UCLA researchers have identified a connection between morning sickness symptoms and the body’s inflammatory response to changes during pregnancy. The study, led by anthropologists and epidemiologists at UCLA, examined how immune system molecules called cytokines are linked to nausea, vomiting, and aversions to certain foods and smells in early pregnancy.
According to the National Institutes of Health, up to 80 percent of women in early pregnancy experience these symptoms. The research suggests that such symptoms are not usually signs of problems for either the mother or fetus but instead reflect a natural immune adjustment unique to pregnant women.
“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 also 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 team believes this balance is achieved through a combination of inflammatory responses that prevent rejection of the fetus along with behavioral adaptations like nausea. These mechanisms may encourage avoidance of potentially harmful foods during periods when fetal vulnerability is highest.
“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 Daniel Fessler, UCLA anthropology professor and co-author.
For their study, researchers collected blood samples from 58 Latina women in Southern California starting early in their pregnancies through postpartum. Participants completed questionnaires about morning sickness symptoms as well as food and smell aversions. Sixty-four percent reported odor or food aversions—mainly tobacco smoke and meat—while 67 percent experienced nausea and 66 percent had vomiting episodes.
Analysis showed those who were averse to tobacco smoke had increased pro-inflammatory cytokine levels. Other symptoms like food aversion, nausea, and vomiting were also associated with more pro-inflammatory immune activity.
Researchers suggest these patterns may be part of an evolutionary adaptation designed to help mothers avoid substances dangerous during pregnancy. However, they note further research is needed before drawing firm 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.
These immunological shifts could lead to nausea which prompts dietary caution as another protective measure for both mother and child.
“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,” Fessler added.
First author Dayoon Kwon noted that understanding these biological underpinnings could help reduce stigma around morning sickness at work by supporting practical accommodations for pregnant employees such as improved health care benefits.
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