Progesterone suppresses innate immune responses [125, 129] . Elevated concentrations of P4 during pregnancy inhibit the development of Th1 immune responses that can lead to fetal rejection and promote production of Th2 immune responses [130, 131] . Progesterone also suppresses antibody production [132] . Recent data illustrate that pregnant women with severe 2009 H1N1 have lower levels of total IgG2 than healthy pregnant women or women with only moderate H1N1 disease [133] . As IgG2 levels are enhanced in a Th1dependent manner, this reduction in total IgG2 may be related to pregnancy-associated modulation of the immune response.