The Adaptive Significance of Morning Sickness


Approximately 66%-75% of all women experience symptoms of nausea and vomiting—commonly referred to as “morning sickness”—early in their pregnancies.  These symptoms were long regarded as deleterious but unavoidable byproducts of normal pregnancy physiology.  Together with Paul Sherman (Department of Neurobiology and Behavior, Cornell University), I have been investigating the potential adaptive significance of morning sickness. In our first paper, we reviewed the literature extensively and showed that available evidence is most consistent with the hypothesis that morning sickness protects pregnant women and developing embryos from harmful chemicals and microorganisms in food.  This sparked a great deal of renewed interest in morning sickness, as well as controversy.  Subsequently, it was suggested that the patterns of occurrence of morning sickness could be explained by an alternative hypothesis, namely, that morning sickness is nothing more than a non-functional byproduct of parent-offspring conflict in the womb.  Hence, we recently formulated a graphical model of mother-embryo conflict in human pregnancy to rigorously develop the latter hypothesis.  We showed that, regardless of whatever realistic assumptions are made, the conflict hypothesis can not explain available data on morning sickness.  These findings appear in an article in the American Naturalist.


Several fascinating questions await our exploration.  First, why does morning sickness apparently occur only in humans?  Second, has the utility of morning sickness been diminished by current medical or food safety practices?  Third, what are the consequences of suppressing the symptoms of nausea and vomiting in early pregnancy?

Relevant publications >>:

  1. 1. Flaxman & Sherman. 2008. American Naturalist.

  2. 2. Sherman & Flaxman. 2002. American Journal of Obstetrics and Gynecology.

  3. 3. Sherman & Flaxman. 2001. American Scientist.

  4. 4. Flaxman & Sherman. 2000. Quarterly Review of Biology.

samuel m. flaxman

Assistant Professor, Department of Ecology and Evolutionary Biology

University of Colorado Boulder