Morning sickness is driven primarily by a sharp rise in hormones during early pregnancy, particularly one called GDF15 that directly triggers nausea and vomiting circuits in the brain. Up to 80% of pregnant women experience some degree of nausea in the first trimester, and while it can feel miserable, the process appears to serve a protective function for both mother and developing embryo.
The Hormone Behind the Nausea
For decades, researchers pointed to hCG (human chorionic gonadotropin) as the main culprit. This hormone is produced by the placenta and peaks around weeks 12 to 14 of pregnancy, which lines up neatly with the worst window of nausea for most women. hCG is still considered a contributing factor, but more recent research has shifted attention to a different player: a protein called GDF15, or growth and differentiation factor 15.
GDF15 rises rapidly during the first trimester and acts on a specific area of the brainstem that controls nausea and vomiting. What makes this protein so interesting is that it doesn’t just correlate with morning sickness. It appears to be the primary cause. The level of GDF15 climbing in your blood during early pregnancy is what makes you feel sick, and how your body responds to that climb determines how severe your symptoms become.
Why Some Women Get It Worse Than Others
Here’s the counterintuitive part: women who experience the most severe nausea often have genetically lower baseline levels of GDF15 before they become pregnant. Because their bodies are accustomed to very little of this protein, the sudden surge in the first trimester hits them harder. Think of it like noise sensitivity. Someone who lives on a quiet farm will find city traffic overwhelming, while someone already used to urban noise barely notices it.
A large genome-wide study of over 50,000 participants first linked genetic variations in the GDF15 gene to severe pregnancy nausea in 2017. Since then, animal research has shown that pre-exposing mice to low doses of GDF15 before giving them a high dose prevented the nausea response entirely. In humans, conditions associated with higher circulating GDF15 before pregnancy appear to reduce the risk of severe morning sickness during pregnancy. This suggests the body can be desensitized to the hormone if it has time to adjust gradually, rather than being hit with a rapid spike.
Genetics also play a role through the fetus itself. If a baby inherits a gene variant that lowers GDF15 production, it contributes less of the protein during pregnancy, which can reduce the mother’s symptoms. So morning sickness severity is shaped by genes from both parents.
Heightened Smell Makes It Worse
Many pregnant women report that their sense of smell becomes almost unbearably sharp in early pregnancy, and this isn’t imagined. Hormonal changes alter how the brain processes odors, making previously neutral or mildly unpleasant smells intensely nauseating. In studies of nausea triggers across different conditions, offensive odors ranked among the top causes, affecting about 52% of people who experience nausea generally.
In pregnancy specifically, this heightened olfactory sensitivity pairs with the already-elevated nausea signals from GDF15 and hCG to create a hair trigger for vomiting. Cooking meat, strong perfumes, coffee, and certain cleaning products are among the most commonly reported triggers. The combination of hormonal nausea and sensory amplification is why morning sickness can feel so unpredictable: you might feel fine one moment and overwhelmed the next after catching a whiff of something from across the room.
The Evolutionary Explanation
Morning sickness looks a lot like a design feature rather than a flaw. The most well-supported evolutionary theory is that nausea and food aversions during the first trimester protect the embryo during its most vulnerable developmental window. Research has found the strongest evidence for the idea that pregnancy nausea steers women away from foods most likely to contain harmful microorganisms or natural toxins, particularly meat products and strong-tasting plants.
This makes sense when you consider the timing. The first trimester is when the embryo’s major organs are forming and when it is most susceptible to damage from pathogens or toxic compounds. By making certain foods repulsive, the body reduces the chance of exposure. Women who experience moderate nausea during pregnancy actually have lower rates of miscarriage on average, which supports the idea that the system is functioning as intended. Knowing this doesn’t make the experience less unpleasant, but it does reframe it: your body isn’t malfunctioning. It’s running a defense system.
Normal Nausea vs. Hyperemesis Gravidarum
Most morning sickness, despite its name, can strike at any time of day and typically peaks between weeks 6 and 14. It’s uncomfortable but manageable, and women with typical nausea continue to gain weight throughout pregnancy and stay hydrated.
Hyperemesis gravidarum is a different situation entirely. It affects a smaller percentage of pregnancies and is distinguished by losing more than 5% of your pre-pregnancy body weight, becoming dehydrated, and developing electrolyte imbalances. Women with hyperemesis often cannot keep any food or liquid down for extended periods, which can require medical treatment including IV fluids. If you’re vomiting so frequently that you can’t stay hydrated, losing weight instead of gaining it, or feeling dizzy and faint, that crosses the line from normal morning sickness into something that needs clinical attention.
The same GDF15 mechanism drives hyperemesis, just at a more extreme level. Women with the genetic profile for very low pre-pregnancy GDF15 are the ones most at risk, and the condition tends to run in families. Having hyperemesis in one pregnancy makes it significantly more likely in subsequent pregnancies, because the underlying genetic sensitivity doesn’t change.
What Actually Helps
Because morning sickness is hormonally driven, there’s no way to eliminate it completely without addressing the hormones themselves, which isn’t practical during pregnancy. But several strategies reduce how often and how intensely nausea hits. Eating small, frequent meals keeps your stomach from being either too empty or too full, both of which worsen nausea. Bland, carbohydrate-rich foods are easier to tolerate than fatty or heavily spiced meals. Many women find that keeping crackers by the bed and eating a few before standing up in the morning prevents the worst early-day episodes.
Ginger has consistent evidence supporting its anti-nausea effects, whether as tea, capsules, or ginger chews. Vitamin B6 is one of the first-line options recommended for pregnancy nausea and is available over the counter. For smell-triggered nausea, some women carry a lemon or a small container of a scent they find pleasant to sniff when an offensive odor hits unexpectedly.
For most women, symptoms improve substantially by weeks 14 to 16 as hormone levels stabilize. GDF15 steadies during the second trimester before rising again later in pregnancy, around weeks 24 to 26, which is why some women experience a return of mild nausea in the third trimester. That second wave is typically less intense than the first-trimester surge.