How Does Morning Sickness Work in Pregnancy?

Morning sickness is driven primarily by hormones that surge during early pregnancy, with a protein called GDF15 now recognized as the central trigger. Produced in large quantities by the placenta and fetus, GDF15 acts directly on a nausea center in the brainstem, and a woman’s sensitivity to it depends partly on how much of the hormone she was exposed to before pregnancy. Other hormones, including hCG, estrogen, and progesterone, pile on by slowing digestion and amplifying sensory responses. The result is a system that feels like a malfunction but appears, from an evolutionary standpoint, to be doing something protective.

The GDF15 Breakthrough

For decades, researchers knew that pregnancy hormones were involved in nausea and vomiting, but the picture was blurry. A 2024 study published in Nature brought the sharpest focus yet. Researchers showed that growth differentiation factor 15, or GDF15, is produced overwhelmingly by the feto-placental unit and that higher levels in a mother’s blood are associated with vomiting in pregnancy and with its most severe form, hyperemesis gravidarum. Using a technique that could distinguish fetal GDF15 from maternal GDF15, the team confirmed that the vast majority of the hormone circulating in a pregnant woman’s blood comes from the developing baby and placenta, not from her own tissues.

What makes GDF15 especially interesting is that the severity of nausea appears to hinge not just on how much of it floods the bloodstream but on how accustomed the mother’s body is to it beforehand. Women who naturally carry low levels of GDF15 before pregnancy seem more vulnerable to severe symptoms when pregnancy causes a sudden spike. By contrast, women with beta-thalassemia, a blood disorder that keeps GDF15 chronically elevated, report very little pregnancy nausea. In mouse experiments, animals previously exposed to sustained GDF15 showed a blunted response when given a sudden dose, consistent with a desensitization effect. The implication is that the brainstem’s nausea circuitry can be “trained” by prior exposure, and pregnancy catches some women’s systems off guard more than others.

This discovery has practical significance beyond explaining who gets sick and who doesn’t. If the brainstem’s sensitivity to GDF15 can be shifted by gradual pre-exposure, it opens the door to preventive therapies. Researchers are already exploring whether slowly ramping up GDF15 levels before or very early in pregnancy could reduce the intensity of nausea when placental production kicks in.

How hCG, Estrogen, and Progesterone Contribute

GDF15 is the newest and arguably strongest piece of the puzzle, but other hormones have long been linked to morning sickness. Human chorionic gonadotropin, or hCG, is the hormone that makes a pregnancy test turn positive, and it rises steeply in the first trimester. Studies have consistently found higher hCG levels in women who experience nausea and vomiting compared with those who don’t.² More recent work has confirmed that elevated free beta-hCG in the first trimester is positively associated with nausea and vomiting symptoms, reinforcing the connection.³

Estrogen and progesterone play a different role. Rather than acting on the brain’s nausea center the way GDF15 does, they slow down the gut. Estrogen stimulates the production of nitric oxide, which relaxes smooth muscle in the digestive tract and delays gastric emptying. Progesterone reduces the contractility of the same smooth muscle. The combined effect is that food sits in the stomach longer, which can worsen feelings of nausea. One study found that women with hyperemesis gravidarum had roughly a quarter higher serum estradiol levels than women without severe symptoms, and the combination of estrogen and progesterone together had an additive effect on slowing gastric movement.⁴

So the picture is layered: GDF15 triggers nausea at the brain level, hCG rises in tandem and may amplify that signal, and estrogen and progesterone make the stomach itself more sluggish. All three pathways ramp up during weeks six through twelve and tend to ease as the placenta matures and hormone levels stabilize, which is why most women feel better entering the second trimester.

Why Smells Become Unbearable

Many pregnant women describe a sudden, almost supernatural ability to detect odors in early pregnancy, often accompanied by intense revulsion toward smells they previously found neutral or pleasant. This isn’t imagined. A prospective study comparing pregnant women with non-pregnant controls found marked distortions in odor identification during the first trimester, suggesting that the sensory processing of smell genuinely changes early in pregnancy.⁵ The researchers proposed that these olfactory changes could be a contributing factor in triggering morning sickness and hyperemesis gravidarum.

