Objective To evaluate the serological response in pregnant Danish women immunized during the 2009 pandemic by serologic infection or by vaccination with influenza A(H1N1) Pandemrix^® and describe levels of passively acquired maternal antibody in their offspring. Design Observational cohort study. Setting Department of Obstetrics, Aarhus University Hospital, Skejby, Denmark, October to December 2009. Population Pregnant women and their offspring Methods Serological analysis of antibodies to influenza A(H1N1)pdm09 by hemagglutination inhibition assay in 197 women and their offspring. Blood samples were collected consecutively at delivery from the mother and the umbilical cord. In a subgroup of 124 of the 197 women, an additional blood sample from gestational weeks 9-12 was available for analysis. Main outcome measures Seroconversion, geometric mean titer, geometric mean-fold rise and protective antibodies. Results 33 of the 124 subgroup women (27%) seroconverted during pregnancy, 79% after vaccination and 17% after serologic infection (p < 0.001). The geometric mean titer after delivery in non-vaccinated, non-serologically infected women was 17.1 (95%CI 15.7-18.6). The geometric mean titer increased significantly after serologic infection with H1N1 [76.5 (95%CI 51.3-113.9), p < 0.001] and after vaccination [589.6 (95%CI 339.3-1024.7), p < 0.001]. The geometric mean-fold rise (mother at delivery/mother early pregnancy) was significantly higher after vaccination [2.23 (1.93-2.54)] than after serologic infection [1.73 (1.59-1.87), p = 0.013]. In newborns of vaccinated mothers, 89.5% had protective antibody levels compared with 15.8% in newborns of serologically infected mothers (p < 0.001). Conclusions Influenza vaccination during pregnancy confers passive immunity to the newborn.