Vomiting during pregnancy is triggered primarily by a hormone called GDF15, produced by the placenta and released into the mother’s bloodstream. Up to 80% of pregnant people experience some degree of nausea or vomiting, typically starting around week six and peaking between weeks eight and ten. Despite the nickname “morning sickness,” it can strike at any hour. The severity depends on how much of this hormone the placenta produces and how sensitive your body is to it before pregnancy even begins.
The Hormone Behind Pregnancy Nausea
A landmark study led by researchers at the University of Cambridge pinpointed GDF15 as the main culprit behind pregnancy nausea. The fetal side of the placenta produces this protein and sends it into the mother’s blood at levels her body isn’t accustomed to. The more sensitive she is to GDF15, the sicker she feels.
What makes this discovery especially interesting is the role of pre-pregnancy exposure. Women who naturally have low levels of GDF15 in their blood before conceiving tend to experience the worst nausea, because the sudden spike during pregnancy is a bigger shock to their system. Women who carry a rare genetic variant linked to very low baseline GDF15 face a much higher risk of severe, unrelenting vomiting. On the flip side, women with the inherited blood disorder beta thalassemia, which causes chronically high GDF15 levels, report little to no nausea during pregnancy. Their bodies are already desensitized to the hormone.
This explains something that has puzzled doctors for decades: why some women sail through pregnancy with barely a queasy moment while others can’t keep food down for months. It’s not about willpower or diet. It’s about the biological gap between how much GDF15 your body is used to and how much the placenta suddenly dumps into your bloodstream.
How Estrogen and Progesterone Make It Worse
GDF15 isn’t acting alone. Estrogen and progesterone, the two hormones that surge to sustain a pregnancy, also contribute to nausea through their effects on your digestive system. Estrogen at high levels is known to induce nausea on its own. Progesterone relaxes smooth muscle throughout the body, including the muscles of the stomach and intestines, which slows digestion. When your stomach empties more slowly, you feel fuller, more bloated, and more prone to vomiting.
Progesterone also disrupts the normal electrical rhythm of the stomach, promoting what’s called slow gastric dysrhythmia. Think of it as your stomach’s contractions falling out of sync, which adds to the discomfort. Progesterone levels tend to be highest in the morning hours, which may partly explain why nausea is often strongest after waking, even though it can persist throughout the day.
Why Smells Become Unbearable
Many pregnant people report that certain smells, ones they previously tolerated or even enjoyed, suddenly trigger waves of nausea. Cooking meat, perfume, coffee, garbage: the list of offending odors varies, but the experience is nearly universal. Whether pregnancy actually sharpens the sense of smell is still debated. What research does support is that the brain can rapidly form associations between a neutral odor and nausea. Once you feel sick around a particular smell even once, your brain can learn to trigger nausea the next time you encounter it, regardless of whether your nose is technically more sensitive. This conditioned response can happen at any odor intensity, meaning even a faint whiff of something your brain has flagged can send you running to the bathroom.
An Evolutionary Safety Net
One prominent theory suggests that pregnancy nausea isn’t a glitch but a feature. The “embryo protection hypothesis” proposes that nausea steers pregnant women away from foods most likely to contain harmful bacteria, parasites, or natural toxins, particularly during the first trimester when the embryo’s organs are forming and most vulnerable. Strong-tasting vegetables, caffeinated drinks, alcohol, and animal products (which, before refrigeration, were often contaminated) are among the most common aversions. By making these foods repulsive, the body may be reducing the chance of exposing the developing embryo to dangerous substances. It’s not a perfect explanation for every case of morning sickness, but it fits the pattern: symptoms tend to be strongest exactly when the embryo is most susceptible to harm.
When Symptoms Start and Stop
Most women notice nausea beginning around the sixth week of pregnancy, with the vast majority experiencing symptoms before the ninth week. The worst stretch is typically between weeks eight and ten, when placental hormone production is ramping up fastest. For most people, symptoms ease significantly once the second trimester begins, around weeks 12 to 14. A smaller percentage of women continue to experience nausea beyond that point, and a few deal with it throughout the entire pregnancy.
When Vomiting Becomes Severe
Normal pregnancy nausea is unpleasant but manageable. Hyperemesis gravidarum is its extreme form, affecting a smaller subset of pregnant women with relentless vomiting that leads to weight loss of more than 5% of pre-pregnancy body weight, dehydration, and an inability to keep food or fluids down. It often requires medical treatment, sometimes including IV fluids and hospital stays. Doctors diagnose it by checking for ketones in the urine (a sign the body is breaking down fat for energy because it’s not getting enough food), along with blood tests for electrolytes and kidney function.
Hyperemesis gravidarum is not simply “bad morning sickness.” It can cause serious complications if untreated, including malnutrition and damage to the esophagus from repeated vomiting. Women with naturally low pre-pregnancy GDF15 levels and those carrying the genetic variants identified in the Cambridge study appear to be at highest risk.
What Helps Reduce the Nausea
Several strategies can take the edge off pregnancy nausea, though nothing eliminates it entirely for everyone. Eating small, frequent meals rather than three large ones keeps your stomach from being either too empty or too full, both of which can trigger vomiting. Bland, starchy foods like crackers, toast, and rice are easier to tolerate than rich or greasy meals. Keeping a few crackers by your bed to eat before getting up in the morning can help, since an empty stomach first thing tends to make nausea worse.
Ginger, in the form of tea, candies, or capsules, has modest evidence supporting its use for mild nausea. Staying hydrated matters, especially if you’re vomiting frequently. Small sips of water, ice chips, or electrolyte drinks throughout the day are easier to keep down than large glasses.
For nausea that doesn’t respond to dietary changes, vitamin B6 is the most commonly recommended first-line option. It can be combined with doxylamine, an antihistamine found in some over-the-counter sleep aids, at a dose of half a 25-mg tablet (12.5 mg). This combination is endorsed by the American College of Obstetricians and Gynecologists as a safe and effective treatment. If vomiting is severe enough to suggest hyperemesis gravidarum, prescription anti-nausea medications and IV fluid replacement become necessary, and your provider will monitor your weight, hydration, and electrolyte levels closely.