A new study has found that baby girls exposed to paracetamol during pregnancy had, on average, smaller ovaries and uteruses and fewer ovarian follicles than those whose mothers did not take the painkiller.
Researchers stressed that the findings do not prove paracetamol caused the differences, or that girls exposed to the medicine will have fertility problems later in life.
The Copenhagen study, published in Human Reproduction Open, examined 302 three-month-old girls whose mothers’ paracetamol use during pregnancy had been monitored through regular urine samples.
Girls exposed to paracetamol in the womb had, on average, 40 per cent smaller ovaries, 13 per cent smaller uteruses and 23 per cent fewer ovarian follicles.
Those exposed before 17 weeks of pregnancy also had lower levels of anti-Müllerian hormone (AMH), which is commonly used as an indicator of ovarian reserve.
The mothers generally took relatively small doses, mainly for headaches or musculoskeletal pain, and none exceeded the recommended daily maximum of 4,000 milligrammes.
Researchers also looked at 1,210 girls followed from infancy into adolescence. Prenatal exposure was associated with smaller uteruses around puberty and smaller ovaries during adolescence.
However, it remains unclear whether these differences have any effect on fertility later in life.
“Whether the differences observed in the study of infants have implications for fertility later in life and age at menopause will require long-term follow-up,” study leader Margit Bistrup Fischer said.
She urged women who took paracetamol during pregnancy not to be alarmed, noting that the study cannot predict outcomes for individual women or children.
Paracetamol, known as acetaminophen in the U.S. and sold as Tylenol, is widely used to treat pain and fever during pregnancy.
Animal studies have previously raised concerns that exposure before birth could affect ovarian development and egg reserves. The new research is among the first to report similar physical differences in humans, although it does not establish cause and effect.
Researchers say longer-term studies are needed to determine whether the changes persist and whether they affect fertility, reproductive lifespan or the timing of menopause.
Current guidance continues to consider paracetamol an option for treating pain and fever during pregnancy. Researchers also warn that untreated high fever or severe pain can pose risks to the mother and fetus.
Dr. Christian De Geyter, a reproductive medicine specialist at the University Hospital of Basel, said the findings were consistent with animal research and called for exposed girls to be followed into adulthood, potentially through menopause.
He also said the findings should prompt a reassessment of recommendations on paracetamol use during pregnancy.
The study comes amid renewed debate over paracetamol use during pregnancy after U.S. President Donald Trump linked Tylenol to autism - a claim widely rejected by medical experts and health organisations as unsupported by sufficient evidence.
The Copenhagen study did not examine autism. Its findings instead point to differences in reproductive development that researchers say require further investigation.
For now, the study is a signal for more research, rather than evidence that pregnant women should stop taking paracetamol.
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