What Causes Morning Sickness During Pregnancy? Science Has the Answer

Scientists may have finally figured out what causes morning sickness during pregnancy, offering hope to millions of women.

Roughly 70 percent of pregnant women are affected by episodes of nausea and vomiting, which can start from just four weeks into the pregnancy.

"This is distressing and inconvenient and interferes with function in family work and social contexts in a large proportion of those [who experience it]," Stephen O'Rahilly, a professor of clinical biochemistry and medicine at the U.K.'s University of Cambridge, told Newsweek.

He continued: "Morning sickness is an old, inaccurate and unhelpful term, as most women feel sick all day. In about 1 to 2 percent of pregnant women, the nausea and vomiting are so severe that the woman is unable to keep herself hydrated and needs to come to hospital for intravenous fluids.

"This severe end of the spectrum is called hyperemesis gravidarum. Without such resuscitative treatment, which also involves essential vitamins, this condition can be fatal," O'Rahilly said.

In the U.S., this condition is the leading cause of hospitalization during early pregnancy and the second most common cause of pregnancy hospitalization overall. Clearly, vomiting and nausea during pregnancy is not a trivial condition. And yet, O'Rahilly said, women are frequently told that it is all "in their head."

"[They] are not given accurate information about what is actually going on," he said. "Understanding the cause in real depth will lead to the development of safe and successful ways of treating and preventing the condition."

In a recent study published in the journal Nature, O'Rahilly and his team identified a potential culprit for this debilitating condition: a hormone called growth differentiation factor 15, or GDF15. The hormone has previously been implicated in nausea and vomiting during pregnancy, but exactly how it may contribute to these effects is still a mystery.

In their study, the team members were able to trace the origins of these elevated hormone levels in the mother's blood. "The GDF15 is made by the fetal component of the placenta, and as the placenta grows, more of it is produced," O'Rahilly said.

He went on: "We have securely established for the first time, using well-validated immunoassays and large sample sizes, that GDF15 levels in maternal blood are significantly higher in women who report vomiting in pregnancy, or have a diagnosis of hyperemesis gravidarum, compared to women who report no or low levels of nausea or vomiting."

However, not everyone with higher GDF15 levels experiences the same symptoms, and women with a condition called beta-thalassemia, which causes chronically high levels of GDF15, reported very low levels of vomiting and nausea during pregnancy.

"The most important factor is how low the mother's GDF15 levels are before she becomes pregnant," O'Rahilly said. "If they are very low, then the shock of being exposed to high GDF15 coming from the fetus causes the most severe symptoms."

In other words, a mother's sensitivity to this hormone is at least partly determined by her pre-pregnancy exposure to it. However, exactly why this hormone-induced nausea evolved has left many scratching their heads.

"The high expression of GDF15 in placenta appears to be a phenomenon restricted to certain higher species, including primates," O'Rahilly said. "We hypothesize that this evolved to send a signal to the mother's brain in early pregnancy that made her averse to ingestion of foodstuffs that had a higher risk of containing [chemicals that can disrupt the embryo] or infectious organisms.

He said that over 90 percent of women describe specific food and beverage avoidance in early pregnancy, most frequently for meat and alkaloid-containing beverages.

"While we have not proven this, it seems likely that GDF15 is involved in signaling such aversions," O'Rahilly said.

He said this would have been critical to the survival chances of the offspring—and also the mother, who is immunosuppressed in early pregnancy and therefore more susceptible to infection—during most of our evolution, when we were largely hunter-gatherers. But in the modern era of safe food production, in developed countries at least, "such a signaling system may be seen as redundant," he said.

"Thus, placental production of GDF15 may be the endocrine equivalent of the appendix, a piece of human biology that has outlived its necessity and now causes us more trouble than it is worth," he said.

These findings have important implications for future treatments for expectant mothers suffering from these symptoms, especially those with hyperemesis gravidarum.

"Blocking the action of GDF15 at its receptor in the mother is very likely to be highly effective in reducing symptoms," O'Rahilly said. "Our data [also] tentatively suggest that a doubling of pre-pregnancy GDF15 levels would reduce [hyperemesis gravidarum] risk by 50 percent.

He said that Metformin, a widely used drug that has been given to people with diabetes for over five years, increases GDF15 levels by two- to threefold and is widely used in pregnancy in many countries.

"Clinical trials of such an intervention are currently being planned, in partnership with patient-led groups," he said.

If you or someone you know is struggling with these symptoms, O'Rahilly has some important advice.

"If the symptoms are disabling then seek medical attention. There are medicines which can help, at least partially. Do not be palmed off by being told to 'eat little and often' or 'try ginger.' These just don't work," he said.

Is there a health issue that's worrying you? Do you have a question about nausea and vomiting during pregnancy? Let us know via health@newsweek.com. We can ask experts for advice, and your story could be featured in Newsweek.