Morning sickness: A mechanism for protecting mother and embryo

Approximately two-thirds of women experience nausea or vomiting during the first trimester of pregnancy. These symptoms are commonly known as morning sickness. hook (1976) and Profet (1988) hypothesized that morning sickness protects the embryo by causing pregnant women to physically expel and subse quently avoid foods that contain teratogenic and abortifacient chemicals, e specially toxic chemicals in strong-tasting vegetables, caffeinated beverag es and alcohol. We examined this hypothesis by comprehensively reviewing th e relevant medical, psychological and anthropological literature. In its su pport, (i) symptoms peak when embryonic organogenesis is most susceptible t o chemical disruption (weeks 6-18), (ii) women who experience morning sickn ess are significantly less likely to miscarry than women who do not (9 of 9 studies), (iii) women who vomit suffer fewer miscarriages than those who e xperience nausea alone, and (iv) many pregnant women have aversions to alco holic and nonalcoholic (mostly caffeinated) beverages and strong-tasting ve getables, especially during the first trimester. Surprisingly, however, the greatest aversions are to meats, fish, poultry, and eggs. A cross-cultural analysis using the Human Relations Area Files revealed 20 traditional soci eties in which morning sickness has been observed and seven in which it has never been observed. The latter were significantly less likely to have ani mal products as dietary staples and significantly more likely to have only plants (primarily corn) as staples than the 20 societies in which morning s ickness occurred. Animal products may be dangerous to pregnant women and th eir embryos because they often contain parasites and pathogens, especially when stored at room temperatures in warm climates. Avoiding foodborne micro organisms is particularly important to pregnancy women because they are imm unosuppressed, presumably to reduce the chances of rejecting tissues of the ir own offspring (Haig 1993). As a result, pregnant women are more vulnerab le to serious, often deadly infections. We hypothesize that morning sicknes s causes women to avoid foods that might be dangerous to themselves or thei r embryos, especially foods that, prior to widespread refrigeration, were l ikely to be heavily laden with microorganisms and their toxins. The alterna tive hypotheses that morning sickness is (i) an epiphenomenon of mother-off spring genetic conflict or hormones associated with viable pregnancies, or (ii) an indicator to potential sexual partners and kin that the woman is pr egnant, resulting in reduced sexual behavior and increased nepotistic aid, were not well supported. Available data are most consistent with the hypoth esis that morning sickness serves an adaptive, prophylactic function.