Pregnancy sickness happens because your baby’s placenta produces hormones that act directly on your brain’s nausea center. About 70 to 85% of pregnant women experience some degree of nausea or vomiting, typically starting around week six and peaking between weeks eight and ten. The good news: for most women, it fades by the end of the first trimester, around week 13.
The name “morning sickness” is misleading. It can hit at any time of day, and its severity ranges from mild queasiness to relentless vomiting. Scientists have long known that pregnancy hormones were involved, but only recently pinpointed exactly which one matters most.
The Hormone Behind the Nausea
For decades, the leading explanation pointed to human chorionic gonadotropin (hCG), the hormone your placenta makes in large quantities during early pregnancy. Women pregnant with twins or multiples have higher hCG levels and are more likely to feel sick, which seemed to confirm the connection. But hCG turned out to be only part of the story.
A 2023 study from 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. GDF15 travels to the brain and directly triggers nausea and vomiting. The severity of your sickness depends on two things: how much GDF15 your baby’s placenta produces, and how sensitive your body is to it.
That sensitivity is shaped by your pre-pregnancy exposure. Women who naturally have low levels of GDF15 in their blood before conceiving are hit hardest when pregnancy floods their system with it. Their bodies simply aren’t used to it. On the flip side, women with the blood disorder beta thalassemia, who carry unusually high GDF15 levels all the time, experience little or no pregnancy sickness. Their brains have essentially built up a tolerance. Researchers believe that finding ways to gradually increase a woman’s GDF15 exposure before pregnancy could one day prevent severe sickness entirely.
How Pregnancy Slows Your Digestion
Nausea isn’t the only digestive change. Progesterone, the hormone that rises sharply to maintain your pregnancy, relaxes smooth muscle tissue throughout your body, including the muscles lining your stomach and intestines. This slows digestion significantly. Food sits in your stomach longer than usual, which can worsen nausea, cause bloating, trigger heartburn, and lead to constipation. These effects layer on top of the direct nausea signal from GDF15, making the whole experience feel more intense than a simple “upset stomach.”
Why Smells Become Overwhelming
Many pregnant women report that certain smells, ones they previously tolerated or even enjoyed, suddenly make them gag. Cooking meat, perfume, coffee, and gasoline are common triggers. Interestingly, research hasn’t found that pregnant women can actually detect fainter odors than non-pregnant women. Their detection thresholds remain about the same. What changes is how they react to smells. Pregnant women tend to rate certain odors as far more unpleasant than they did before, a shift in odor perception rather than odor sensitivity. This altered response likely works alongside the GDF15-driven nausea, making you more reactive to environmental triggers that might otherwise go unnoticed.
Who Gets Hit Hardest
Some women sail through pregnancy with barely a wave of nausea. Others are bedridden for weeks. Several factors raise your risk of more severe symptoms:
- Multiple pregnancy. Carrying twins or more means higher hormone levels across the board.
- Previous pregnancy sickness. If you had it before, you’re very likely to have it again.
- Family history. If your mother or sister had severe nausea during pregnancy, your risk is higher.
- History of motion sickness or migraines. Both suggest a brain that’s already more sensitive to nausea triggers.
- Carrying a female fetus. This association has shown up in multiple studies, though the reason isn’t fully understood.
- Low pre-pregnancy GDF15 levels. Women with a rare genetic variant linked to lower baseline GDF15 are at significantly higher risk of severe vomiting.
When Sickness Becomes Dangerous
About 1 to 3% of pregnant women develop hyperemesis gravidarum, the severe end of the spectrum. The defining feature is vomiting so persistent that you lose more than 5% of your pre-pregnancy body weight and become dehydrated. At that point, your body starts breaking down fat for energy because it can’t keep food or fluids down, and your electrolyte balance shifts in ways that can cause a rapid heart rate and drops in blood pressure.
Hyperemesis gravidarum is not just “bad morning sickness.” It often requires treatment with IV fluids and can lead to hospitalization. If you’re vomiting multiple times a day, can’t keep any liquids down for 24 hours, feel dizzy when standing, or notice your urine has become very dark, those are signs you’ve crossed from uncomfortable to potentially unsafe. Women with hyperemesis gravidarum have significantly higher hCG levels than average, and the Cambridge research suggests they also tend to have had very low GDF15 exposure before pregnancy, making the sudden spike feel overwhelming to their system.
The Typical Timeline
Most women notice nausea beginning around week six, though some feel it as early as week four. Symptoms tend to be worst between weeks eight and ten, which lines up with when placental hormone production is ramping up fastest. By week 13, the end of the first trimester, the majority of women feel noticeably better. A smaller group continues to experience nausea into the second trimester, and a few unlucky women deal with it throughout their entire pregnancy.
If your nausea disappears very suddenly before week 10, that alone isn’t necessarily a sign of a problem. Symptom intensity can fluctuate day to day. But a dramatic, overnight change in how you feel is worth mentioning at your next appointment.
Why Sickness May Signal a Healthy Pregnancy
There’s a well-documented pattern: women who experience nausea and vomiting in the first trimester have lower rates of miscarriage. One theory, published in the journal Trends in Ecology and Evolution, proposes that pregnancy sickness evolved to protect the developing embryo during its most vulnerable period by steering mothers away from foods that could contain toxins or pathogens. The timing fits: nausea peaks exactly when organ formation is happening and the embryo is most susceptible to damage.
A competing explanation argues that the nausea itself isn’t protective. Instead, strong nausea signals a robust placenta producing plenty of hormones, which also happens to mean the pregnancy is developing normally. Embryos with chromosomal defects tend to produce less hormonal signaling, leading to lower nausea and a higher chance of miscarriage. In other words, sickness may be a marker of a healthy pregnancy rather than the cause of one. Either way, the correlation is real: feeling sick is, paradoxically, a reassuring sign that things are progressing as they should.