Morning sickness happens primarily because of a hormone called GDF15 that the placenta produces in large quantities during early pregnancy. Up to 74% of pregnant women experience nausea, and about 50% also have vomiting. The severity depends on a combination of hormonal surges, genetic sensitivity, and changes in how the digestive system functions during the first trimester.
The Hormone Behind the Nausea
For decades, doctors pointed to hCG (human chorionic gonadotropin) as the main culprit behind morning sickness. And hCG does play a role: women pregnant with twins or multiples have higher hCG levels and are more likely to experience nausea. But newer research has identified a more direct trigger, a protein called GDF15 (growth/differentiation factor 15).
GDF15 circulates at low levels in everyone’s body, but during pregnancy, the placenta produces it in abundance. Some women have no notable response to this spike. For those with a heightened sensitivity to GDF15, nausea and vomiting are the result. Both the tolerance and the sensitivity are genetically determined, which explains a pattern many women notice: morning sickness can be severe in one pregnancy and mild or absent in the next. In a surprising twist, the fetus’s own genes can influence how much GDF15 the placenta produces. If the fetus carries certain genetic variants the mother doesn’t have, it can shift her body’s response to the hormone entirely.
How Pregnancy Hormones Slow Your Digestion
GDF15 isn’t working alone. Estrogen and progesterone, both of which rise sharply in early pregnancy, have a direct effect on your gastrointestinal tract. Estrogen slows gastric emptying, meaning food sits in your stomach longer than usual. Progesterone relaxes smooth muscle throughout the body, including in the esophagus, stomach, and small intestine, which reduces the normal contractions that move food along. The combined effect is a sluggish digestive system that contributes to nausea, bloating, and the queasy feeling many women describe even before they vomit.
This hormonal cocktail peaks during the same window that morning sickness is at its worst. Women who are especially sensitive to estrogen tend to report more severe symptoms, and higher estrogen levels are independently linked to worse nausea.
When It Starts, Peaks, and Ends
Morning sickness typically begins around the sixth week of pregnancy, though most women notice it before week nine. Symptoms hit their worst point between weeks eight and ten, then gradually improve. By the 13th week, around the end of the first trimester, most women feel significantly better or are symptom-free.
The name “morning sickness” is misleading. Nausea can strike at any time of day. Some women feel worst in the evening, others deal with low-grade queasiness around the clock. The timing varies from person to person and even between pregnancies.
An Evolutionary Explanation
One influential theory proposes that morning sickness isn’t a glitch but a protective mechanism. The idea, first advanced in the late 1970s, is that nausea steers pregnant women away from foods most likely to contain harmful substances during the critical early weeks of organ development. Strong-tasting vegetables, caffeine, alcohol, and especially meat and fish (which, before refrigeration, were often contaminated with dangerous bacteria) tend to be the very foods that trigger the strongest aversions.
This makes biological sense for another reason: pregnancy suppresses parts of the immune system to prevent the body from rejecting fetal tissue. That immune suppression makes pregnant women more vulnerable to foodborne infections that might otherwise be manageable. Nausea that keeps you away from risky foods could, in evolutionary terms, offer a real survival advantage for both mother and baby.
Who Gets It Worse
Several factors increase the likelihood of more intense symptoms:
- Carrying twins or multiples. Higher hCG levels mean a stronger hormonal trigger for nausea.
- Genetic sensitivity to GDF15. If your body isn’t accustomed to even baseline levels of this protein, the pregnancy surge hits harder.
- A history of motion sickness or migraines. Both suggest a nervous system that’s more reactive to nausea triggers in general.
- Previous pregnancies with severe nausea. The pattern tends to repeat, though fetal genetics can change the equation.
Ethnicity and socioeconomic factors also appear in the data as risk modifiers, likely reflecting a mix of genetic predisposition, dietary patterns, and access to early symptom management.
When Nausea Becomes Hyperemesis Gravidarum
About 1 to 3% of pregnant women develop hyperemesis gravidarum, the severe end of the morning sickness spectrum. The defining feature is weight loss exceeding 5% of pre-pregnancy body weight, along with dehydration significant enough that the body starts breaking down fat for energy (producing compounds called ketones that show up on urine tests). Women with hyperemesis gravidarum have measurably higher hCG levels than those with typical morning sickness.
This condition requires medical treatment, usually intravenous fluids and medication to control vomiting. Left untreated, the dehydration and nutritional deficits can become dangerous. Research from King’s College London found that severe pregnancy sickness raises the risk of mental health conditions by over 50%, a connection that was historically dismissed by the medical community but is now recognized as clinically significant. Notably, the psychiatric impact doesn’t always track with physical severity. Even women with moderately severe cases can experience significant anxiety and depression.
Managing Mild to Moderate Symptoms
For typical morning sickness, two remedies have the strongest evidence behind them: ginger and vitamin B6. The Society for Obstetric Medicine of Australia and New Zealand recommends up to 1,000 mg per day of standardized ginger extract, or 600 mg of ginger combined with 37.5 mg of vitamin B6. Ginger capsules are generally better studied than ginger tea or ginger ale, which contain inconsistent amounts of the active compounds.
Beyond supplements, practical strategies make a real difference. Eating small, frequent meals keeps your stomach from being either too empty or too full, both of which worsen nausea. Cold or room-temperature foods tend to be better tolerated than hot meals because they produce less aroma. Staying hydrated matters, but sipping small amounts throughout the day works better than drinking large quantities at once. Many women find that bland, starchy foods like crackers or toast first thing in the morning help bridge the gap between waking up and being able to eat a real meal.
If these approaches aren’t enough, prescription options exist that are safe during pregnancy. The key is addressing symptoms early rather than waiting for them to escalate, since nausea that progresses to repeated vomiting is harder to bring back under control.