The idea, first proposed by physician Ernest Hook at New York’s Albany Medical College in 1976, is that a pregnant woman’s sickness significantly narrows the amount and variety of things she eats. That protects the unborn baby — and its host, the mother — from bacteria, toxins and other nasty things that can be contained in food products. Think salmonella in raw eggs or Toxoplasma gondii in undercooked meat. For most women, morning sickness lasts for only the first trimester, which also happens to be the most vulnerable period for a fetus — the period when its brain’s neurons and its organs are formed.
“Morning sickness and the aversion to potentially harmful foods is the body’s way of preserving wellness of the mother at a time when her immune system is naturally suppressed,” evolutionary biologists Samuel M. Flaxman and Paul W. Sherman of Cornell University wrote in a 2000 paper.
This mechanism still remains just a theory, and a somewhat controversial one at that. Perhaps the biggest question about it has been whether pregnancy sickness is actually linked to better pregnancy outcomes.
A new analysis published Monday in JAMA Internal Medicine by researchers at the National Institutes of Health sides squarely with Hook in providing evidence that nausea and vomiting in pregnant women may indeed be a positive sign for the baby’s prognosis.
The study involved tracking nearly 800 women, and it’s important to note that there are a few unusual things about the cohort. One is that the group was highly homogeneous. Most of the women were white (96 percent), very young (63 percent age 29 and younger), married (95 percent) and employed (72 percent). All the women in the study had also experienced one or two prior pregnancy losses and, as might be expected, the number of miscarriages in the group is higher than might be expected in the general population. Moreover, the women were also part of a randomized clinical trial about the effectiveness of low-dose aspirin in protecting against pregnancy loss.
Nonetheless, it stands as perhaps the most rigorously conducted research on the topic. Previous studies involved asking women well along in their pregnancies to try to recall how much nausea and vomiting they had experienced — a methodology that is error-prone because it relies on people’s imperfect memories. But this one began following women trying to get pregnant and was therefore able to get information on them — via daily diaries — from almost the first day of conception.
The women’s diaries showed that by the eighth week, nearly 85 percent reported having nausea alone or nausea with vomiting. Of the 797 women who had positive pregnancy tests, 188 of the pregnancies ended in loss. An analysis of that data found that women who had either nausea alone or nausea accompanied by vomiting were 50 to 75 percent less likely to experience a miscarriage. Those are huge differences.
“The results should be reassuring to women who are going through these symptoms and suffering through this hard time,” researcher Stefanie Hinkle, a staff scientist in the National Institute of Child Health and Human Development’s epidemiology branch, said in an interview.
Hinkle and her colleagues wrote that it’s possible there may be “evolutionary advantage to change one’s dietary intake, increase consumption of carbohydrate-rich foods.” That explains the ice cream many women ingest by the pint when they are pregnant. Not so much the pickles.
Encouraging women to eat foods they dislike during pregnancy will not improve the pregnancy outcome and could increase the embryo’s exposure to pathogens and harmful chemicals, according to the researchers.
The NIH study also provides some evidence countering another hypothesis proposed by scientists about the function of pregnancy sickness — that it might have to do with forcing women to avoid risky behaviors such as smoking and consuming alcohol and caffeine. The researchers noted that “our modeling strategy accounted” for this idea and suggested “the mechanism is likely not through avoidance of such substances.”
A third theory — that pregnancy sickness may be due to effects of human growth hormone — wasn’t tested in the study.
Hinkle emphasized that the findings shouldn’t lead the lucky women who are not experiencing pregnancy-related nausea and vomiting to panic.
“Every pregnancy is different,” she said, “and at an individual level this is not indicative of having or not having a healthy pregnancy.”