Morning sickness is primarily caused by hormones produced during early pregnancy, especially a protein made by the placenta that triggers nausea centers in the brain. It affects up to 80% of pregnant people, typically starting around week six and peaking between weeks eight and ten before fading near the end of the first trimester. While the exact interplay of causes is complex, researchers have identified several hormonal and biological factors that work together to produce those waves of nausea.
The Hormones Behind the Nausea
For decades, the leading explanation centered on human chorionic gonadotropin (hCG), a hormone the placenta begins producing shortly after a fertilized egg implants in the uterine lining. hCG levels rise rapidly during the first trimester, and the timing of that spike lines up closely with when nausea is at its worst. People pregnant with twins or multiples have higher hCG levels and are more likely to experience morning sickness, which strengthens the connection.
But hCG isn’t the whole story. A landmark 2023 study led by researchers at the University of Cambridge identified a protein hormone called GDF15 as a primary driver of pregnancy nausea. GDF15 is produced by the fetus and placenta, and it acts directly on a part of the brain that controls the vomiting reflex. The severity of nausea a woman experiences depends on two things: how much GDF15 the placenta produces and how sensitive her body is to it. Women who naturally have low levels of GDF15 in their blood before pregnancy are less accustomed to the hormone, making them more reactive when fetal production floods their system. This discovery was significant because it finally explained why some women sail through pregnancy with mild queasiness while others are incapacitated.
Estrogen and progesterone also play supporting roles. Both hormones surge during the first trimester. Estrogen is known to induce nausea even outside of pregnancy, and rising estrogen levels appear to heighten sensory sensitivity in early pregnancy, making smells and tastes more intense and more likely to trigger a gag reflex. Progesterone, meanwhile, relaxes smooth muscle throughout the body, including the muscles of the digestive tract. This slows the rate at which your stomach empties and increases the frequency of irregular stomach contractions, both of which contribute to that persistent queasy feeling.
Why Some People Get It Worse
Several factors raise the odds of more severe nausea. Carrying twins or triplets tops the list, largely because of higher hormone levels. A personal history of motion sickness or migraines also increases risk, likely because both conditions involve the same brain pathways that GDF15 activates. Family history matters too: if your mother or sister had severe pregnancy nausea, you’re more likely to experience it yourself. And if you had significant nausea in a previous pregnancy, it tends to return in subsequent ones.
The most extreme form, hyperemesis gravidarum, goes well beyond ordinary morning sickness. It’s characterized by vomiting so persistent that it causes weight loss of more than 5% of body weight, dehydration, and disrupted electrolyte balance. Women with hyperemesis gravidarum consistently show higher hCG levels than other pregnant women. Left untreated, the dehydration can become serious enough to cause a rapid heart rate and drops in blood pressure. This condition requires medical treatment, and early intervention can prevent hospitalization.
How Digestion Changes in Early Pregnancy
Beyond the hormonal triggers, the physical changes in your digestive system amplify the discomfort. Progesterone’s muscle-relaxing effect doesn’t just slow stomach emptying. It also loosens the valve between your esophagus and stomach, making acid reflux more common, and slows the movement of food through your intestines. The result is a digestive system that feels sluggish and irritable.
At the same time, your sense of smell becomes significantly sharper during the first trimester, thanks to rising estrogen. Foods and odors you tolerated easily before pregnancy can suddenly provoke intense nausea. This heightened sensitivity is part of what makes mornings especially difficult: an empty stomach combined with amplified sensory input creates a perfect storm for nausea, though the name “morning sickness” is misleading since symptoms can strike at any time of day.
The Protective Theory
One of the more reassuring findings in this area is that morning sickness may actually serve a purpose. A well-supported evolutionary theory, sometimes called the “maternal and embryo protection hypothesis,” suggests that nausea steers pregnant women away from foods most likely to contain harmful substances during the critical early weeks of organ development. Research published in the American Journal of Obstetrics and Gynecology found the strongest evidence for the idea that pregnancy nausea protects against pathogenic microorganisms in meat and toxins in strong-tasting plants.
This doesn’t mean nausea is pleasant or that it should go untreated. But understanding that normal levels of pregnancy nausea reflect a functioning defense system, rather than something going wrong, can be genuinely reassuring. It also helps explain the pattern: nausea peaks during weeks eight to ten, when the embryo is most vulnerable to toxic disruption, and fades around week thirteen, when the major organ systems have formed.
The Typical Timeline
Morning sickness follows a fairly predictable arc for most people. It starts as early as week six, and the vast majority of women notice symptoms before week nine. The worst stretch is usually between weeks eight and ten, when hCG and GDF15 levels are climbing fastest. By around week thirteen, the end of the first trimester, symptoms improve significantly or disappear entirely for most women.
A smaller percentage of women experience nausea that lingers into the second trimester or, rarely, throughout the entire pregnancy. This doesn’t necessarily indicate a problem, but persistent vomiting that causes weight loss or prevents you from keeping fluids down is worth addressing early. Treatment in the initial stages can prevent the cycle from escalating into something more serious. Your own perception of how severe your symptoms feel is a valid and important factor in deciding whether to seek help.