One of the best things about being a gay man is that one doesn’t have to worry about accidentally impregnating his partner, or, for that matter, getting knocked up. That’s probably a good thing in my case, since I have a very low tolerance for any form of pain and discomfort (except the good kind) and such inconveniences tend to go along with the long, tumultuous gestation of a human conceptus gathering mercilessly inside a woman’s abdominal cavity. It’s all very wonderful bearing a child, I’m sure, and life-affirming; but on the other hand, one of the worst parts of being pregnant -- and I was informed concretely of this very fact by some vomitus landing on my sandaled foot -- is what is commonly referred to as morning sickness.
This term for the nausea and vomiting accompanying pregnancy is something of a misnomer, actually, since such gastrointestinal issues certainly aren’t limited to the morning hours. Rather, for those women who do get green around the gills (and not all do; more on that later) sudden bouts of toilet-hugging can happen morning, noon and night. But much as mild fevers may help boil away bacterial infections, some degree of nausea and vomiting, at least during the early stages of pregnancy, may be helpful and adaptive -- an evolved mechanism that protects both the fetus and mother.
One of the first to notice the salubrious effects of this “pernicious vomiting of pregnancy” was a Boston-based physician named Frederick Irving. In 1940, Irving reported that women in his clinic who experienced strong food aversions early in their pregnancies were less likely to suffer miscarriages than were women whose first trimesters were easier to stomach.
In 1976, Ernest Hook, an endocrinologist at Albany Medical College, postulated a functional theory of the condition, suggesting that nausea and vomiting protected the developing fetus from foods that could compromise its anatomical development. In embryological jargon, elements that can cause mutations early in development are known as teratogens; these can be any of a host of things -- with Chernobyl-level radiation being an extreme example -- but they include caffeinated beverages, tobacco, and alcohol, and these are the types of items that Hook focused his attention on. Many pregnant women exhibit an aversion to these products, he pointed out, particularly during the crucial first trimester when the fetus’s basic body plan (limbs, appendages, digits, and other things that make us look like standard-issue human beings) is particularly vulnerable to deleterious foreign substances invading the womb. Such teratogens can be avoided by nausea or expelled by vomiting.
A proper evolutionary accounting of pregnancy sickness was formulated in 1992, when the biologist Margie Profet articulated the compelling argument that it is an intricate adaptive mechanism. (Profet, who was awarded a MacArthur “genius grant” in 1993, was in the news recently for emerging after a long and mysterious absence.) She noted that, while teratogenic goodies like absinthe and mocha lattes weren’t exactly a threat to our pregnant ancestors in the African savannas hundreds of thousands of years ago, such products do have high concentrations of so-called secondary plant compounds. These phytochemicals deter or kill plant enemies such as insects, fungi, and bacteria. They’re usually harmless to humans, but when consumed in large quantities, they can be allergens, carcinogens, mutagens and, in pregnant women, teratogens and even abortion-inducing agents.
Animal products also would have posed dietary problems for our ancestral mothers. Meat is a perfect hiding place for dangerous microorganisms. Salmonella, for example, thrive in eggs, and roundworms can inhabit seafood. Toxoplasma gondii, which can be found in raw or undercooked meat and, notoriously, in cat feces (which hopefully you’re not snacking on even when you’re not pregnant), has been linked to spontaneous abortions, congenital brain defects, and even schizophrenia.
Not only does nausea and vomiting shield the developing embryo from toxins, Profet reasoned, but these responses may also protect the mother. Her immunological defenses are lowered during pregnancy, especially during the first trimester, which allows her to accommodate the half-foreign genome of the lovely little beast incubating in her womb. If her immune system operated at full speed, it might reject the fetus. By avoiding certain foods, she simultaneously decreases the risk of her offspring’s exposure to teratogens while protecting herself from those toxins and parasites that she is in no shape to fight.
