So-called morning sickness is often the first (sometimes doubly unwelcome) sign of conception. Bleeding resembling weak menstruation can occur during early pregnancy, so a woman may experience daytime queasiness before she has any other reason to suspect conception. Mild to pronounced nausea, typically lasting six weeks, affects three-quarters of women in early pregnancy. In two-thirds of them nausea is severe enough to provoke actual vomiting.
What is “morning sickness”?
Queasiness during pregnancy is not confined to mornings, so the household name is misleading. The official name is Nausea and Vomiting of Pregnancy (NVP), but I prefer “Bilious All-day Recurring Feelings” (BARF). A careful study of daily records from 160 pregnant women by Renée Lacroix and colleagues in Canada found nausea restricted to mornings in less than 2% and occurring throughout the day in 80%.
Other features that occur mainly during early pregnancy are cravings for unusual food or drink and aversions to previously palatable dietary items. Both occur in 50-60% of women. Most frequently craved items are fruits or juices, chocolates, other sweets, biscuits and desserts. Aversions are most commonly triggered by beverages containing caffeine, spicy or highly-flavoured foods and animal foods such as poultry, red meat, eggs and fish.
A hormonal basis for BARF?
BARF and aversions/cravings have traditionally been interpreted as side-effects of marked hormonal changes during pregnancy. Indeed, this is still a standard explanation in online guidance to “morning sickness”. However, no direct link between nausea and hormones has been established. Curiously, steroid hormones, notably oestrogens, are most often emphasized, partly because nausea sometimes accompanies Pill use. But levels of oestrogens and progesterone actually rise continuously throughout pregnancy, peaking in late pregnancy when BARF, aversions and cravings are all relatively rare. Moreover, steroid levels apparently do not differ significantly between women with or without symptoms.
By contrast, production of the pregnancy-specific hormone hCG (human chorionic gonadotropin) rises and falls, peaking at about the same time as BARF. Moreover, some studies have indicated that hCG levels are higher in women with pronounced pregnancy sickness. Intriguingly, BARF is prevalent with hydatiform mole — an over-large placenta with no embryo. Diagnosis of this condition includes high hCG levels.
Alternative explanations for BARF
Taking an entirely different approach, in 1988 author Margie Profet proposed that an increased tendency to vomit is a biological adaptation predisposing mothers to eliminate ingested toxins threatening embryonic/fetal development. Embryos and early fetal stages are likely to be susceptible to toxins as major organs such as the brain develop. In two guidance books for mothers-to-be published in 1995 and 1997 Profet particularly emphasized potential toxins in plant-based foods: strong-tasting vegetables, alcohol and caffeine-containing beverages. But this “veggie hypothesis” specifically linking defensive plant toxins to birth defects was fiercely attacked by some authors. Most notably, taking new data for more than 500 women, epidemiologist Judith Brown and colleagues analysed relationships between BARF, pregnancy outcome and intake of supposedly noxious vegetables. They found no statistical association between suspect vegetables and nausea or vomiting in early pregnancy. More importantly, no correlation was found between pungent-tasting vegetables and adverse pregnancy outcomes.
Animal products rather than plant foods
In 2000, neurobiologists Samuel Flaxman and Paul Sherman presented evidence for a modified interpretation that morning sickness serves to protect both the mother and the developing baby from ingested toxins of animal origin. They reinforced Profet’s claim that women afflicted with BARF are significantly less likely to miscarry than women who do not, stating that fewer miscarriages occur with actual vomiting than with nausea alone.
Flaxman and Sherman showed that, although many pregnant women abruptly dislike alcohol, caffeine-containing beverages and strong-tasting vegetables, it is animal products — poultry, other meats, fish and eggs — that provoke the greatest aversions. They reported results from a cross-cultural study covering twenty traditional societies for which BARF had been recorded and seven for which it had never been noted. Animal products were not major dietary items in any society without BARF, while plants, most notably corn, were significantly more likely to be staples.
Connecting BARF to hCG
Neither Profet nor Flaxman and Sherman explicitly discussed how hCG might be involved in BARF. In fact, pregnancy poses a basic challenge because a developing embryo is effectively a foreign body with many proteins differing from the mother’s. So her immune response must be temporarily down-regulated to reduce rejection risk. As a result, pregnant women and their developing offspring are more exposed to serious infections. In fact, hCG plays a key rôle in regulating immunological interactions between the mother and her embryo/fetus by promoting local tolerance, particularly during the first third of pregnancy. So it is easy to imagine a compensatory adaptation with hCG enhancing the mother’s ability to detect and avoid dietary toxins by provoking nausea and aversions.
