Common Pregnancy Pain Reliever Linked to Smaller Ovaries and Uterus in Baby Girls, Study Finds

For more than half of pregnant women, Tylenol is the go-to fix for headaches, fevers, and body aches, the one painkiller doctors have long considered the safe choice. But a new study out of Denmark is asking an uncomfortable question: what if “safe” isn’t the whole story for baby girls?

Researchers tracked pregnant women and their daughters from the first trimester through infancy, publishing their results in the journal Human Reproduction Open. They found that babies exposed to paracetamol (known as acetaminophen in the United States and sold under brand names like Tylenol) in the womb had smaller ovaries, fewer egg-containing structures called follicles, altered hormone levels, and a smaller uterus, with the specific effect depending on when during pregnancy the exposure happened. These differences showed up even at mild to moderate doses, the kind of everyday use that rarely raises a second thought.

These weren’t findings from a single group of babies. Scientists checked for similar patterns in a completely separate group of girls followed from birth into their teenage years, and saw some of the same signals: smaller uteruses at puberty and smaller ovaries in adolescence among girls whose mothers used paracetamol during pregnancy. Animal studies have pointed to these effects for years. This appears to be the first human study built specifically to test whether that link, well established in rodents, also shows up in people.

How Researchers Tracked Paracetamol Exposure in Pregnancy

COPANA, as the study is called, followed 685 healthy pregnant women in Copenhagen from their first trimester onward. Every two weeks, the women reported which mild painkillers they’d used. Researchers also tested their urine for actual traces of the drug, so the study had both a self-reported record and a lab-verified one.

From that group, 302 infant daughters were examined around three months old, during a stretch of infancy pediatric researchers call “minipuberty.” For a few weeks, a baby girl’s hormone system briefly switches on in a way that resembles adult reproductive activity, giving doctors a rare early look at how her ovaries are working. Researchers used ultrasound to measure the girls’ ovaries and uterus, counted visible follicles, and drew blood to check hormone levels, including one called AMH, a marker of ovarian activity that also reflects a woman’s egg reserve.

Researchers split the girls into three groups: those whose mothers used paracetamol before 17 weeks of pregnancy, those exposed later, and those never exposed. About half the mothers had used the drug at some point, most often for headaches or migraines rather than fever or illness.

Timing seemed to matter. Girls exposed early in pregnancy had smaller ovaries and uteruses than unexposed girls. Girls exposed later instead had fewer follicles, the small fluid-filled sacs where eggs mature. Girls exposed only during that early window also had lower AMH levels, the hormone tied to egg reserve.

That pattern held beyond simple yes-or-no comparisons. Mothers with higher paracetamol levels in their urine tended to have daughters with smaller ovaries, smaller uteruses, and smaller breast tissue, pointing to a dose-related pattern rather than an all-or-nothing one. Even so, the doses mothers reported were generally modest, not the kind associated with heavy or long-term use.

To see whether these early differences lasted, researchers turned to an older, separate group called the Copenhagen Mother-Child Cohort, which had followed more than 1,200 girls from birth into their teens for unrelated reasons. Mothers in that study had only been asked once during pregnancy whether they had taken any medication, with paracetamol offered as one example, and the exposed group ultimately included mothers who reported using paracetamol and/or other mild anti-inflammatory painkillers at any point in pregnancy. Even with that much broader and rougher measure of exposure, girls in that group had smaller uteruses at puberty and smaller ovaries in adolescence, along with lower levels of a hormone called inhibin B, another marker of ovarian function.

Why Paracetamol in Pregnancy Might Affect Ovary Development

None of this surprises scientists who study reproduction. Animal studies have shown for years that fetal exposure to paracetamol disrupts ovary development in female offspring, cutting their supply of egg-containing follicles and shortening their reproductive lifespan. Researchers suspect the drug interferes with the rapid cell division that has to happen correctly for a fetus to build up its lifetime egg supply. Females are born with every egg they’ll ever have, and that number only goes down from there. Paracetamol is known to block certain enzymes involved in that process, and aspirin and other anti-inflammatory drugs have shown similar effects in past research. This human study, though, wasn’t built to pin down exactly how the drug produces the differences it found.

During minipuberty, a baby girl’s ovaries send out estrogen-like signals. The study’s authors suggest that if the ovaries are less active during that window, it could explain why exposed girls also had smaller breast tissue and uterus size, both of which respond directly to those hormone signals.

One detail stood out: paracetamol showed up in every single urine sample tested, including from mothers who said they hadn’t taken the drug. The authors think this comes from low-level environmental exposure. A chemical called aniline, found in various industrial products, breaks down into paracetamol inside the body. That raises the possibility that some of the paracetamol showing up in urine comes from sources other than taking the medication itself.

What the Findings Don’t Prove

Because this was an observational study rather than a controlled experiment, it can’t prove paracetamol directly caused the smaller ovaries and uteruses researchers saw. Whatever prompted a mother to take the drug in the first place, whether that was pain or fever, could theoretically be the real driver. But researchers specifically tested what happened when they excluded mothers who took the drug for fever, and the results didn’t change. Most women in the study used it for headaches or migraines, not illness, which makes it harder to argue that sickness itself explains the pattern.

Some results also didn’t turn up consistently in both groups of girls, and the researchers acknowledge that infant ovaries are notoriously small and hard to measure by ultrasound. That means some findings need to be repeated before anyone treats them as settled. Nobody yet knows whether smaller ovaries or fewer follicles in infancy actually lead to fertility problems or earlier menopause decades down the line. Answering that would mean following these girls for another twenty or thirty years.

Still, it’s hard to dismiss a well-designed human study that lines up so closely with nearly a decade of animal research. Paracetamol remains one of the only pain and fever medications considered appropriate during pregnancy, and there are real situations, like a dangerously high fever, where taking it is the safer option for both mother and baby. One detail the authors flag directly gives the study its weight: the exposure levels they measured fell within current pregnancy guidelines, and the differences still turned up. That is why they argue current guidance may not go far enough on how much of this drug to take and for how long, rather than treating an over-the-counter label as proof of no effect at all.

Disclaimer: This article summarizes findings from a single observational study and is intended for general informational purposes only. It is not medical advice. Observational research can identify associations but cannot prove that one thing causes another. Paracetamol (acetaminophen) remains one of the few pain and fever relievers considered appropriate during pregnancy, and in some situations, such as a high fever, treating it carries its own health benefits. Anyone with questions about pain relief or medication use during pregnancy should consult a doctor or qualified healthcare provider before making changes.

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