About 70% of women suffer from nausea and vomiting during pregnancy — and scientists have finally figured out why.

A hormone produced by the fetus plays a key role in morning sickness, according to a study published in the medical journal Nature. The hormone, called GDF15, exists at low levels in people who aren’t pregnant, but it increases significantly during pregnancy. Particularly high levels of the hormone, and a person’s sensitivity to it, increase a woman’s risk of developing hyperemesis gravidarum, an extreme form of morning sickness that sends some women to the hospital.

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Women with lower levels of GDF15 prior to pregnancy tended to experience more severe morning sickness symptoms once pregnant, the study found. That suggests exposing women to the greater doses of the hormone before they become pregnant may protect them from bad bouts of morning sickness, researchers said. 

“Most women who become pregnant will experience nausea and sickness at some point, and while this is not pleasant, for some women it can be much worse – they’ll become so sick they require treatment and even hospitalization,” said study author Stephen O’Rahilly, the director of the medical research council metabolic diseases unit at the University of Cambridge. “We now know why. Knowing this gives us a clue as to how we might prevent this from happening.”

The study involved researchers from the United Kingdom, United States, Sri Lanka and Croatia. In addition to analyzing genetic data from more than 18,000 people, the researchers also conducted experiments on mice. The animals, which were not pregnant, received either a placebo or a long-lasting dose of GDF15. Then, three days later, the researchers injected all of the mice with the hormone. Those that received the placebo had lower appetites and lost weight, but those that were “primed” with previous doses of GDF15 ate normally.

This experiment was not conducted on people, but that could be on the horizon. Study author Marlena Fezjo, an assistant professor of population and public health sciences at the University of Southern California, is seeking funding to test a drug that increases GDF15 levels on patients with a history of hyperemesis gravidarum. Drugs that block GDF15 from binding to receptors in the brain also may be useful in treating the condition, but they, too, require further testing.

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