Morning sickness happens because your growing baby produces a hormone called GDF15 that triggers nausea and vomiting. About seven in ten pregnant women experience it, and how sick you feel depends on both how much of this hormone your baby’s placenta releases and how sensitive your body is to it. Rising levels of estrogen and progesterone also slow your digestion, compounding the queasy feeling. Symptoms typically start around week 6 and resolve by the start of the second trimester.
The Hormone Behind the Nausea
The primary driver of morning sickness is a protein called GDF15, produced by the fetal side of the placenta and released into the mother’s bloodstream. Your body already makes small amounts of GDF15 outside of pregnancy, but during pregnancy those levels spike dramatically. The hormone activates a receptor in the brain that controls nausea and appetite suppression, and the result is that familiar wave of queasiness.
Here’s what makes it personal: how sick you feel is directly tied to how much GDF15 your body was exposed to before you got pregnant. Women who naturally have low baseline levels of the hormone tend to be more sensitive to the sudden surge, which means worse nausea. Women with naturally high levels, on the other hand, are already somewhat accustomed to it. Researchers at the University of Cambridge found that women with the blood disorder beta thalassemia, which causes chronically high GDF15 levels, experience little or no pregnancy nausea at all. A rare genetic variant that sharply increases the risk of severe pregnancy sickness was also linked to unusually low pre-pregnancy GDF15.
How Estrogen and Progesterone Slow Your Gut
GDF15 isn’t working alone. The steep rise in estrogen and progesterone during early pregnancy has a direct effect on your digestive system. Both hormones relax the smooth muscle lining your gastrointestinal tract, reducing the contractions that normally push food through your stomach and intestines. Estrogen in particular slows gastric emptying, meaning food sits in your stomach longer than usual. This sluggish digestion contributes to bloating, acid reflux, and nausea. Progesterone also relaxes the valve at the top of your stomach, making it easier for stomach acid to creep upward.
These effects aren’t unique to pregnancy. Studies show that even outside of pregnancy, gastric emptying slows during the phase of the menstrual cycle when estrogen and progesterone peak. Pregnancy just amplifies those hormone levels far beyond what a normal cycle produces.
When It Starts, Peaks, and Ends
Most women first notice nausea around week 6 of pregnancy, though it can begin as early as 8 to 10 days after ovulation. Symptoms tend to peak between weeks 8 and 10, which lines up with the highest levels of hCG (another pregnancy hormone that rises rapidly in the first trimester). Despite the name “morning sickness,” nausea can strike at any time of day.
For most women, symptoms improve noticeably by weeks 12 to 14 as the second trimester begins. By week 20, the majority are symptom-free. Some women experience mild lingering nausea until around week 16, which is still within the normal range. A smaller number deal with nausea well into the second or even third trimester.
Why Some Women Get It Worse
Several factors influence severity. Genetics plays a major role. A large study analyzing DNA from more than 10,000 women identified 10 genes linked to hyperemesis gravidarum, the most severe form of pregnancy sickness. The strongest genetic link was to GDF15 itself, along with the gene that produces its receptor in the brain. Other identified genes are involved in placental development and metabolic regulation. If your mother or sister had severe morning sickness, your own risk is higher.
Carrying twins or multiples also raises the odds. hCG levels roughly double in twin pregnancies compared to singleton pregnancies, and higher hormone levels generally mean more intense symptoms. First pregnancies, a history of motion sickness, and migraine headaches are also associated with worse nausea.
It May Actually Be Protective
From an evolutionary standpoint, morning sickness appears to serve a purpose. The leading hypothesis is that nausea steers pregnant women away from foods most likely to contain harmful bacteria or natural toxins during the critical early weeks of embryonic development. Research supports this: the strongest aversions tend to target meat products (which carry a higher risk of foodborne pathogens) and bitter or strong-tasting plants (which are more likely to contain natural toxins). The timing also fits, since the first trimester is when the embryo is most vulnerable to disruption.
This doesn’t make the experience any less miserable, but it does mean that typical morning sickness reflects a functioning defense system rather than something going wrong with your body.
When Nausea Becomes Something More Serious
Normal morning sickness is unpleasant but manageable. Hyperemesis gravidarum is a more extreme version that affects a smaller percentage of pregnancies and requires medical attention. The distinguishing features are weight loss greater than 5% of your pre-pregnancy weight, significant dehydration, and the inability to keep food or fluids down consistently. You might notice dark urine, dizziness when standing, or a rapid heartbeat.
Hyperemesis gravidarum often requires treatment to restore fluids and correct electrolyte imbalances. It typically improves as the pregnancy progresses, but some women deal with it for much longer than standard morning sickness. If you’re losing weight, can’t keep water down for more than 24 hours, or feel faint regularly, those are signs that your nausea has moved beyond the normal range.
What Helps With Everyday Nausea
Small, frequent meals work better than three large ones. Keeping your stomach completely empty tends to make nausea worse, so eating a few crackers or a piece of toast before getting out of bed in the morning can help. Cold foods are often better tolerated than hot ones because they produce less smell, and strong odors are a common trigger during early pregnancy.
Ginger has modest evidence behind it for reducing mild nausea, whether as tea, chews, or capsules. Staying hydrated matters, especially if you’re vomiting. Sipping water, diluted juice, or electrolyte drinks throughout the day is easier on your stomach than drinking large amounts at once. Some women find that vitamin B6 reduces nausea, and it’s often the first thing recommended before stronger options. If simple strategies aren’t enough, prescription treatments are available that are considered safe during pregnancy.