We report the immunogenicity of trivalent influenza immunization in 29 pregnant women compared with 22 nonpregnant women. We obtained blood specimens on day 0 prior to 2011-2012 influenza vaccine administration and day 28 after immunization. Hemagglutination inhibition (HAI) geometric mean titers were similar before immunization but were significantly reduced by 40%-50% in pregnant women after immunization for influenza A/California(H1N1) (P = .027) and A/Perth(H3N2) (P = .037). Postimmunization HAI titers were similar between groups for influenza B/Brisbane (P = .390). The geometric mean ratio (fold increase) for influenza A(H1N1) was nonsignificantly reduced in pregnant participants (P = .089). The percentages of participants who seroconverted and achieved seroprotection were similar between groups.