The “Hook-Profet embryo-protection hypothesis” has formed the analytical scaffolding for a slew of scholarship. Biologists Paul Sherman and Samuel Flaxman of Cornell University have rounded up the evidence in several review articles. For example, questionnaire studies reveal that women who do experience nausea and vomiting report an onset around the fifth week; queasiness then peaks between the sixth and 12th weeks, and is rare after the 18th week. These patterns overlap with critical periods in which embryonic cells divide rapidly and differentiate.
When the authors asked which foods the women found off-putting, the most disgusting things were meat (that lair of microorganisms) followed by vegetables, coffee, tea (all of which are usually laced with secondary plant compounds) and alcohol (a teratogen).
The most important question for the theory, of those reviewed by Sherman and Flax, is the extent to which pregnancy sickness equates to better pregnancy outcomes. The results were mixed, and for that reason some scholars question the veracity of the Hook-Profet hypothesis. After performing an impressive meta-analysis using data from tens of thousands of pregnancies, the authors found that women who experienced nausea and vomiting were, just as Irving noticed back in 1940, significantly less likely to miscarry compared to women who had more comfortable pregnancies. On the other hand, there was no clear relationship between morning sickness and birth defects, which, at least on the surface, poses problems for the protection model. In fact, although the causality is unclear, women who were prescribed antihistamines (which happen to have an anti-nausea effect) for varying reasons during their first trimesters were statistically less likely to have children born with birth defects. (The use of thalidomide in the 1950s to suppress pregnancy sickness resulted in a notorious teratogenic catastrophe, however.) Given that these medicated women did not have the proposed protective benefits of morning sickness, and yet had healthy babies still -- and perhaps even healthier babies than their nauseated cohorts -- these findings fail to support the Hook-Profet model. In such cases, however, it’s often difficult to rule out other confounds that often go along with prescription drugs, such as maternal socioeconomic status and related access to prenatal health care, which may also underlie this difference in infant health.
There’s another question that cannot be answered comfortably by the protection model, and that is simply why, if it is indeed an evolutionary adaptation, does pregnancy sickness not occur in all (or at least, almost all) pregnant women? Women who neither gag nor barf in any substantive way during their first trimester may be in the minority, but they’re not a small minority. Profet and others didn’t ignore this lack of universality, and they postulated some factors that may account for individual differences, yet the absence of morning sickness in so many women (even within the same society and with very similar diets) is difficult to reconcile with the strong adaptationist protection model.
One scientist who has begun poking holes in the Hook-Profet hypothesis and piecing together an alternative theory is SUNY-Albany psychologist Gordon Gallup. (Full disclosure: Gallup and I are like-minded, and I’m often partial to his ideas. I even gave his work on human penis adaptations the titular spot in my new essay collection.) Gallup claims that even the best evidence of categorical food aversions in early pregnancy is not, in fact, as compelling as it’s usually made out to be.
First, pregnant women aren’t the only ones to say that animal products such as meat, fish, poultry, and eggs are the most disgusting foodstuffs -- nonpregnant women answer surveys the same way. Also, the embryo-protection account doesn’t really distinguish between the relative dangers of the two main food categories -- both plant toxins and meat-borne pathogens, after all, can disrupt organogenesis and make the immunosuppressed mother ill. But it’s also common for women to report meat cravings during this period, much more so than they do for vegetables. Furthermore, there is conflicting evidence regarding the universality of these aversion patterns; women in some cultures may actually be more likely to develop morning sickness to common starch-based carbohydrates that have a low potential for toxicity.
Another problem for the Hook-Profet model, Gallup argues, is the fact that the evidence for pregnancy nausea and vomiting in other species is extremely weak, at best. Not only would similar selection pressures have been operating on the evolution of other species’ pregnancies for exactly the same reasons, more importantly, it’s unlikely to have arisen for the first time in hominids without some phylogenetic precedent that can still be easily observed in modern nonhuman animals. Yet nausea and vomiting in pregnancy have only been reported -- and anecdotally so -- in two other species: domestic dogs and rhesus macaques.
