Nausea and vomiting of pregnancy in an evolutionary perspective

doi:10.1067/mob.2002.122593

American Journal of Obstetrics and Gynecology

Volume 186, Issue 5, Supplement 2, May 2002, Pages S190-S197

https://doi.org/10.1067/mob.2002.122593 Get rights and content

Abstract

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. (Am J Obstet Gynecol 2002;186:S190-7.)

Section snippets

Occurrence

Across the world, an average of 66% of pregnant women experience nausea and vomiting of pregnancy, but there is considerable variation among countries (35% to 84% of women¹). Typically, symptoms appear about 5 weeks after the last menstrual period, peak during weeks 8 to 12, and gradually decline thereafter (Fig 1, a ).

. Time course of nausea and vomiting of pregnancy and its relationship to critical periods in embryogenesis. a, Proportion of pregnant women experiencing nausea and vomiting of

Cause

Why does nausea and vomiting of pregnancy occur? As with all questions in biology, this one can be analyzed from multiple perspectives or "levels of analysis."⁵ Answers to questions about how the symptoms are brought about (ie, the proximate, physiological mechanisms that underlie nausea and vomiting of pregnancy) and why the symptoms occur in the first place (ie, the ultimate, evolutionary significance of nausea and vomiting of pregnancy) are complementary, not mutually exclusive. Complete

Testing the maternal-and-embryo-protection hypothesis

This hypothesis yields 6 critical predictions, which can be evaluated by use of information in the medical, psychological, and anthropological literature.1, 2, 6

Alternative hypotheses

Three hypotheses alternative to maternal-and-embryo-protection have been proposed. First, nausea and vomiting may be inevitable side-effects of hormone titers associated with viable pregnancies (eg, human chorionic gonadotropin). If so, the symptoms themselves have no function.³⁰ However, contrary to this nonadaptive hypothesis, nausea and vomiting of pregnancy is neither necessary nor sufficient for a viable pregnancy (at least in developed countries). Among 5235 pregnancies in which nausea

Nausea and vomiting of pregnancy in other species

There is anecdotal evidence of nausea and vomiting of pregnancy-like symptoms in only two other mammals, domestic dogs³⁴ and rhesus macaques.³⁵ Perhaps nausea and vomiting of pregnancy actually is more widespread in nature, but it has not been noticed because of difficulties of detecting symptoms in free-living mammals. Zoo specimens receive bland diets that may not trigger symptoms. Alternatively, wild mammals may be protected in other ways. For example, early in gestation herbivores may avoid

Is nausea and vomiting of pregnancy still useful?

Nowadays food-borne pathogens are reduced by chemical preservatives and packaging of raw products, cooking and microwaving of meals, and refrigeration and freezing of leftovers. Commercial vegetables typically have minimal secondary compounds because of artificial selection for bland taste (ie, reduction in secondary compounds). In this modern environment is nausea and vomiting of pregnancy just an evolutionary anachronism?

We think not. After all, the relationship between the occurrence of

Implications

Because we are not physicians, it is not appropriate for us to make medical recommendations. However, our results may help health care providers answer some frequently-asked questions about nausea and vomiting of pregnancy:

Conclusions

When we began our inquiries, a key question was whether nausea and vomiting of pregnancy is a cause or a consequence of a viable pregnancy-that is, are nausea and vomiting of pregnancy symptoms themselves functional or just uncomfortable, superfluous side effects of a hormonally mediated tug-of-war between vigorous embryos and mothers over maternal resources?30, 37 Our analyses1, 2 and an independent synthesis of complementary information⁶ strongly favor the functional interpretation.

The

Acknowledgements

We thank Mark E. Hauber and George C. Williams for comments on a preliminary draft, Daniel M. T. Fessler for sharing his preliminary manuscript, and Roberto Romero and Thomas M. Goodwin for inviting us to participate in the NVP Symposium. The order of authorship was determined by a coin flip.

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^☆: Support was provided by the National Science Foundation, the Howard Hughes Foundation, the Olin Foundation, and the Agricultural Experiment Station at Cornell University.

^☆☆: Reprint requests: Paul W. Sherman, Department of Neurobiology & Behavior, Cornell University, Ithaca, NY 14853.

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Understanding and Treating Nausea and Vomiting of PregnancyNausea and vomiting of pregnancy in an evolutionary perspective☆,☆☆