UCLA researchers have identified a link between morning sickness symptoms and the body’s natural inflammatory response to pregnancy. According to the National Institutes of Health, up to 80 percent of women in early pregnancy experience nausea, vomiting, and aversions to certain foods and smells. These symptoms are generally not considered signs of health problems for either the mother or fetus.
“During pregnancy, a mother’s immune system faces a tricky challenge: it has to protect both her and the fetus from infection, but without accidentally attacking the fetus, whose genetic identity is half-foreign because it is half derived from the father. Normally, the immune system attacks anything that seems foreign, so in pregnancy, it has to carefully adjust to keep the fetus safe while still defending against infection,” said UCLA anthropology professor Molly Fox.
Fox is the corresponding author of “Of scents and cytokines: How olfactory and food aversions relate to nausea and immunomodulation in early pregnancy,” published in Evolution, Medicine and Public Health.
The study suggests that this immune balance may be maintained through specific inflammatory responses that prevent fetal rejection while prompting behaviors such as nausea that discourage consumption of potentially harmful foods during vulnerable stages of pregnancy.
“Nausea, vomiting or aversions to foods or smells are not indications that something is going wrong for the mother or the fetus. It’s likely an indication that everything is moving along normally, and a reflection of the body’s healthy and helpful immune response,” said UCLA anthropology professor Daniel Fessler.
Researchers collected blood samples from 58 Latina women in Southern California during early pregnancy through postpartum periods. They measured cytokines—proteins involved in regulating inflammation—and gathered self-reported data on symptoms like food aversion and nausea. Sixty-four percent reported odor or food aversions (mainly tobacco smoke and meat), 67 percent experienced nausea, and 66 percent experienced vomiting.
Women who reported an aversion to tobacco smoke showed higher levels of pro-inflammatory cytokines. Food aversions, nausea, and vomiting were also linked with more pro-inflammatory immune profiles.
The findings support a theory that these symptoms could be evolutionary adaptations aimed at minimizing exposure to harmful substances during critical periods of fetal development. However, researchers noted further investigation is needed before drawing definitive conclusions.
“In many mammals, the fetal compartment has barriers separating it from the mother’s blood supply, where her immune cells are. But in humans, we have a unique setup — fetal cells are bathed in maternal blood. Humans have the most invasive of all placentas, burrowing deep into maternal tissue. So humans need unique strategies to prevent the mother’s immune system from attacking the fetus,” said Fox.
According to Fessler: “Nowadays, you will see labels on packages of ground beef or soft cheese that warn pregnant women to be cautious about these products because of the risks of foodborne illness during pregnancy. Aversions to certain odors and foods, and nausea and even vomiting, appear to be evolution’s way of achieving that same objective.”
First author Dayoon Kwon noted that understanding these biological underpinnings could reduce stigma around morning sickness at work by supporting workplace accommodations for pregnant employees.
The research was funded by the National Institutes of Health.
