The intricate mechanisms regulating fetomaternal interactions are still largely uncharacterized. Recent papers have revealed a major role for the innate immune system during abortion. Different experimental conditions—deletion of a complement regulator, injection of anti-phospholipid antibodies into mothers, or allorecognition of fetuses in the presence of an IDO inhibitor—all lead to complement activation, inflammation, and fetal loss. These observations also raise new questions on the relationship between the adaptive and innate systems during pregnancy.