Morning sickness is primarily caused by a hormone called GDF15, produced by the fetal side of the placenta and sent into the mother’s bloodstream, where it acts on the brain to trigger nausea and vomiting. About seven in ten pregnant women experience some degree of morning sickness, and the severity depends on both how much GDF15 the fetus produces and how sensitive the mother’s body is to it. Hormonal shifts, immune system changes, and even a common stomach bacterium also play roles in how intense symptoms become.
The Hormone Behind the Nausea
Research from the University of Cambridge identified GDF15 as the central driver of pregnancy nausea. The fetal placenta produces this protein and releases it into the mother’s blood, where it reaches a specific receptor in the brain that controls the urge to vomit. The more GDF15 circulating in the bloodstream, the worse the nausea tends to be.
What makes the picture more interesting is that your sensitivity to GDF15 during pregnancy depends on how much of it your body was already exposed to before you got pregnant. Women who naturally have low levels of the hormone outside of pregnancy are essentially “naive” to its effects, so when the placenta starts flooding their system with it, the response is more intense. A rare genetic variant associated with very low pre-pregnancy GDF15 levels puts women at significantly higher risk for hyperemesis gravidarum, the severe form of morning sickness that sometimes requires hospitalization.
This discovery also helps explain why morning sickness varies so dramatically from one woman to the next, and even between pregnancies in the same woman. It’s not about pain tolerance or mental toughness. It’s about a measurable hormonal mismatch between what your body is used to and what the pregnancy demands.
Why Symptoms Peak Around Weeks 8 to 10
Morning sickness typically begins around week 6 of pregnancy, though some women notice it as early as 8 to 10 days after ovulation. Symptoms climb in intensity and peak between weeks 8 and 10, which coincides with the highest levels of hCG (human chorionic gonadotropin), another key pregnancy hormone. hCG stimulates the placenta and is closely linked to the surge in GDF15 during this window.
For most women, the nausea begins to ease by weeks 12 to 14, as the second trimester starts and hormone levels stabilize. By week 20, the majority are symptom-free. A smaller percentage of women experience nausea that persists well into the second or even third trimester, which is more common in cases where GDF15 levels remain elevated longer than usual.
The Immune System’s Role
Pregnancy triggers significant changes in the immune system, and those changes contribute to nausea independently of GDF15. When an embryo implants in the uterine wall, the body mounts a pro-inflammatory immune response. This is a normal part of early pregnancy, but the surge in inflammatory signaling molecules can directly affect the brain’s nausea and vomiting center, which sits in a region that’s exposed to whatever is circulating in the bloodstream.
This inflammatory response may also explain why nausea in early pregnancy often comes with a general feeling of being unwell, similar to how you feel during a cold or flu. The immune signals involved are some of the same ones your body uses to fight infection.
Why Smells Become Unbearable
One of the most distinctive features of morning sickness is the sudden, extreme sensitivity to odors. Cooking meat, perfume, coffee, garbage: smells that never bothered you before can become instantly nauseating. Interestingly, pregnancy does not actually sharpen your ability to detect faint odors. Your nose isn’t more powerful in a technical sense. What changes is how intensely you perceive the smells you do detect, and how unpleasant they feel.
This shift in odor perception likely works alongside the inflammatory and hormonal changes happening simultaneously. Your brain is already primed to trigger nausea more easily, so smells that would normally register as neutral or mildly unpleasant can push you over the threshold. The result is a powerful system of avoidance: you instinctively steer away from anything that smells “off.”
An Evolutionary Protection System
The timing of morning sickness is not random. It overlaps almost exactly with the period of embryonic development when organs are forming and the fetus is most vulnerable to toxic substances. This has led researchers to propose that nausea and food aversions function as a protective mechanism, steering pregnant women away from foods most likely to contain harmful chemicals or pathogens.
The pattern of food aversions supports this idea. While strong-tasting vegetables, caffeinated drinks, and alcohol are commonly avoided (all of which contain compounds that could interfere with fetal development), the strongest aversions are actually to meat, fish, poultry, and eggs. Before refrigeration, these were the foods most likely to carry dangerous bacteria and parasites. The aversions track the historical risk profile of foods, not just their taste.
Factors That Make It Worse
Several factors can amplify morning sickness beyond what GDF15 alone would predict. Carrying twins or other multiples increases the risk of severe nausea, likely because more placental tissue means more GDF15 production. Pregnancies with a female fetus are also associated with more intense symptoms, suggesting that something produced in higher quantities by female fetuses contributes to the severity.
A less obvious factor is infection with Helicobacter pylori, a bacterium that colonizes the stomach lining. A large study of over 5,500 women found that those who tested positive for H. pylori were 44% more likely to experience daily vomiting during pregnancy. A broader analysis of multiple studies found that H. pylori colonization was associated with more than triple the odds of severe nausea and vomiting. Many women carry this bacterium without symptoms outside of pregnancy, but the added burden on the digestive system during pregnancy can tip the balance. If your morning sickness is unusually severe, H. pylori is worth investigating, since it’s treatable.
Other known risk factors include a history of motion sickness or migraines (both of which suggest a more reactive nausea center in the brain), morning sickness in a previous pregnancy, and a family history of severe pregnancy nausea. The genetic component is strong: if your mother or sister had hyperemesis gravidarum, your risk is substantially higher.
Why It’s Called “Morning” Sickness
The name is misleading. While nausea is often worst in the morning, likely because blood sugar is low and the stomach is empty after a night of sleep, most women experience symptoms throughout the day. Studies consistently show that nausea limited to mornings is actually the minority experience. For many women, it’s an all-day phenomenon that simply starts worst in the morning and fluctuates with meals, smells, and fatigue levels.
Eating small, frequent meals can help because an empty stomach tends to worsen nausea. This isn’t a psychological trick. When your stomach is empty, gastric acid has nothing to work on, and the resulting irritation adds a physical trigger on top of the hormonal one. Bland, starchy foods are easier to tolerate because they absorb acid and digest without producing strong odors.