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Monday, Nov. 20, 2000

NATURAL SELECTIONS

No, really: morning sickness benefits mothers, babies alike


Most women would find it hard to believe that morning sickness -- vomiting and nausea during pregnancy -- is a good thing, but the evidence is growing that it helps protect the mother and her baby.

In a paper in the Quarterly Review of Biology, Samuel Flaxman and Paul Sherman of the Department of Neurobiology and Behavior at Cornell University have reviewed a wealth of evidence from medical, psychological and anthropological studies. They conclude that morning sickness is adaptive. That is, it has a selective function: to prevent women from ingesting foods which might be dangerous to themselves or their embryos.

Around two-thirds of women suffer from morning sickness, usually during the first trimester (the first 13 weeks) of pregnancy. Despite its name it can occur throughout the day and is more accurately termed nausea and vomiting during pregnancy (NVP).

Neural control of vomiting is mediated by areas in the brainstem. Sickness symptoms are stimulated by two pathways, from the chemoreceptor trigger zone, and from gastrointestinal centers. The chemoreceptor trigger zone also controls food intake and determines aversions to certain foods. So food intake, aversion and vomiting are all controlled by the same areas of the brain.

It's been 60 years since it was reported that there were fewer miscarriages in pregnancies which had shown symptoms similar to morning sickness. Following him, other researchers proposed that NVP was beneficial because it led to the expulsion of dangerous compounds in food.

The embryo protection hypothesis subsequently developed concentrated on the response of the pregnant woman to the harmful effects of alcohol, caffeine and tobacco, and was detailed by Margie Profet in her 1995 book "Protecting Your Baby-to-Be."

Still, a rigorous test of the opposing arguments was not made until Flaxman and Sherman's recent paper. They aimed to test five critical predictions of the embryo protection hypothesis: that NVP is associated with positive outcomes to pregnancy; that foods which lead to NVP contain harmful compounds; that NVP should occur at developmentally vulnerable stages; that women should show aversion to foods containing toxins at sensitive periods of fetal development, and that NVP should be uncommon in populations where the staple diet rarely contains substances toxic to embryos.

The survey supported all the predictions, and furthermore suggested that NVP also functions to protect the mother, as well as the embryo.

They reviewed studies on the frequency of pregnancy sickness in 75 groups of women from 16 countries worldwide, with a total of 79,146 pregnancies. Among all the groups, the average proportion of women experiencing NVP was 66 percent. Interestingly, the highest frequency for a single country was Japan with 84 percent. In Britain 75 percent of women experienced symptoms, and in the United States, 64 percent.

The periods of embryonic development are well known. Cell differentiation and organ development peak during a critical period in weeks six to 12, and the embryo's central nervous system remains sensitive until week 18. Flaxman and Sherman's survey revealed a clear match between the peak timing of NVP and the sensitive periods of embryonic development.

They also reported on previous work on 18,464 pregnancies which showed that women who experienced NVP were significantly less likely to suffer a miscarriage than women who didn't show sickness symptoms.

Most women, in interviews, reported that aversions to particular foods started during pregnancy, and that they weren't becoming averse to foods they'd disliked before pregnancy. The most disliked foods were meat, fish, poultry and eggs. The second most disliked group of foods was nonalcoholic drinks, followed by vegetables. Aversion to chocolate and sweets was (not surprisingly) rare, and aversion to grains and starchy foods, and to fruit and fruit juice, was very rare.

In contrast, women's cravings during pregnancy were almost exact opposites of their aversions: The most popularly craved foods were fruit and fruit juice and chocolates and sweets. Aversions to animal products (meat, fish, poultry and eggs) were twice as common among pregnant as among nonpregnant women.

Why are some foods avoided? What is it that pregnancy sickness is protecting the embryo from? Profet claimed that many vegetables and caffeinated drinks contain mutagens harmful to the developing embryo. For example, Brussels sprouts and cabbage contain isothiocyanates, which can cause chromosomal disruptions. Consistent with this idea, Flaxman and Sherman found that there were more aversions than cravings to food groups containing these compounds.

An unexpected finding of the study was that pregnant women were more averse to meat, fish, poultry and eggs than to any other food category. The authors noted that meats are more dangerous to pregnant women than vegetables because they spoil faster, and therefore have a higher risk of carrying disease. A woman's cellular immune response is naturally weakened during pregnancy to allow the development of an embryo containing foreign tissues. This leads to higher rates of infection from a suite of different organisms for pregnant compared to nonpregnant women.

The researchers looked at 20 traditional societies in which morning sickness had been reported and seven where it had not. Those cultures where NVP had never been reported were statistically less likely to have animal products as part of the staple diet, and more likely to rely mainly on corn and plants as staples.

Flaxman and Sherman conclude that the hypothesis of embryo protection must be extended to cover the mother also. Sickness during pregnancy evolved to protect both the mother and her embryo from toxins in the mother's diet, and to avoid foods that in the past, and in traditional societies, were likely to carry high doses of harmful micro-organisms and toxins.



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