But there is another problem: BARF is seemingly unique to humans. Nausea or unusual dietary aversions/cravings during early pregnancy have not been reported for other mammals, even for our closest primate relatives. Why should such adaptations in early pregnancy occur only in our own species? One possibility is that they arose as a novel development during human evolution because our diet moved away from the typical primate focus on fruits, becoming very flexible and often including various animal foods.
At first sight, the take-home message regarding BARF seems to be: “Don’t fight it, it’s good for the baby.” Because BARF is so widespread, it certainly seems likely that it serves to protect, but it is crucial to avoid alarming mothers-to-be with hasty conclusions. For instance, it might be thought that any attempt to alleviate BARF might reduce protection of the embryo/fetus. More insidiously, a pregnant woman who does not suffer from BARF might fear that her infant is more likely to be born with a birth defect. But in fact available evidence does not warrant those conclusions. In the first place — as revealed by a comprehensive literature survey by Roger Gadsby and Tony Barnie-Adshead — no connection between BARF and birth defects has been established. Additionally, the unstated assumption that miscarriage occurs because of developmental defects has not been tested. Yet again, correlation does not necessarily reflect causation. (See my July 12, 2013 post: The Stork-and-Baby Trap.) We simply do not know whether BARF reduces the likelihood of developmental defects in the embryo or early fetus.
I robustly support the founding principle of Darwinian medicine: Proper understanding of human disease requires knowledge of evolutionary biology. But the principal rôle of evolutionary thinking is surely to generate serious investigation in new directions, not facile “explanations” of medical conditions. So what we need is more research into underlying causes of BARF and possible ways of mitigating associated problems. For instance, might avoidance of animal products and caffeinated beverages during early pregnancy ward off BARF?
References
Brown, J.E., Kahn, E.S. & Hartman, T.J. (1997) Profet, profits, and proof: do nausea and vomiting of early pregnancy protect women from 'harmful' vegetables? Am. J. Obstet. Gynecol. 176:179-181.
Chan, R.L., Olshan, A.F., Savitz, D.A., Herring, A.H., Daniels, J.L., Peterson, H.B. & Martin, S.L. (2010) Severity and duration of nausea and vomiting symptoms in pregnancy and spontaneous abortion. Hum. Reprod.25:2907-2912.
Czeizel, A.E., Puho, E., Acs, N. & Banhidy, F. (2006) Inverse association between severe nausea and vomiting in pregnancy and some congenital abnormalities. Am. J. Med. Genet. 140A:453-462.
Einarson, T.R., Piwko, C. & Koren, G. (2013) Quantifying the global rates of nausea and vomiting of pregnancy: A meta analysis. J. Popul. Ther. Clin. Pharmacol.20:e171-183.
Fessler, D.M.T. (2002) Reproductive immunosuppression and diet. An evolutionary perspective on pregnancy sickness and meat consumption. Curr. Anthropol. 43:19-61. [An excellent overview.]
Flaxman, S.M. & Sherman, P.W. (2000) Morning sickness: a mechanism for protecting mother and embryo. Quart. Rev. Biol. 75:113-148.
Flaxman, S.M. & Sherman, P.W. (2008) Morning sickness: Adaptive cause or nonadaptive consequence of embryo viability? Am. Nat. 172:54-62.
Gadsby, R. & Barnie-Adshead, A (last updated in 2011) Nausea and Vomiting of Pregnancy: A Literature Review.https://www.pregnancysicknesssupport.org.uk/documents/NVP-lit-review.pdf
Kaupilla, A., Huhtaniemi, I. & Ylikorkala, O. (1979) Raised serum human chorionic gonadotrophin concentrations in hyperemesis gravidarum. Brit. Med. J.I:1670-1671.
Lacroix, R., Eason, E. & Melzack, R. (2000) Nausea and vomiting of pregnancy: A prospective study of its frequency, intensity and patterns of change. Am. J. Obstet. Gynecol.182:931-937.
Profet, M. (1988) The evolution of pregnancy sickness as protection to the embryo against Pleistocene teratogens. Evol. Theory 8:177-190.
Profet, M. (1995) Protecting Your Baby-to-be: Preventing Birth Defects in the First Trimester. Reading, MA: Addison-Wesley.
Profet, M. (1997) Pregnancy Sickness: Using Your Body's Natural Defenses to Protect your Baby-to-Be. Reading, MA: Perseus.
Tortora, G. & Derrickson, B. (2009) Principles of Anatomy & Physiology, 12th Edition. New York: John Wiley.