Nausea, vomiting, and a powerful dislike of foods and smells are some of the discomposing effects of pregnancy known as morning sickness, which is common to an estimated 80 percent of pregnant women across the globe. Scientists have been asking themselves for decades, why such a widespread symptom would have developed at the cost of causing pain. An innovative study of the University of California, Los Angeles (UCLA) and Duke University of 2025 has introduced a new dimension: the immune system is potentially the cause of all those unpleasant reactions.
The study was published in Evolution, Medicine, and Public Health and examines the interaction between immune signaling molecules known as cytokines and olfactory aversion and food aversion during early pregnancy. The results indicate that nausea and aversions could not be arbitrary but a component of a time-honored biological mechanism affording the mother and fetus of being hurt.
The Immune Shift during Early Pregnancy.
One of the greatest pieces of immune balancing in the nature is the pregnancy. The body is forced to take in an embryo that is still developing, and is genetically different, and protect itself against infection. To cope with this, the immune system undergoes different stages in the course of the pregnancy.
At the beginning of pregnancy, it is a transient pro inflammatory response which is predominated by T-helper 1 (Th1) cytokines such as IL-1b, IL-2, and TNF-a. This immunological response assists the embryo to implant onto the lining of the uterus but in addition, causes systemic inflammation. Scientists think that this inflammatory burst at such a young age could affect the nausea centre in the brain, which leads to nausea in the morning and aversion to food.
Subsequently in the second trimester, the immune system changes to an anti-inflammatory Th2 condition, which is important to maintain fetal development and prevent rejection. This novel study fills the gap between behavior and physiology in the course of pregnancy by relating immune action and sensory alterations.
The Research: The Association of Smells with Food and Cytokines.
The investigators sampled 58 Latina women in Southern California at the early stages of pregnancy (5-17 weeks of gestation). The respondents were asked to fill in the comprehensive questionnaires, concerning the nausea, vomiting, and aversions, including the aversion to certain odors and food. Their blood samples were then analyzed by the scientists using a panel of cytokines in all Th1 (pro-inflammatory) and Th2 (anti-inflammatory) types.
Key Findings
There were high aversions by 64% of women the greatest being to tobacco smoke and meat.
67 percent experienced nausea and 66 percent vomited.
Odor aversion was also associated with higher levels of pro inflammatory Th1 cytokines as well as a greater Th1:Th2 ratio, which is an immune shift.
It was found that women who did not like tobacco smoke particularly exhibited a stronger Th1-dominant immune pattern, which is an indicator of increase in immune sensitivity to environmental toxins.
The same tendency was observed among women having food aversions with a specific inclination to meat which has always been associated with the possibility of a greater number of pathogens.
That is, nausea, vomiting and food aversions were not accidental. They were directly associated with immune modification in early pregnancy especially markers of inflammation.
Why Meat and Smoke? The Logic of Protection of Evolution.
The so called maternal and embryonic protection hypothesis has long been put forward by evolutionary biologists. According to this theory, the morning sickness developed to enable mothers to avoid foods that could contain toxins, pathogens or even cancer causing agents at the critical stage of their organ formation.
As an example, the undercooked meat in the ancient setting could contain parasites whereas smoke and strong fumes might indicate chemical contaminations. This opinion is reinforced by the new study as it demonstrates that the same immune molecules that defend against infections can also make the body more sensory in other words, teach it to evade danger before it attacks.
The role of Cytokines: The Chemicals of the Body.
Cytokines refer to small proteins which mediate immune communication. During pregnancy, they act as defenders and regulators they ensure status quo during implantation and also prevent infection.
Accordingly, the research discovered that more averted pregnant women had higher levels of Th1 cytokines (IL-1b, IL-2, IL-12, TNF-a, and IFN-g), which are associated with inflammation. The same cytokines are also reported to influence the central nervous system and vomiting reflex which may be the reason why inflammation causes nausea.
In comparison, women who possessed fewer symptoms possessed more Th2 cytokines (IL-4, IL-10, IL-13), which inhibit inflammation. To a certain extent, this equilibrium, or lack of it, may precondition the severity of the symptoms of pregnancy.
Nausea as an Adaptive Signal
Morning sickness can be very dismal but according to data collected in various studies, it may rather indicate healthier pregnancies. The moderate nausea and vomiting are linked to the low rate of miscarriage and birth defects. The well-functioning immune response, and not malfunction, may be reflected in these symptoms according to the lead author Dr. Dayoon Kwon.
The new observations confirm the hypothesis that pregnancy sickness is a kind of behavioral immunity. The immune defenses increase to send sensory signals making some smells and tastes unbearable, and drive the mother away to possible danger.
The Mind-Body Connection
Interestingly again, a psychological aspect is also brought up in the research. Inflammation and immune control may interact with stress, depression and food insecurity. Acute stress may increase cortisol, thereby breaking the balance of cytokines, leading to increasing nausea. Physical responses may also be enhanced by social or emotional factors in certain instances, making adaptive symptoms severe conditions such as hyperemesis gravidarum – excess pregnancy nausea that has to be treated by a doctor.
Learning about this immune-psychological interaction would aid clinicians in making more effective treatment interventions in extreme cases without violating that mild nausea may be biologically very useful.
Future Research and Maternal Care implications.
This research opens up the possibilities of new directions in prenatal care. Early pregnancy cytokine monitoring could be useful in identifying the risk of women who will develop complications at the time of implantation, inflammatory disorders, or severe morning sickness. Besides, the results may be used to create personalized nutrition plans personalizing the diet and lifestyle to optimize the immune functions and alleviate discomfort without eliminating the protective effect of natural aversions.
Researchers accentuate that nausea and aversions may be painful, yet are the manifestation of the superior capacities of the body to regulate itself and preserve the developing life.
Conclusion
The 2025 Evolution, Medicine and Public Health research alters the way we perceive the morning sickness not as a negative condition but rather as an evolutionary protection. The association among olfactory sensitivity, aversions to food, and immune modulation demonstrates that the biological system is fine tuned to ensure protection and adjustment.
Studying the interaction between cytokines and sensory responses, researchers will be able to reveal new methods of determining the health of early pregnancy and transforming common symptoms into useful biological signals.
Reference
Kwon D., Fessler D.M.T., Knorr D.A., Wiley K.S., Sartori J., Coall D.A., & Fox M.M. (2025). Of scents and cytokines: How olfactory and food aversions relate to nausea and immunomodulation in early pregnancy. Evolution, Medicine, and Public Health, 13(1), 269–280. https://doi.org/10.1093/emph/eoaf01