Morning sickness may be evolution’s clever way of protecting mothers and babies.
Morning sickness isn’t just random misery—it’s a biological defense system shaped by evolution to protect the fetus. By linking immune responses to nausea and food aversions, UCLA researchers show these symptoms are signs of a healthy pregnancy.
Key takeaways
- A UCLA (University of California - Los Angeles) study has found that "morning sickness" symptoms, including nausea, vomiting and aversions to certain foods and smells, are linked to the body's natural, but complex, immune response during pregnancy.
- In the early stages of pregnancy, a unique mix of inflammatory responses alongside behavioral mechanisms that researchers believe are adaptive, like nausea, achieves a delicate balance, allowing the mother to tolerate and nourish the half-foreign fetus while also avoiding potentially harmful foods.
- The study could have workplace implications for pregnant women, helping to widen recognition that these symptoms are healthy and normal, both reducing stigma and paving the way for common-sense workplace accommodations.
- UCLA researchers have uncovered a link between "morning sickness" symptoms and the body's natural, but complex, inflammatory response to biological and bodily changes during pregnancy.
- According to the National Institutes of Health, up to 80% of early-stage pregnant mothers experience some nausea, vomiting and aversions to certain foods and smells. While uncomfortable, these symptoms are not typically a sign that anything is wrong with the health of the mother or the developing fetus, but rather an indication of a delicate balance 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," UCLA anthropology professor Molly Fox is reported to have said.
- 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," recently published in the journal Evolution, Medicine and Public Health.
- The researchers believe that this delicate balance, which protects mother and fetus, is achieved by a unique mix of inflammatory responses. They function to prevent the mother's body from rejecting the fetus, alongside adaptive behavioral mechanisms, like nausea, that encourage the mother to avoid foods that are potentially harmful, especially in the first and second trimesters when the fetus is most vulnerable.
"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," says UCLA anthropology professor and paper co-author Daniel Fessler.
Methodology and findings
For the study, the UCLA-led team of anthropologists and epidemiologists collected and analyzed blood samples to measure immune system molecules called cytokines. Cytokines are proteins that send signals to help the body launch a quick defense against sickness and regulate inflammation. Participants also filled out questionnaires that asked about morning sickness-related symptoms and food and smell aversions during the early stages of pregnancy. The participants were 58 Latina women in Southern California who were followed beginning in early pregnancy through the postpartum.
Sixty-four percent of study participants experienced odor or food aversions, primarily to tobacco smoke and meat. Sixty-seven percent reported nausea and 66% experienced vomiting.
The study team measured cytokines that promote inflammation (pro-inflammatory) as well as cytokines that suppress inflammation (anti-inflammatory). They found that women who experienced an aversion to tobacco smoke showed a noticeable shift toward a more inflammatory response. Food aversions, nausea and vomiting were also associated with a more pro-inflammatory immune balance.
Natural selection?
The correlation is consistent with researchers' theory that these symptoms may be part of an evolutionary adaptation that helps pregnant mothers' bodies minimize exposure to harmful substances, though the study's authors caution that the evidence is not definitive and more research is needed.
They emphasized that the study allowed the team to look at both human biological and behavioral responses during pregnancy.
"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," says Fox.
These immunological changes may induce nausea, which in turn encourages food avoidance that might act as an additional layer of protection, according to the researchers
"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," says Fessler.
Practical implications
The researchers, including first author Dayoon Kwon, who just completed her Ph.D. in epidemiology at UCLA (and is now a postdoctoral fellow at Stanford), said that the study could help bolster recognition that nausea and vomiting are normal symptoms with biological underpinnings associated with healthy pregnancies. The study's results could help in paving the way for common-sense workplace accommodations, such as more efficient deployment of health care benefits and other helpful resources to reduce stigma, excessive absences and lost productivity.
They also encourage other researchers to continue to look into the questions raised by the study, to not only explore the evolutionary questions, but to work toward providing clinicians with non- or low-invasive measures of prognoses.
Funding for this study was provided by the National Institute of Health.
Story Source:
University of California - Los Angeles. "Scientists finally explain the real reason pregnant women get morning sickness." ScienceDaily 29 September 2025.