Morning sickness is driven primarily by hormones produced during early pregnancy, especially one made by the developing placenta. About seven in ten pregnant women experience some degree of nausea and vomiting, typically starting around the sixth week of pregnancy and peaking between weeks eight and ten. While the exact interplay of causes is complex, recent research has identified the specific protein responsible and explained why some women suffer far more than others.
The Hormone Behind the Nausea
For decades, the leading explanation pointed to human chorionic gonadotropin, or hCG, the same hormone that turns a pregnancy test positive. From the moment the embryo implants, hCG rises exponentially through the first seven weeks and peaks around week ten. That timeline maps almost perfectly onto when nausea is worst, peaking around week nine. Women pregnant with twins or multiples produce higher hCG levels and are more likely to experience morning sickness. Women with the most severe form, hyperemesis gravidarum, consistently show higher hCG levels than average.
But hCG alone didn’t fully explain the picture. Some women with high levels felt fine, while others with normal levels were miserable. A 2023 breakthrough from researchers at the University of Cambridge identified the real culprit: a protein called GDF15, produced by the fetal side of the placenta and released into the mother’s bloodstream.
Why Some Women Get It Worse
The severity of morning sickness depends on two things: how much GDF15 the fetus produces and how sensitive the mother’s body is to it. That sensitivity is shaped by how much GDF15 she was exposed to before pregnancy. Women whose bodies naturally produce low levels of GDF15 outside of pregnancy are essentially blindsided by the sudden flood of it, and they tend to get much sicker. A rare genetic variant linked to very low pre-pregnancy GDF15 levels dramatically increases the risk of hyperemesis gravidarum.
The reverse is also true. Women with the inherited blood disorder beta thalassemia have naturally high GDF15 levels throughout their lives. Their bodies are already accustomed to the protein, and they experience little or no nausea during pregnancy. As one of the Cambridge researchers put it: the baby is producing a hormone at levels the mother is not used to, and the more sensitive she is to it, the sicker she becomes.
How Pregnancy Hormones Slow Your Gut
Estrogen and progesterone, both of which surge in early pregnancy, contribute to nausea through a separate mechanism. Receptors for both hormones are found throughout the digestive tract, and together they relax the smooth muscle lining the esophagus, stomach, and small intestine. This slows the rate at which your stomach empties, leaving food sitting longer and creating that persistent queasy feeling. Estrogen specifically reduces signals that control stomach contractions, which can make digestion feel sluggish well beyond mealtimes.
These same hormones also heighten your sense of smell, sometimes dramatically. Odors that never bothered you before, like cooking meat, coffee, or certain perfumes, can suddenly trigger waves of nausea. This heightened sensitivity appears to be a direct hormonal effect on the brain’s processing of scent, and it tends to fade as hormone levels stabilize later in pregnancy.
Thyroid and Blood Sugar Effects
Very high hCG levels can overstimulate the thyroid gland, causing a temporary form of hyperthyroidism in early pregnancy. This is most common in women with severe morning sickness and can amplify symptoms like nausea, vomiting, and a racing heart. It typically resolves on its own as hCG levels drop in the second trimester.
Blood sugar fluctuations also play a role. In early pregnancy, expanding blood volume can cause glucose levels to dip, especially after fasting overnight. That drop may partly explain why nausea is often worst in the morning, though it’s rarely the sole cause. Eating small, frequent meals helps stabilize blood sugar and can take the edge off symptoms for many women.
Risk Factors That Increase Severity
Several factors raise your chances of more intense morning sickness:
- Twin or multiple pregnancy, which produces higher hormone levels across the board
- History of morning sickness in a previous pregnancy, suggesting a consistent hormonal sensitivity
- Family history, particularly if your mother or sister had severe nausea during pregnancy
- Genetic variants that result in lower baseline GDF15 levels before pregnancy
- History of motion sickness or migraines, both of which signal a lower nausea threshold in the brain
The Typical Timeline
Morning sickness starts as early as the sixth week, and most women notice symptoms before week nine. It feels worst between weeks eight and ten, then gradually improves. By the end of the first trimester, around week thirteen, symptoms have resolved for the majority of women. Some experience lingering nausea into the early second trimester, through weeks fourteen to sixteen, before it fully fades.
Despite the name, morning sickness can strike at any hour. Many women feel worst on an empty stomach, but nausea in the afternoon or evening is equally common.
When It Becomes Hyperemesis Gravidarum
About 1 to 3 percent of pregnant women develop hyperemesis gravidarum, a severe form that goes well beyond typical morning sickness. The hallmarks are losing more than 5 percent of your pre-pregnancy weight, persistent vomiting that prevents you from keeping food or fluids down, and signs of dehydration like dark urine, dizziness, or a rapid heartbeat. This condition requires medical treatment, usually intravenous fluids, and can persist well into the second trimester or occasionally throughout pregnancy.
Hyperemesis is strongly linked to both elevated hCG levels and the GDF15 sensitivity pattern identified in the Cambridge research. Women who have it once are very likely to experience it again in future pregnancies.
The Evolutionary Explanation
One of the more compelling theories for why morning sickness exists at all comes from evolutionary biology. Research published in the American Journal of Obstetrics & Gynecology found the strongest support for the idea that nausea protects the developing embryo during its most vulnerable weeks. By steering pregnant women away from potentially harmful substances, particularly bacteria in meat and toxins in strong-tasting plants, morning sickness may act as a built-in food safety mechanism. The timing supports this: the first trimester is when the embryo’s organs are forming and it’s most susceptible to damage from toxins, and that’s precisely when nausea peaks.