Vaccine During Every Pregnancy. JAMA. 2012;308(19):1960. doi:10.1001/jama.2012.54092