Morning sickness is triggered primarily by a hormone called GDF15, produced by the placenta and sent into the mother’s bloodstream, where it acts on the brain to cause nausea and vomiting. But GDF15 isn’t the whole story. A combination of rising hormones, a slowed digestive system, and heightened sensory responses all converge during the first trimester to create what most pregnant people experience as persistent, sometimes debilitating nausea.
The Hormone Behind the Nausea
For years, researchers pointed to hCG (human chorionic gonadotropin) as the main driver of morning sickness, since it peaks during the first trimester right when nausea is at its worst. But a landmark study led by the University of Cambridge identified a more direct culprit: a protein hormone called GDF15, produced by the fetal side of the placenta.
GDF15 acts on a specific area of the brain that controls nausea and vomiting. The degree of sickness a woman experiences is directly related to two things: how much GDF15 the placenta produces and how sensitive her body is to it. That sensitivity, it turns out, depends on how much GDF15 she was exposed to before pregnancy. Women who naturally have low levels of GDF15 in their blood before conceiving are more vulnerable to severe nausea once that hormone spikes. Women with the blood disorder beta thalassemia, who carry naturally very high GDF15 levels before pregnancy, experience little or no morning sickness at all.
This explains something that puzzled doctors for decades: why some women sail through pregnancy with barely a queasy moment while others are incapacitated. It’s not about toughness or diet. It’s largely about pre-pregnancy hormone exposure and genetic makeup.
How Your Digestive System Slows Down
Progesterone, which rises sharply in early pregnancy to support the uterine lining, has a side effect: it relaxes smooth muscle throughout the body, including the muscles lining the digestive tract. This means food moves through your stomach and intestines more slowly than usual. The result is a higher prevalence of slow gastric rhythm, a pattern of sluggish stomach contractions that contributes to bloating, discomfort, and nausea.
This slowdown is part of why nausea often feels worse on an empty stomach. When digestion is already sluggish, long gaps between meals can leave stomach acid sitting without much to work on, intensifying that queasy feeling. Eating small, frequent meals helps partly because it keeps the slowed system from running on empty.
Why Smells Become Unbearable
Many pregnant women report that certain odors, ones they previously tolerated or even enjoyed, suddenly trigger waves of nausea. Coffee, cooking meat, perfume, and garbage are common offenders. This shift in odor perception happens early in pregnancy and is most dramatic during the first trimester.
The timing doesn’t match rising estrogen, which climbs steadily throughout pregnancy and peaks near delivery. Instead, the changes in how odors are perceived, particularly whether a smell registers as pleasant or revolting, track more closely with hCG levels, which peak during weeks 11 to 13 and then decline. hCG is considered a strong candidate for driving these shifts in odor response, and since nausea itself also correlates with hCG levels, the two likely reinforce each other. A smell that suddenly registers as disgusting can push an already-nauseated person over the edge.
The Typical Timeline
Morning sickness starts as early as the sixth week of pregnancy, with most women noticing symptoms before week nine. It feels worst around weeks eight to ten, when multiple hormonal changes overlap at their most intense. By week 13, the end of the first trimester, symptoms improve or disappear for most women.
The name “morning sickness” is misleading. Nausea can strike at any hour. It tends to be more noticeable in the morning partly because overnight fasting leaves the stomach empty while progesterone-driven digestive slowdown is already in effect. But for many women, nausea persists throughout the day.
Genetics and Risk Factors
Severe nausea and vomiting during pregnancy runs in families. If your mother or sister had a rough first trimester, your chances of the same experience are higher. Research from UCLA identified DNA variations around the GDF15 gene (and a related gene called IGFBP7) that are associated with hyperemesis gravidarum, the most extreme form of pregnancy sickness that can require medical treatment.
The Cambridge research pinpointed a rare genetic variant that causes women to have unusually low GDF15 levels before pregnancy. Carrying this variant significantly raises the risk of severe symptoms, because the body hasn’t built any tolerance to the hormone before the placenta starts flooding the bloodstream with it. Carrying twins or multiples also increases nausea, likely because a larger placenta produces more GDF15 and hCG.
An Evolutionary Purpose
Morning sickness may feel like a malfunction, but a large body of evidence suggests it’s actually a defense mechanism. A review published in the American Journal of Obstetrics and Gynecology found the strongest support for the hypothesis that nausea and vomiting protect both the mother and embryo from harmful substances in food, particularly pathogenic microorganisms in meat and toxins in strong-tasting plants.
The timing makes sense from this perspective. The first trimester is when the embryo is most vulnerable to disruption, as organs are forming and cells are differentiating rapidly. The food aversions that come with morning sickness tend to target exactly the categories of food most likely to carry pathogens or natural toxins: meat, fish, bitter vegetables, and pungent spices. By the second trimester, when the major organ systems are established and the fetus is more resilient, nausea typically fades.
The researchers behind this analysis were clear in their conclusion: morning sickness indicates a functioning defense system, not a bodily malfunction. That reframing doesn’t make the symptoms less miserable, but it does help explain why nausea during pregnancy is so common across cultures and throughout recorded history. It persists because, on a population level, it works.
Why Severity Varies So Much
Putting all of this together, the wide range of experiences makes more sense. A woman’s morning sickness severity is shaped by how much GDF15 her placenta produces, how much GDF15 she was exposed to before pregnancy, her genetic variants around GDF15 and related genes, how strongly progesterone affects her digestive motility, and how her brain responds to hCG-driven shifts in odor perception. These factors combine differently in every pregnancy, which is why the same woman can have vastly different experiences from one pregnancy to the next.
About 70 to 80 percent of pregnant women experience some degree of nausea. Roughly half also vomit. At the extreme end, about 1 to 3 percent develop hyperemesis gravidarum, with vomiting severe enough to cause dehydration and weight loss. For most women, though, symptoms stay within a manageable range and resolve on their own by the start of the second trimester.