If you’ve ever felt queasy at the smell of coffee or turned away from a burger you once loved, you’re not alone. According to the National Institutes of Health, up to 80% of pregnant people experience nausea, vomiting, and food or smell aversions in early pregnancy. And according to a 2025 UCLA study published in the journal Evolution, Medicine and Public Health, those uncomfortable symptoms might actually be your body’s way of protecting both you and your baby.

For many, morning sickness can feel confusing or even isolating—especially when you’re not sure when morning sickness starts or how long it will last. But researchers say it may actually be a sign of a healthy, finely tuned immune system working behind the scenes. Rather than signaling trouble, your body may be sending helpful warnings designed to keep you—and your developing baby—safe.

Related: When does morning sickness start?

What causes morning sickness?

The exact cause of morning sickness isn’t fully understood, but scientists believe it’s tied to the dramatic hormonal and immunological shifts that happen in early pregnancy. Rising levels of human chorionic gonadotropin (hCG), estrogen, and progesterone all play a role—as do changes in your immune system that are designed to keep your body from rejecting the developing fetus.

The 2025 UCLA study, led by anthropology professor Molly Fox and colleagues Daniel Fessler and Dayoon Kwon, followed 58 pregnant women in Southern California, collecting blood samples and tracking symptoms such as nausea, vomiting, and aversions to certain foods and smells.

They measured immune proteins called cytokines—molecules that regulate inflammation and immune responses—and found that women experiencing nausea or food aversions showed a shift toward a pro-inflammatory immune response. In plain language, their bodies were actively protecting themselves while also preventing an immune attack on the fetus—a delicate act of biological diplomacy.

Nausea and aversions may even encourage pregnant people to avoid foods or substances that could be harmful during the early, most vulnerable stages of fetal development. While the findings are correlational and more research is needed, the pattern supports a long-standing theory that pregnancy nausea is a protective adaptation.

Is morning sickness a good sign?

It might be. While morning sickness definitely doesn’t feel like a positive development when you’re in the thick of it, research suggests that experiencing nausea during early pregnancy is associated with a lower risk of miscarriage. The UCLA study’s findings add to that picture: the inflammatory immune shifts linked to nausea appear to be markers of a pregnancy in which the immune system is successfully adjusting to protect the fetus.

That said, not everyone gets morning sickness—and that’s completely fine. Roughly 20 to 30% of pregnant people have little to no nausea and go on to have perfectly healthy pregnancies. The absence of nausea is not a reason to worry. And conversely, severe or persistent nausea that keeps you from eating or drinking may be a sign of hyperemesis gravidarum, which requires medical attention.

Related: 4 lies I believed about morning sickness—before I had it myself

The evolutionary perspective: why your body makes you nauseous

Humans have one of the most invasive placentas of any species, meaning fetal cells are bathed in maternal blood. This intimacy requires special immune strategies to prevent the body from rejecting its own developing baby.

Scientists believe that morning sickness may be one such adaptation: a behavioral mechanism shaped by natural selection to minimize exposure to harmful foods or environmental toxins. As early as 2000, Cornell University evolutionary biologists found that nausea and food aversions peak precisely when embryonic organ development is most susceptible to disruption—between weeks 6 and 18 of pregnancy—and that pregnant people who experience morning sickness are significantly less likely to miscarry.

Odors and tastes that trigger nausea—like tobacco smoke, strong cheeses, or undercooked meat—may not be random irritations. They could reflect evolution’s way of nudging pregnant people away from potential dangers, a survival strategy written into our biology.

Practical takeaways for parents

Reframing morning sickness as meaningful can help reduce stigma and guilt. Morning sickness is your body’s built-in shield, actively working to protect new life. Recognizing this can empower pregnant people to:

  • Advocate for accommodations at work. While the UCLA study didn’t directly test workplace outcomes, understanding that morning sickness has biological underpinnings could support requests for flexible schedules, remote work options, or more frequent breaks.
  • Communicate with healthcare providers. Knowing that symptoms are often normal can guide conversations about safe coping strategies.
  • Practice self-compassion. Your body is performing an extraordinary feat. Acknowledge that it’s okay to slow down when needed.

If you’re looking for concrete relief strategies, read 12 steadying strategies for pregnancy nausea that actually help. And for a candid look at what first-trimester nausea actually feels like day to day, check out one mom’s honest account of surviving morning sickness in the first trimester.

Related: It’s science: Probiotics in early pregnancy may reduce morning sickness

Why it matters

Each bout of morning sickness is a sign of the remarkable resilience and protection your body provides. Seeing nausea and aversions as part of a larger story of evolutionary and immunological strategy can help you feel validated, empowered, and connected to your own body’s work.

The next time you turn away from a smell or feel queasy in the morning, remember: your body is acting in your and your baby’s best interest—a fascinating testament to the ways humans have evolved to protect new life. Morning sickness may be uncomfortable, but it may also be a sign of care, resilience, and survival written into your biology.

Sources:

  1. Gastroenterology Clinics of North America. 2011.Nausea and Vomiting of Pregnancy
  2. Evolution, Medicine, and Public Health. 2025. “Of scents and cytokines: How olfactory and food aversions relate to nausea and immunomodulation in early pregnancy
  3. The Quarterly Review of Biology. 2000. Flaxman & Sherman, “Morning Sickness: A Mechanism for Protecting Mother and Embryo”

A version of this story was originally published on Sep. 29, 2025. It has been updated.