Nausea, vomiting, disgust for certain smells... Pregnancy comes with its share of unpleasantness, but it's for a good reason. Some cases can be serious, such ashyperemesis gravidarum, which affects a minority of women and can lead to hospitalizations. But nausea in most pregnant women is benign, even protective. Indeed, morning sickness is associated with beneficial effects for pregnancy and to a lower risk of miscarriage. A new study, published on September 24, 2025 in the journal Evolution, Medicine, and Public Health by researchers from the University of California, Los Angeles (USA), shows that these morning nauseas are caused by an adaptation of the immune system, which becomes more wary of food in order to protect the unborn baby.
Avoid infections to protect the baby
Pregnancy is a very special time for the human body. A new, very fragile being is in full formation, which must be protected from any infection. But the mother's body cannot simply put its immune system on high alert, otherwise it risks attacking the fetus, recognizing it as foreign to the mother's organism.It's a challenge: the immune system must protect the mother and fetus from infections, but without accidentally attacking the fetus, which has a half-foreign genetic identity, because it comes from the father.", explains in a press release Molly Fox, study director. Normally, the immune system attacks anything it perceives as foreign, but during pregnancy, it must find the right balance between fighting infections while maintaining a moderate immune response. To achieve this, the best approach is to avoid any risk of infection, especially food-related.
The pregnant woman's body reacts as if it were infected
Fetal development begins with the implantation of the embryo into the uterine wall. When this implantation occurs, the maternal immune system responds with an increase in the production of type 1 helper T lymphocytes, pro-inflammatory immune cells (cells that become activated, particularly during an infection). Hypothesizing that these cells could influence the emergence of morning sickness, the researchers investigated whether there was a link between the production of these cells and these characteristic discomforts of pregnancy.
Nausea is associated with this immune response
To do this, they analyzed blood samples from a total of 58 pregnant women. These samples were collected between weeks 5 and 17 of pregnancy, between the first and second trimesters. Nearly two-thirds of them reported experiencing nausea (67%) and vomiting (66%), as well as disgust for certain odors (mainly cigarettes) and certain foods (particularly meat). These discomforts were associated with each other: the women who suffered the most disgust were also those who had the longest nausea, and the duration of this nausea was linked to a greater frequency of vomiting.
These worries were also associated with a higher level of proteins produced by these type 1 helper T lymphocytes, showing that women who react most to smells and food are also those who activate this pro-inflammatory response the most. Nausea, vomiting, or disgust for certain foods or smells are not indicators of a problem. Rather, they are a sign that everything is going normally, reflecting a healthy and beneficial immune response., concludes Daniel Fessler, co-author of the study. Today, we see labels on certain foods, such as ground beef, warning pregnant women of the potential risks of food poisoning during pregnancy. Disgust for these types of foods or smells, along with nausea and vomiting, may be evolution's way of achieving this same goal. However, knowing that your nausea is a good sign won't necessarily make it more pleasant. So know that if it becomes too debilitating, there are tips to decrease the frequency.
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