There may very well be logical explanations for these apparent empirical shortfalls. For example, modern food regulations and prenatal care may counteract the effects of a deleterious diet today, but there may indeed have been more frequent birth defects in ancestral infants whose mothers weren’t susceptible to pregnancy sickness. And perhaps other species do exhibit these symptoms, but they are simply more difficult to detect in the wild than in the more familiar lab monkeys or dogs. Yet Gallup suspects we’ve been barking up the wrong tree with the Hook-Profet hypothesis. “Diet may only be a small part of the picture,” he wrote to me by email.
So what does Gallup say is the real culprit behind nausea and vomiting in early pregnancy? Semen. More specifically, unfamiliar semen. To understand where he’s coming from, we need to think back to the maternal immune system’s response to the fetus. Because half of the DNA the fetus is carrying comes from the father, the mother’s body may initially treat the organism as foreign tissue or an infection. This response, Gallup says, triggers an immune reaction that is commonly experienced as nausea, vomiting, and malaise (aka morning sickness). The best cure for this type of sickness, says Gallup, is, strangely enough, the same thing as its cause. The more exposure a woman has to her partner’s semen -- that is to say, the more often she’s inseminated prior to conception and during the early stages of the pregnancy -- the more tolerance her body develops to his genetic material. This tolerance generalizes to a tolerance for the fetus and leads to successful maternal immunosuppression -- and subsequently allows her to feel less like an infected zombie with serious stomach troubles.
Here is where Gallup’s reputation as an innovative -- if often highly speculative -- evolutionary theorist comes into the picture. Gallup surmises that pregnancy sickness is not itself an adaptation, but instead a side effect of a broader maternal adaptation for favoring the best possible mates. He suggests that this broader adaptation serves primarily to facilitate reproduction with males that are likely to support mother and child (in evolutionary terms, to invest in the offspring), while weeding out the players. In previous work, Gallup has shown that women are more likely to develop preeclampsia -- and thus have a higher infant mortality risk -- in pregnancies resulting from unfamiliar semen. Historically, these would have included rape and “dishonest mating strategies” (tactics in which the man lies to the woman about his long-term intentions just to get into her pants) as well as unplanned conception occurring in a new, still-fragile relationship. From the point of view of Mother Nature’s cold, cold heart, spontaneous abortions due to a reaction against unfamiliar semen might have been biologically adaptive. This is because conception and childbirth historically meant that a woman foreclosed on any other reproductive opportunities for 2 to 4 years, so pregnancies in which paternal investment was improbable would have meant an enormous gamble. Today, however, technological innovations such as barrier contraceptives (condoms reduce a woman’s exposure to semen that would otherwise become familiar) and artificial insemination mimic some ancestral conditions. The maternal immune system has no way to distinguish between, say, conception by in vitro fertilization and rape.
Gallup’s evolutionary reinterpretation of pregnancy sickness is quite new -- so new, in fact, that it hasn’t been put to a test. But at the 2012 meeting of the Northeastern Evolutionary Psychology Society in Plymouth, N.H., he and graduate student Jeremy Atkinson laid out a set of explicit predictions that, if borne out by data, would support their model and may lead scholarship away from the traditional embryo-protection account. First, the authors predict that the intensity of pregnancy sickness should be directly proportional to the frequency of insemination by the child’s father. “Risk factors for morning sickness,” they reason, “should include condom use, infrequent insemination, and not being in a committed relationship.” In fact, Gallup and Atkinson believe that lesbians with little (if any) previous exposure to semen who are impregnated by artificial insemination should have some of the worst cases of nausea and vomiting. Also, pregnancy sickness should wane in severity from one consecutive pregnancy to the next, but only assuming that the same man sires each successive offspring. By contrast, a change in paternity between offspring should reinstate pregnancy sickness.
Although the Hook-Profet view has long been considered to be the solution to the nausea gravidarum mystery, the full evolutionary story may have yet to be told. Still, while it may very well work wonders in curtailing nausea and vomiting in your next pregnancy, it’s probably too early to suggest imbibing copious amounts of your partner’s semen, either vaginally or orally (there is some evidence, believe it or not, that fellatio may be just as effective as vaginal insemination for priming the woman’s body with the man’s protein, activating maternal immunosuppression in preparation for a child.) Your partner might think it’s worth a shot, though.