Scientists Finally Explain the Real Reason for Morning Sickness in Pregnant Women

Researchers at UCLA have discovered a link between the symptoms of “morning sickness” and the body’s natural but complex inflammatory response to biological and physical changes during pregnancy.

Morning Sickness May Be A Clever Evolutionary Strategy to Protect Mothers and Babies

According to the National Institutes of Health, up to 80% of pregnant women experience nausea, vomiting, and aversion to certain foods and smells in the early stages of pregnancy. While these symptoms are unpleasant, they are not usually a sign that something is wrong with the health of the mother or the developing fetus, but rather an indication of a delicate balance that only occurs in pregnant women.

“During pregnancy, the mother’s immune system faces a difficult challenge: it must protect both her and the fetus from infection without accidentally attacking the fetus, whose genetic identity is half foreign, as it comes half from the father. Normally, the immune system attacks anything that appears foreign. During pregnancy, it must therefore carefully adapt to protect the fetus while warding off infection,“ explains Molly Fox, professor of anthropology at UCLA. Fox is the corresponding author of the study ”Of scents and cytokines: How olfactory and food aversions relate to nausea and immunomodulation in early pregnancy” , which was recently published in the journal Evolution, Medicine and Public Health.

The researchers believe that this delicate balance, which protects both mother and fetus, is achieved through a unique combination of inflammatory responses. These prevent the mother’s body from rejecting the fetus and are complemented by adaptive behavioral mechanisms such as nausea, which cause the mother to avoid potentially harmful foods, especially in the first and second trimesters when the fetus is most vulnerable. “Nausea, vomiting, or aversion to certain foods or smells are not signs that something is wrong with the mother or fetus. Rather, they are likely a sign that everything is proceeding normally, and reflect the body’s healthy and helpful immune response,” said Daniel Fessler, professor of anthropology at UCLA and co-author of the study.

Methodology and Results

For the study, the UCLA-led team of anthropologists and epidemiologists collected and analyzed blood samples to measure molecules of the immune system called cytokines. Cytokines are proteins that send signals to help the body mount a rapid defense against disease and regulate inflammation. Participants also completed questionnaires asking about symptoms related to morning sickness and aversion to certain foods and smells in early pregnancy. The participants were 58 Latin American women in Southern California who were followed from the beginning of their pregnancy until after birth. 64% of the study participants suffered from aversions to certain smells or foods, especially tobacco smoke and meat. 67% reported suffering from nausea, and 66% suffered from vomiting.

The study team measured both cytokines that promote inflammation (pro-inflammatory) and cytokines that suppress inflammation (anti-inflammatory). They found that women who had an aversion to tobacco smoke showed a significant shift toward a stronger inflammatory response. Aversion to certain foods, nausea, and vomiting were also associated with a more pro-inflammatory immune imbalance.

Natural Selection?

The correlation is consistent with the researchers’ theory that these symptoms may be part of an evolutionary adaptation that helps the bodies of pregnant women minimize exposure to harmful substances. However, the study’s authors point out that the evidence is not conclusive and further research is needed. They emphasized that the study allowed the team to examine both the biological and behavioral responses of humans during pregnancy.

“In many mammals, the fetus is separated from the mother’s blood supply, which contains her immune cells, by barriers. In humans, however, we have a unique constellation: fetal cells are surrounded by maternal blood. Humans have the most invasive placenta of all, which implants itself deep into the maternal tissue. Therefore, humans need unique strategies to prevent the mother’s immune system from attacking the fetus,” Fox said. These immunological changes can cause nausea, which in turn leads to certain foods being avoided, which, according to the researchers, could serve as an additional protective measure. “Nowadays, packages of ground beef or soft cheese usually carry warnings for pregnant women to consume these products with caution due to the risk of foodborne illness during pregnancy. Aversion to certain smells and foods, as well as nausea and even vomiting, appear to be an evolutionary strategy to achieve the same goal,” Fessler said.

The researchers said the study could help raise awareness that nausea and vomiting are normal symptoms with biological causes associated with a healthy pregnancy. The study’s findings could help pave the way for meaningful workplace accommodations, such as more efficient use of health services and other helpful resources, to reduce stigma, excessive absenteeism, and lost productivity. They also encourage other researchers to continue exploring the questions raised by the study, not only to investigate evolutionary issues, but also to work toward providing physicians with non-invasive or minimally invasive methods for prognosis.