The hormonal shifts described above likely explain why smell perception warps so dramatically. Estrogen in particular is known to influence the olfactory system, and when estrogen levels climb sharply in the first trimester, it may amplify how the brain processes scent signals. For many women, the combination of a sensitized nose and a GDF15-primed nausea center means that a whiff of cooking meat or coffee can trigger a wave of nausea before the conscious mind has even registered what the smell is.

The Evolutionary Angle

Morning sickness looks like a design flaw, but a significant body of research argues it’s actually a feature. The idea, sometimes called the “maternal and embryo protection hypothesis,” is that nausea steers pregnant women away from foods most likely to carry dangerous microorganisms or natural plant toxins during the window when embryonic organ development is most vulnerable.

Two thorough reviews of the evidence support this interpretation. One, published in the American Journal of Obstetrics and Gynecology, concluded that the strongest evidence points toward nausea and vomiting protecting the pregnant woman and embryo from harmful substances in food, particularly pathogens in meat and toxins in pungent plants.⁶ An earlier, comprehensive review in the Quarterly Review of Biology found multiple lines of supporting evidence: symptoms peak during weeks six through eighteen, precisely when organogenesis is most susceptible to chemical disruption; women who experience morning sickness are less likely to miscarry in every study examined; women who vomit have even fewer miscarriages than those who feel only nausea; and food aversions during pregnancy cluster around alcohol, caffeinated drinks, and strongly flavored vegetables.⁷

A secondary analysis of a randomized clinical trial put numbers on the miscarriage link. Among women with a history of prior pregnancy losses, those who experienced nausea alone in a given week had roughly half the risk of losing the pregnancy that week, and those with nausea plus vomiting had about a quarter of the risk, compared with women who had neither symptom.⁸ The association held even after excluding very early losses, suggesting it isn’t simply that healthier pregnancies produce more hCG and therefore more nausea. This doesn’t mean that a woman who sails through pregnancy without nausea should worry; plenty of healthy pregnancies produce minimal symptoms. But the population-level pattern is consistent enough across studies that researchers treat it as real.

When Morning Sickness Becomes Hyperemesis Gravidarum

Ordinary morning sickness is uncomfortable but manageable. At the far end of the spectrum sits hyperemesis gravidarum, or HG, a condition involving relentless vomiting, dehydration, weight loss, and sometimes hospitalization. Despite how debilitating it can be, there is no universally agreed-upon set of diagnostic criteria. A systematic review found that definitions of HG vary across studies, with some relying on symptoms alone and others combining symptoms with lab markers, making research and clinical recognition inconsistent.⁹

HG affects a small percentage of pregnancies, with estimates typically running between one and three percent, but its impact is outsized. It is one of the leading causes of hospitalization in early pregnancy. The GDF15 research discussed earlier helps explain why some women develop HG while others experience only mild queasiness: women whose bodies are especially unaccustomed to GDF15 before pregnancy appear to be hit hardest by the sudden placental flood of the hormone.

Treatments That Actually Help

Most women manage mild morning sickness with lifestyle adjustments like eating small meals, avoiding triggers, and staying hydrated. When symptoms are persistent enough to interfere with daily life, several treatments have evidence behind them.

Ginger

Ginger is the most studied non-pharmaceutical remedy for pregnancy nausea. A Cochrane-style review found that in the first trimester, ginger could improve nausea and vomiting by about four points on a forty-point symptom scale and stopped vomiting entirely for about one in three women within six days.¹⁰ A randomized, placebo-controlled trial found that ginger reduced vomiting by about half and nausea by just under half, significantly outperforming placebo.¹¹ Ginger is generally considered safe at doses under one gram per day, and it’s available as capsules, teas, or candies. It won’t solve severe cases, but for mild to moderate symptoms, it’s a reasonable first step.

