Came across this fascinating example of natural selection right at the early phase of human birth. It explains the logic behind the pregnancy sickness. Despite all the hardship pregnant women have to undergo, one safe assumption you can make is: the baby is probably just fine.
An interesting example is a new theory of pregnancy sickness (traditionally called “morning sickness”) by the biologist Margie Profet. Many pregnant women become nauseated and avoid certain foods. Though their sickness is usually explained away as a side effect of hormones, there is no reason that hormones should induce nausea and food aversions rather than, say, hyperactivity, aggressiveness, or lust. The Freudian explanation is equally unsatisfying: that pregnancy sickness represents the woman’s loathing of her husband and her unconscious desire to abort the fetus orally.
Profet predicted that pregnancy sickness should confer some benefit that offsets the cost of lowered nutrition and productivity. Ordinarily, nausea is a protection against eating toxins: the poisonous food is ejected from the stomach before it can do much harm, and our appetite for similar foods is reduced in the future. Perhaps pregnancy sickness protects women against eating or digesting foods with toxins that might harm the developing fetus. Your local Happy Carrot Health Food Store notwithstanding, there is nothing particularly healthy about natural foods. Your cabbage, a Darwinian creature, has no more desire to be eaten than you do, and since it can’t very well defend itself through behavior, it resorts to chemical warfare. Most plants have evolved dozens of toxins in their tissues: insecticides, insect repellents, irritants, paralytics, poisons, and other sand to throw in herbivores’ gears. Herbivores have in turn evolved countermeasures, such as a liver to detoxify the poisons and the taste sensation we call bitterness to deter any further desire to ingest them. But the usual defenses may not be enough to protect a tiny embryo.How the Mind Works – by Steven Pinker
So far this may not sound much better than the barf-up-your-baby theory, but Profet synthesized hundreds of studies, done independently of each other and of her hypothesis, that support it. She meticulously documented that (1) plant toxins in dosages that adults tolerate can cause birth defects and induce abortion when ingested by pregnant women; (2) pregnancy sickness begins at the point when the embryo’s organ systems are being laid down and the embryo is most vulnerable to teratogens (birth defect—inducing chemicals) but is growing slowly and has only a modest need for nutrients; (3) pregnancy sickness wanes at the stage when the embryo’s organ systems are nearly complete and its biggest need is for nutrients to allow it to grow; (4) women with pregnancy sickness selectively avoid bitter, pungent, highly flavored, and novel foods, which are in fact the ones most likely to contain toxins; (5) women’s sense of smell becomes hypersensitive during the window of pregnancy sickness and less sensitive than usual thereafter; (6j) foraging peoples (including, presumably, our ancestors) are at even higher risk of ingesting plant toxins, because they eat wild plants rather than domesticated crops bred for palatability; (7) pregnancy sickness is universal across human cultures; (8) women with more severe pregnancy sickness are less likely to miscarry; (9) women with more severe pregnancy sickness are less likely to bear babies with birth defects. The fit between how a baby-making system in a natural ecosystem ought to work and how the feelings of modern women do work is impressive, and gives a measure of confidence that Profet’s hypothesis is correct.