Morning Sickness

I was one of those women who was convinced that motherhood would be a dreamy time of watching my body do its natural, beautiful thing. In actuality, it was 9 months of nausea, swollen feet and carpal tunnel syndrome. If there was a possible weird pregnancy symptom, I had it (except for hemorrhoids, I hasten to add). I ate whole jars of pickles, a couple small packages of mustard, threw up at a conference, and lived off of apples. I got up in the middle of the night every night for a bowl of cereal, the better to avoid early morning nausea. Turns out that pregnancy was not a magical, lovely time. It was disgusting.

Morning sickness, which strikes approximately 2/3 of women, may have an evolutionary advantage. Nausea in the first three months of gestation (and sometimes longer) is prevalent in cultures around the world, and commonly involves aversion to meat, poultry and dairy products (especially HAM and the smell of skin care products from Bath and Bodyworks). Women who experience morning sickness, especially vomiting, are significantly less likely to miscarry, and vomiting peaks during the weeks where the fetus is most vulnerable to exposure to weird toxins that may be a result of food poisoning. Interestingly, morning sickness is not commonly found in parts of the world where meat consumption is much lower. All this suggests that morning sickness is an evolutionary adaptation to protect the fetus from food borne pathogens, especially those in meat (Flaxman and Sherman, 2000, Morning Sickness: A Mechanism for Protecting Mother and Embryo. Quarterly Review of Biology, 75:113).

This made me wonder if I could blame the rest of my obscure pregnancy symptoms on evolution. With a little searching I stumbled upon the fascinating world of mother-child genetic conflict. Common pregnancy complications and side effects can be considered from an evolutionary perspective; there is evidence that gestational diabetes and preeclampsia can be the result of genetic conflict between mother and fetus.

We usually consider pregnancy and childbirth to be a cooperative endeavor between mother and child. However, as was elegantly pointed out in Richard Dawkins’ seminal work the Selfish Gene, the picture is much different if you consider human procreation from the point of view of the gene. Genes are the primary vehicles of inheritance, and the only thing that matters from a gene’s perspective is propagation into the next generation. Both the mother and child have a genetic interest in the survival of the child: the mother shares 1/2 of her genome with her, therefore has a strong stake in her survival. However, the mother also has the opportunity of making more offspring, so the investment in current offspring has to be balanced against the potential for investing in future offspring. The fetus has some interest in the survival of siblings, as she shares 1/2 of her genome with them, but has a 100% stake in the survival of her own genes. Thus, it can be hypothesized that the child might be interested in acquiring more of the mother’s resources than the mother is interested in giving.

So the baby wants as much food as possible, and the mother would like to not have to nap in the car after she parks at work each morning. I stumbled upon an older paper that reviews some of the evidence for this sort of conflict: Genetic Conflicts in Human Pregnancy (1993, Haig, Quarterly Review of Biology, 68:495). At the beginning of pregnancy, fetal tissue embeds in the maternal endometrium and alters the tiny blood vessels it contacts to make them more porous and less able to constrict. This means that the mother is no longer able to regulate blood flow to this area and thus is unable to restrict nutrient uptake by the hungry baby. To add insult to injury, the fetus is now able to release hormones directly into the mother’s blood stream such as human placental lactogen (hPL). It acts to increase the mother’s resistance to insulin, thus increasing blood concentrations of glucose. If the mother is not able to release enough extra insulin to combat this, then gestational diabetes results. Did I have this? CHECK! It really meant that I had to be annoyed by ignorant dietitions (Her: Maybe you should get up two hours after your middle of the night cereal to check your blood sugar. Me: Maybe you should go fuck yourself.) It is also hypothesized that the fetus can manipulate the mother’s blood pressure to increase vascular flow to the baby, resulting in preeclampsia. Again, CHECK!

After nine months of nausea, vomiting, diabetes and preeclampsia, I then delivered the most beautiful baby girl in the world who then goes and looks more like her dad than me. That leads to another interesting evolutionary question: is there a selective advantage for babies to look more like their fathers than their mothers? The identity of the mother is not in question, however, the father’s continued support of mother and child is a lot more likely if the father is certain that the child is his. BUT – what if approximately 20-30% of babies are actually fathered by the non-resident male? This percentage is largely true for most human cultures, and also most monogamous birds. If this is the case, might there also be a selective disadvantage to looking like your father, if your father is not the one who’s currently bringing home the bacon? (Daly and Wilson, Selfish Genes and Family Relations. In: Richard Dawkins: How a Scientist Changed the Way We Think. Eds, Grafen and Rifley.) Stew on that one for a while.

Was all that nausea and time in the doctor’s office worth it? No question. I miss my little darling if I’m away from her for even a few hours and 1/2 of my genome is especially happy. It’s just nice to be able to blame evolution for the rougher aspects of the experience.