Study Finds Maternal Tylenol Use Associated With Smaller Ovaries And Uteruses In Infant Daughters — Long‑Term Effects Unknown

New research suggests that maternal use of acetaminophen (Tylenol/paracetamol) during pregnancy was associated with measurable differences in the reproductive organs of infant daughters, although the study's authors emphasize that their findings do not prove the drug caused those changes and say the long‑term implications remain unknown.

The Copenhagen team, led by Margit Bistrup Fischer of Rigshospitalet, examined 302 three‑month‑old girls whose mothers’ acetaminophen use was tracked through pregnancy. Ninety‑two infants were first exposed in utero before 17 weeks’ gestation, 67 were first exposed after 17 weeks, and 143 were born to mothers who reported no use of the drug during pregnancy.

Exposure was assessed using urine samples provided by the mothers at regular intervals. Reported dosages were relatively low and none of the women exceeded the recommended daily maximum of 4,000 milligrams. Most women took the medication for headaches or musculoskeletal pain.

Main Findings: On average, infants exposed in utero had smaller ovaries, fewer ovarian follicles (the structures that contain immature eggs), smaller uteri and lower circulating levels of certain reproductive hormones. Specifically, the researchers reported a 40% smaller ovarian volume, a 13% smaller uterine volume and 23% fewer ovarian follicles in exposed infants compared with unexposed infants. Those first exposed before 17 weeks also showed lower levels of Anti‑Müllerian hormone (AMH), a marker commonly used to estimate ovarian egg quantity and quality.

The investigators also analyzed a separate cohort of 1,210 girls followed from infancy into adolescence whose mothers reported acetaminophen use during pregnancy. In that group, fetal exposure was associated with smaller uterine size at puberty and smaller ovarian size during adolescence.

Interpretation and Limitations: The study is observational and cannot establish causation. Fischer and colleagues note that many daughters of women who used acetaminophen in pregnancy had ovarian measurements similar to those of unexposed peers. The authors call for long‑term follow‑up — ideally into adulthood and menopause — to determine whether these early structural differences affect fertility or age at menopause.

Animal studies have previously suggested that fetal acetaminophen exposure may affect ovarian reserve and later reproductive function, and an accompanying commentary by Dr. Christian De Geyter (University Hospital of Basel) says the new human data are consistent with animal findings. De Geyter recommends extended follow‑up and suggests clinical guidance on acetaminophen use in pregnancy may need re‑evaluation in light of accumulating evidence.

Practical Context: Current medical guidelines continue to recommend acetaminophen as the first‑line option to treat pain and fever in pregnancy because untreated high fever or severe pain can pose risks to both mother and fetus. The researchers urge pregnant women not to be alarmed but to consult their health care providers about symptoms and medication choices.

Additional Note: The report mentions that U.S. President Donald Trump has previously linked prenatal acetaminophen use to autism; major health organizations and the medical community have dismissed that specific claim as lacking scientific evidence.

What To Take Away

• The study found associations between prenatal acetaminophen exposure and smaller ovarian and uterine measurements in infant girls, but it did not prove a causal link.
• Early pregnancy exposure (before 17 weeks) was linked to lower AMH levels, a marker of ovarian reserve.
• Longer follow‑up into adolescence and menopause is needed to understand whether these differences affect fertility or menopause timing.
• Current guidance still supports acetaminophen for pregnant women when indicated; patients should discuss treatment with their clinicians.