Morning Sickness: A Mechanism for Protecting Mother and Embryo

Abstract

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 subsequently avoid foods that contain teratogenic and abortifacient chemicals, especially toxic chemicals in strong-tasting vegetables, caffeinated beverages and alcohol. We examined this hypothesis by comprehensively reviewing the relevant medical, psychological and anthropological literature. In its support, (i) symptoms peak when embryonic organogenesis is most susceptible to chemical disruption (weeks 6-18), (ii) women who experience morning sickness 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 experience nausea alone, and (iv) many pregnant women have aversions to alcoholic and nonalcoholic (mostly caffeinated) beverages and strong-tasting vegetables, 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 societies in which morning sickness has been observed and seven in which it has never been observed. The latter were significantly less likely to have animal products as dietary staples and significantly more likely to have only plants (primarily corn) as staples than the 20 societies in which morning sickness occurred. Animal products may be dangerous to pregnant women and their embryos because they often contain parasites and pathogens, especially when stored at room temperatures in warm climates. Avoiding foodborne microorganisms is particularly important to pregnant women because they are immunosuppressed, presumably to reduce the chances of rejecting tissues of their own offspring (Haig 1993). As a result, pregnant women are more vulnerable to serious, often deadly infections. We hypothesize that morning sickness causes women to avoid foods that might be dangerous to themselves or their embryos, especially foods that, prior to widespread refrigeration, were likely to be heavily laden with microorganisms and their toxins. The alternative hypotheses that morning sickness is (i) an epiphenomenon of mother-offspring genetic conflict or hormones associated with viable pregnancies, or (ii) an indicator to potential sexual partners and kin that the woman is pregnant, resulting in reduced sexual behavior and increased nepotistic aid, were not well supported. Available data are most consistent with the hypothesis that morning sickness serves an adaptive, prophylactic function.

Nausea and vomiting in early pregnancy: Effects on food intake and diet quality

“…It has been suggested that NVP confers functional advantages by preventing intake of substances that may be harmful to the mother or fetus (Flaxman & Sherman, 2000; Pepper & Craig Roberts, 2006) (the maternal and embryo protection hypothesis), or by decreasing nutrient intake thereby stimulating placental growth (Huxley, 2000) (the placental growth and development hypothesis). This latter hypothesis is based on the assumption of a reduction in appetite and overall energy intake resulting from NVP which may be supported by the current study in which severely nauseous women reduced their energy intake.…”

Why is the neurobiology of nausea and vomiting so important?

“…Importantly, the first trimester is a period of rapid fetal growth, and includes critically the development of the CNS, which is highly susceptible to toxicosis. Pregnant women also appear to be picky eaters during this period and tend to avoid meat and fish products, which are more likely to contain pathogens that might harm the fetus (Flaxman & Sherman, 2000). In humans, the presence of pregnancyinduced nausea and vomiting in the first trimester is correlated with a healthy pregnancy (Weigel & Weigel, 1989).…”

Menstrual cycle phase modulates cognitive control over male but not female stimuli

“…Despite the vast number of functional neuroimaging studies, surprisingly little research has investigated the neural basis of sex differences in brain activation patterns underlying cognitive-affective processing and how these differences might be modulated by the menstrual cycle. Studies have reported that female preference for male characteristics varies with the probability of conception across the menstrual cycle, with women manifesting more attraction to men with masculine traits during the late follicular phase of the menstrual cycle (when fertility is high), than at other times (Jones et al, 2005a andPenton-Voak et al, 1999) These studies support the existence of beneficial adaptations whereby ancestral females increased their reproductive success by increasing attraction to masculine traits in men when fertility probability was high (Penton-Voak et al, 1999) and increasing preferences for people believed to be trustworthy (DeBruine et al, 2005), to be free of contagion (Fessler, 2002;Flaxman and Sherman, 2000;Jones et al, 2005b) and to display social cues associated with relationship commitment (Gangestad et al, 2002) when fertility had a low probability. Protopopescu et al(2005) reported that inhibitory control does vary across the cycle but neuroscientists have yet to determine if this modulation is stimulus-specific.…”