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 foetus from infection, but without accidentally attacking the foetus, 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 foetus safe while still defending against infection,” said UCLA anthropology professor Molly Fox, corresponding author of the study published in Evolution, Medicine and Public Health.
The researchers believe that this delicate balance, which protects mother and foetus, is achieved by a unique mix of inflammatory responses. They function to prevent the mother’s body from rejecting the foetus, alongside adaptive behavioural mechanisms, like nausea, that encourage the mother to avoid foods that are potentially harmful, especially in the first and second trimesters when the foetus is most vulnerable.
“Nausea, vomiting or aversions to foods or smells are not indications that something is going wrong for the mother or the foetus. 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 and paper co-author Daniel Fessler.
Methodology and findings
For the study, the UCLA-led team of anthropologists and epidemiologists collected and analysed blood samples to measure immune system molecules called cytokines. Cytokines are proteins that send signals to help the body launch a quick defence 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 odour or food aversions, primarily to tobacco smoke and meat. Sixty-seven percent reported nausea and 66% experienced vomiting.
The study team measured pro-inflammatory cytokines as well as anti-inflammatory cytokines. 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.
An evolutionary process?
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 emphasised that the study allowed the team to look at both human biological and behavioural responses during pregnancy.
“In many mammals, the foetal compartment has barriers separating it from the mother’s blood supply, where her immune cells are. But in humans, we have a unique setup – foetal 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 foetus,” said 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 odours and foods, and nausea and even vomiting, appear to be evolution’s way of achieving that same objective,” said Fessler.
Practical implications
The researchers, including first author Dayoon Kwon, who just completed her PhD 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.
