When in 1999 at 31, geneticist Marlena Fejzo was pregnant for the second time and expecting some of the nausea that affects about 70 per cent of pregnancies, she didn’t expect to be so flattened by it that she couldn’t eat or drink. Her GP initially brushed it off as normal and, by the time he said she needed a feeding tube, it was too late. She had become so weak she had lost the baby.

It might be an evolutionary adaptation, but severe morning sickness is not normal or harmless. Credit: Getty Images Twenty-five years later, Fejzo, now a mother of three, is a world-leading researcher in morning sickness and she is one of Time magazine’s Women of the Year . Her previous research found that “morning sickness” during pregnancy is the result of a spike in the levels of a stress response hormone called growth and differentiation factor 15 (GDF15).

In a paper published in Nature in 2023, she found that women who are genetically predisposed to low levels of GDF15 before pregnancy can become hypersensitive to its rapid rise during pregnancy, leading them to experience severe morning sickness, otherwise known as hyperemesis gravidarum (HG). Moreover, the levels of GDF15 are significantly higher if the mother is carrying a female fetus: about 72 per cent of women report nausea, versus about 42 per cent who are carrying a male. And if the fetus carries the mutation that leads to the GDF15 hypersensitivity, the mother does not experience HG.

But when the fetus does not have the.