Bordetella pertussis infection may cause severe illness in newborns. Mothers with B. pertussis infection during delivery can infect newborns. The seroprevalence of B. pertussis infection in pregnancy was measured in pregnant women by detection of immunoglobulin G against pertussis toxin; 6.3% had serological evidence of infection. Maternal vaccination should be considered to prevent pertussis in newborns.