- A Danish study of 302 infants found that girls exposed to acetaminophen (Tylenol) in the womb had 40% smaller ovarian volume, 13% smaller uterine volume and 23% fewer ovarian follicles on average
- The research, published in Human Reproduction Open, marks the first human evidence confirming what animal studies had previously suggested about reproductive effects
- Exposure before 17 weeks of pregnancy was linked to lower levels of Anti-Müllerian hormone, a key indicator of egg quantity and quality
- A separate analysis of 1,210 girls followed from infancy to adolescence found smaller uteruses at puberty and smaller ovaries during adolescence associated with prenatal exposure
- Researchers emphasized the study does not prove causation and said women should not be alarmed, while calling for long-term follow-up into menopause and a reconsideration of current guidelines
For decades, acetaminophen—sold under the brand name Tylenol—has been considered the safest pain and fever relief option during pregnancy, recommended by doctors when alternatives like ibuprofen carry their own risks. But a growing body of evidence is challenging that assumption. A Danish study published Sept. 9, 2026, in Human Reproduction Open examined 302 three-month-old girls and found that those exposed to the drug in the womb had, on average, significantly smaller reproductive organs and fewer eggs in reserve—the first human evidence confirming what animal studies had long suggested.
The findings arrive amid wider scrutiny of acetaminophen's safety during pregnancy. More than 90 scientists and public health experts previously called for regulatory reassessment after studies linked prenatal exposure to increased risks of ADHD and autism. With up to 65% of pregnant women reporting use of the drug, the implications reach millions of families annually.
The study: First human evidence of reproductive impact
Researchers at Rigshospitalet in Copenhagen tracked 302 mother-daughter pairs, using questionnaires and urine samples to measure acetaminophen exposure during pregnancy. None of the mothers exceeded the recommended daily maximum of 4,000 milligrams, with most taking the drug for headaches or musculoskeletal pain.
The results were striking. Girls exposed in utero had 40% smaller ovarian volume, 13% smaller uterine volume and 23% fewer ovarian follicles—the structures containing immature eggs. Infants exposed before 17 weeks of pregnancy also showed lower levels of Anti-Müllerian hormone, a marker of egg quantity and quality.
A separate analysis of 1,210 girls followed from infancy to adolescence found that prenatal exposure was associated with smaller uteruses at puberty and smaller ovaries during adolescence, suggesting the effects persist beyond infancy.
Study leader Margit Bistrup Fischer emphasized that the findings do not prove causation and said women should not be alarmed. She noted that many exposed infants had measurements similar to unexposed peers.
Historical context: The shifting safety landscape
Acetaminophen's reputation as the go-to pain reliever in pregnancy dates to the early 2000s, when the U.S. Food and Drug Administration began advising against nonsteroidal anti-inflammatory drugs like ibuprofen after 20 weeks of pregnancy due to risks of fetal kidney damage. With limited options, doctors and patients turned to Tylenol.
But concerns emerged over the past decade. A 2020 study found that infants with the highest acetaminophen levels in umbilical cord blood had twice the risk of ADHD and up to three times the risk of autism. Other research linked the drug to lower testosterone levels in male fetuses and language development delays in girls.
The drug is now classified as an endocrine disruptor, meaning it interferes with hormone signaling critical for brain and reproductive development. A woman's reproductive function is established entirely in the womb—girls are born with all the eggs they will ever have. Any disruption during this sensitive window could have lifelong consequences.
What this means for pregnant women
Untreated high fever during pregnancy carries its own risks, including neurological effects in offspring. Chronic pain can lead to anxiety, depression and high blood pressure. The dilemma facing pregnant women is real.
Current guidelines continue to recommend acetaminophen as safe, but researchers increasingly call for updated warnings. Dr. Christian De Geyter, a reproductive medicine specialist at the University Hospital of Basel, Switzerland, wrote in an accompanying commentary that the findings fit well with animal data and that recommendations "should be reconsidered."
Kenvue, which owns the Tylenol brand, said the study had methodology limitations and does not support a causal relationship. The company advised pregnant women to consult health professionals before use.
A call for caution, not panic
The study does not answer whether these physical changes affect future fertility or age at menopause. Researchers called for long-term follow-up extending into menopause before drawing conclusions about reproductive outcomes.
But the pattern is clear: the most commonly used drug in pregnancy is facing unprecedented scrutiny. Animal studies, epidemiological data and now human physiological evidence converge on a common theme of caution. Until regulatory agencies issue updated guidance, the prudent path for expectant mothers may be to use acetaminophen at the lowest effective dose for the shortest duration necessary, reserving it for high fevers and severe pain rather than routine discomfort. The burden of proof, once assumed to favor the drug's safety, has now shifted.
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