Maternal pertussis vaccination is associated with higher levels of pertussis antibodies at birth. We assessed the persistence of pertussis antibodies until primary vaccination in infants whose mothers received Tdap (tetanus, diphtheria, acellular pertussis) vaccine during pregnancy. Infants were born at the Hospital Clinic of Barcelona (Spain) in November 2014. Anti-PT IgG was determined by ELISA at delivery, between the first and second month of life, and estimated at 2months of age. The study included 37 infants whose mothers received Tdap between 21 and 38weeks of gestation. Infants presented a decline in GMC of anti-PT IgG between peripartum and follow-up levels, 52.7 (95% CI 34.7-80.2) versus 7.5 (95% CI 4.2-13.3) at 2months of age (p<0.001). The median half-life of maternal antibodies was 47days. More than half (51.4%) the infants presented detectable anti-PT IgG before the start of primary infant vaccination.