Introduction: Illness and death from influenza increase during pregnancy. In the United Kingdom pregnant women were targeted in a national programme for vaccination during the H1N1 2009-10 pandemic. Methods: In this study, pregnant women were recruited in labour from November 9, 2009 to March 10, 2010. Pandemic vaccination status was determined. Venous cord blood collected at delivery was evaluated for transplacental transfer of antibodies by measurement of haemagglutination inhibition and microneutralization titres. Results: Samples were collected from 77 vaccinated and 27 unvaccinated women. Seroprotection (HI titre ≥1:40) was detected in 58 (75.3%, 95% CI 64.2-84.4) cord blood samples from vaccinated women and 5 (18.5%, 95% CI 6.3-38.1) from unvaccinated women (P<0.0001). There was evidence of transplacental seroprotection 8 days after maternal immunization (77.9%, 95 CI 66.2-87.1), maintained in most cases for at least 16 weeks. Discussion: Immunization of pregnant women with AS03A-adjuvanted vaccine is followed by transplacental transfer of passive immunity at titres consistent with clinical protection in three-quarters of new-born infants. The findings support national and international pandemic H1N1 2009 recommendations for immunization during pregnancy.