For decades, women who experienced severe illness during pregnancy often received the message, either subtly or overtly, that it was "in their head." Now, a landmark genetic study has turned that assumption on its head, offering the clearest evidence yet that the most severe form of pregnancy sickness is driven not by emotion, but by biology.

In the largest study of its kind, researchers from the Keck School of Medicine at USC, working with international collaborators, analysed genetic data from 10,974 women suffering from hyperemesis gravidarum (HG) and compared it with 461,461 others. The findings, published in Nature Genetics, identify 10 genes linked to the condition—six of them never before associated with it.

HG affects about 2% of pregnant women, but its impact can be devastating. It is not typical “morning sickness”. Women with HG can vomit so frequently that eating becomes nearly impossible, leading to dangerous weight loss, dehydration, and malnutrition. In severe cases, both mother and baby face serious risks.

“Because this is the largest study of HG ever conducted, we’ve been able to tease out important new details that were previously unknown,” said Marlena Fejzo, PhD, who led the research. “The fact that we’ve studied women from multiple ancestry groups suggests that these results may be generalisable across a broad population.”

At the center of the discoveries is a gene called GDF15, which produces a hormone that rises sharply during pregnancy. Scientists found that women who had lower exposure to this hormone before pregnancy—due to certain genetic variations—were more likely to experience severe symptoms when levels suddenly increased.

One way to understand the situation is to imagine the body having a “volume knob” for hormones. If the knob is set too low before pregnancy, the sudden surge can feel overwhelming—almost like the body is being blasted with signals it cannot process. The result: intense nausea and vomiting.

But the study goes further. Some of the newly identified genes are linked to how the brain learns and adapts. Researchers believe the brain may begin to associate certain smells or foods with sickness, almost like saving a faulty memory. Over time, even the thought of food can trigger a strong physical reaction, such as nausea or aversion, due to the brain's learned associations between specific smells or foods and past experiences of sickness.

In laboratories that analyse genome-wide data—where scientists scan the entire DNA sequence for patterns—this connection became clearer after months of statistical modelling and cross-checking across populations. The signals were consistent. The result was not random.

Another striking finding is the role of a gene called TCF7L2, already known as one of the strongest genetic risk factors for type 2 diabetes. Its presence in HG opens up a surprising link between metabolism, blood sugar regulation, and pregnancy sickness.

“This gene is a brand-new target, and it’s not yet clear what it’s doing in pregnancy,” Fejzo noted.

That uncertainty, however, comes with opportunity. Because the gene is already well studied in diabetes, researchers are now exploring whether existing medicines could help. One such drug is metformin, widely used to manage blood sugar levels. Early plans are underway to test whether taking it before pregnancy could act as a kind of biological shield—reducing sensitivity to the hormone surge and preventing severe illness.

At present, treatment options for HG remain limited. Even commonly prescribed medicines provide only partial relief, and only for some patients. For many women, the advice has remained frustratingly basic: rest, small meals, and endurance.

This new evidence changes that narrative. It shifts the conversation from coping to prevention.

The implications are significant. Genetic screening could, in the future, identify women at risk before pregnancy begins. Targeted therapies could replace guesswork. Most importantly, the long-standing stigma surrounding the condition may finally begin to fade.

What was once dismissed as anxiety or fear is now understood as a complex biological response shaped by genes, hormones, and brain signals.

In simple terms, it is not a matter of willpower. It is a matter of wiring.

And for millions of women, that distinction could make all the difference—not just in how they are treated, but in whether they suffer at all.