The proximate mechanisms underlying gestational nausea and vomiting have been intensively studied, but the possibility that the symptoms themselves serve a useful function has only recently been considered seriously. We synthesized evidence to evaluate various hypotheses for the adaptive significance of nausea and vomiting of pregnancy, as well as the possibility that symptoms are nonfunctional byproducts of pregnancy hormones. We found greatest support for the hypothesis that normal levels of nausea and vomiting of pregnancy (excluding hyperemesis) protect pregnant women and their embryos from harmful substances in food, particularly pathogenic microorganisms in meat products and toxins in strong-tasting plants. We discuss the data that support critical predictions of this "maternal and embryo protection hypothesis" (and contradict other hypotheses), as well as appropriate implications of these results. Knowledge that normal nausea and vomiting of pregnancy indicates the functioning of a woman's defense system, rather than a bodily malfunction, may reassure patients and enable health care providers to develop new ways of minimizing the uncomfortable symptoms.