Vitamin B6 and Doxylamine

Vitamin B6 (pyridoxine) has been used against morning sickness for decades. A trial comparing it head-to-head with ginger found both produced significant symptom reductions of similar magnitude.¹² The active form that appears to do the work in the body is pyridoxal-5′-phosphate (PLP). Research suggests that the commonly taken forms of B6 are converted into PLP before they exert their antiemetic effect, and steady PLP levels correlate with symptom improvement.¹³

When B6 alone isn’t enough, the combination of doxylamine (an antihistamine) with pyridoxine is the most commonly prescribed medication for pregnancy nausea in many countries. Large epidemiological studies have shown it poses no increased risk of birth defects.¹⁴ In a randomized placebo-controlled trial, serious adverse event rates were essentially the same in the treatment and placebo groups (about three to four percent in each), and the combination was well tolerated at the recommended dose of up to four tablets daily.¹⁵ The most common side effect is drowsiness, which is why many women take it at bedtime.

For women with hyperemesis gravidarum who cannot keep pills down, treatment escalates to intravenous fluids, ondansetron (an anti-nausea drug originally developed for chemotherapy patients), and sometimes corticosteroids. These carry more complex risk-benefit profiles and are managed in close consultation with a physician.

The Psychological Toll of Severe Symptoms

Morning sickness is often treated casually in popular culture, as a punchline or a rite of passage. For women with hyperemesis gravidarum, the reality is far grimmer. A prospective case-control study found that women with HG were roughly five and a half times more likely to meet criteria for depression and nearly seven times more likely to have an anxiety disorder compared with pregnant women who did not have HG.¹⁶

The effects extend beyond mood. Research has shown that women with hyperemesis gravidarum report lower acceptance of pregnancy and lower acceptance of the role of motherhood during the period of illness, and their overall quality of life drops significantly.¹⁷ A separate comparative study confirmed that this reduced quality of life feeds back into lower pregnancy acceptance, creating a cycle that can strain the mother’s emotional bond with the pregnancy itself.¹⁸ Some women with HG have described feeling suicidal or choosing to terminate otherwise wanted pregnancies because the suffering was unbearable. These outcomes are a strong argument for taking severe pregnancy nausea seriously rather than dismissing it as normal discomfort.

Does Severe Morning Sickness Affect the Baby Long-Term?

Most children born after pregnancies complicated by morning sickness, even moderate cases, are perfectly healthy. But research into hyperemesis gravidarum specifically has raised some flags worth knowing about, even if the findings require careful interpretation.

A systematic review and meta-analysis found that children born to mothers with HG had higher odds of anxiety disorders and sleep problems. All six studies in the review that looked at attention-related and autism spectrum conditions reported an increase among children of mothers with HG, though the data were too variable to produce a single reliable pooled estimate.¹⁹ A separate study found that children born after HG-affected pregnancies had modestly lower insulin sensitivity, which could theoretically raise their lifetime risk of metabolic disease.²⁰

However, a large population-based study using a sibling design offers an important counterpoint. It found that while children born after HG pregnancies initially appeared to have slightly higher rates of hospitalization and slightly lower academic scores in reading, spelling, and numeracy, most of these associations disappeared when the researchers compared siblings from the same mother, some born after HG pregnancies and some not.²¹ That pattern suggests that the apparent risks may be driven less by HG itself and more by underlying family-level factors like genetics, socioeconomic conditions, or chronic health issues shared across pregnancies. The take-home is that HG is not a known cause of developmental harm to the child, even if some studies find statistical associations.

Morning Sickness in Other Species

Humans are not the only animals that seem to experience pregnancy-related changes in appetite and food intake. A 2025 preprint examined marmosets and mice during pregnancy and found suggestive parallels. About one in five marmoset pregnancies showed a transient drop in body weight during the phase of placental development, and some individual marmosets experienced this weight loss repeatedly across pregnancies, hinting at individual variation much like in humans. Mice did not lose weight but did slow their food intake and change their activity patterns during the equivalent gestational window.²² These findings are preliminary, but they matter because having reliable animal models would let researchers test interventions and study mechanisms in ways that are impossible in human pregnancies. If marmosets experience something functionally similar to human morning sickness, driven by the same GDF15 pathway, it could accelerate the development of treatments considerably.