Some degree of morning sickness is experienced by almost every pregnant woman (in interviews conducted after the peak period of pregnancy sickness, up to 90% of women reported some amount of nausea or food sensitivity). Because it is so common, the concept of morning sickness as a 'sickness' (read 'malfunction') is suspect.

One theory that is starting to gain acceptance is that the food aversions, nausea, and vomiting associated with pregnancy sickness serve as a mechanism to prevent the pregnant mother from consuming toxins that might harm the developing fetus.

Many of the foods humans commonly eat are loaded with carcinogens and other toxins. For example, black pepper contains sarole, a substance which is both carcinogenic and mutagenic. Tea consumption during the first trimester been associated with the development of spina bifida in fetuses. Grilled meats are loaded with carcinogens. The foods most commonly cited by pregnant women as causing nausea, vomiting, or being generally aversive are foods that have the highest concentrations of substances that could be harmful to a developing fetus.

The developing fetus is most vulnerable to toxins during the period from about 2 weeks after conception to roughly 10 weeks into development. During this period, the major organ systems are being formed. Pregnancy sickness is most pronounced during this period, and it drops off and often disappears afterwards.

Perhaps the most telling evidence in support of this theory is that women who report no pregnancy sickness are about 3 times more likely to experience spontaneous abortion (i.e. miscarriage) than women who do have pregnancy sickness.

It's not an illness, it's an adaptation!

This theory was developed by Margie Profet. For more information, see her article: Profet, M. (1992). Pregnancy sickness as adaptation: A deterrent to maternal ingestion of teratogens. In J. Barkow, L. Cosmides, & J. Tooby (Eds.), The adapted mind (pp. 327-366). New York: Oxford University Press.

factgirl: In response to your various comments: All scientists develop hypotheses using induction. People do not require graduate degrees to have good ideas. Margie Profet may not have done scientific research herself, but that does not invalidate her theory. Some people do research, some people process it and form hypotheses, few people are good at both. Here is some relevant research in support of Profet's theory of morning sickness:

*regarding the toxicity of various foods: Buttery, R.G., Guadagni, D.G., Ling, L.C., Siefert, R.M., and Lipton, W. (1976). Additional volatile components of cabbage, broccoli, and cauliflower. Journal of Agricultural and Food Chemistry, 24, 829-832; Nesse, R.M., and Williams, G.C., (1994). Why We Get Sick. New York: Times Books Random House.
*studies on the foods pregnant women find aversive: Tierson, F.D., Olsen C.L., and Hook, E.B. (1985). Influence of cravings and aversions on diet in pregnancy. Ecology of Food and Nutrition, 17, 117-129; Dickens, G., and Threthowan, W.H. (1971). Cravings and Aversions during pregnancy. Journal of Psychosomatic Research, 15, 259-268.
*describing the typical time course of morning sickness: Gadsby, R. (1994). Pregnancy sickness and symptoms: your questions answered. Professional Care of Mother and Child, 4(1), 16-17.
*review of fetal development: Vasta, R., Haith, M.M., and Miller, S.A. (1995). Child Psychology: the Modern Science, 2nd Edition. New York: John Wiley and Sons, Inc. (Chapter 5)
*describing the connection between pregnancy sickness and spontaneous abortion: Yerushalmy, J., and Milkovich, L. (1965). Evaluation of the teratogenic effects of meclizine in man. American Journal of Obstetrics and Gynecology, 93, 553-562.
*recent review article supporting Margie Profet's theory of morning sickness: Flaxman, S.M., and Sherman P.W. (2000). Morning sickness: a mechanism for protecting mother and embryo. Quarterly Review of Biology, 75(2), 113-148.
*association of tea and spina bifida: Correa, A., Stolley, A., Liu, Y. (2000). Prenatal tea consumption and risks of anencephaly and spina bifida. Annals of Epidemiology, 10(7), 476-477.

factgirl: oops, sorry, you were right about the potatoes. Only potatoes that have been exposed to potato blight increase the risk of spina bifida (which explains the prevalence in Ireland). I've removed the offending potato comment. But I did find an interesting article that linked first trimester tea consumption and spina bifida. Dickens and Threthowen (1971) found that about 1/3 of pregnant women reported tea aversions, so I'm replacing potatoes with tea and adding another reference to the reference farm above.