Prenatal Tylenol Exposure Linked to Variations in Infant Female Reproductive Organs

Female reproductive organ development in infants may be influenced by prenatal exposure to Tylenol, according to new research, though investigators stress that the study does not prove the painkiller is the cause or demonstrate any impact on future fertility.

Margit Bistrup Fischer, lead author at Rigshospitalet in Copenhagen, said determining whether these physical variations in infants affect adult fertility or the timing of menopause will require extended, long-term evaluation.

Fischer noted that women who took Tylenol—also sold as acetaminophen, or paracetamol outside the U.S.—during pregnancy should not be alarmed by the findings.

Her research team evaluated 302 three-month-old female infants whose mothers' Tylenol consumption was recorded throughout pregnancy.

Of the total group, 92 infants were first exposed to Tylenol in utero prior to 17 weeks of gestation, 67 were first exposed after 17 weeks, and 143 were born to mothers who did not take the drug.

Researchers determined Tylenol levels using periodic urine samples provided by the women. The mothers took relatively small amounts, with none exceeding the maximum recommended daily limit of 4,000 milligrams, mostly using the medication for headaches or musculoskeletal discomfort.

On average, infants exposed in utero exhibited smaller ovaries, fewer ovarian follicles—the cellular structures containing immature eggs—smaller uteruses, and reduced levels of reproductive hormones, the team reported Wednesday in Human Reproduction Open.

Researchers explained that a woman's lifelong reproductive capacity is established while still in the womb, as females are born with all the eggs they will ever possess.

While animal studies previously suggested that prenatal exposure to Tylenol—the most widely used medication during pregnancy—might alter egg reserves and later reproductive health, no similar human findings had been recorded until now.

President Donald Trump has linked Tylenol use in pregnancy to autism, a claim widely rejected by the medical community and leading health authorities as lacking scientific proof.

Infants at three months old who were exposed to Tylenol in utero showed, on average, a 40% reduction in ovarian volume, a 13% smaller uterine volume, and 23% fewer ovarian follicles, researchers discovered.

Infants exposed earlier in pregnancy—before 17 weeks—also demonstrated lower concentrations of Anti-Müllerian hormone, a marker measuring egg quantity and quality.

The investigators additionally evaluated a distinct group of 1,210 girls followed from infancy into adolescence whose mothers reported using Tylenol while pregnant. In that cohort, prenatal exposure was linked to smaller uteruses at puberty and smaller ovaries during adolescence.

Fischer highlighted that the study cannot forecast outcomes for individual mothers or children, pointing out that many women who took Tylenol had daughters with ovarian dimensions similar to those born to unexposed mothers.

Medical guidelines currently continue to recommend Tylenol as a safe option for treating pain and fever during pregnancy. Leaving severe pain or high fever untreated presents its own health risks to both mother and fetus, Fischer noted.

In an accompanying commentary, Dr. Christian De Geyter, a reproductive medicine specialist at University Hospital of Basel in Switzerland, said the findings align well with existing animal data.

He noted that long-term tracking of girls exposed to Tylenol in utero should continue through menopause, adding that current recommendations regarding its use during pregnancy ought to be reconsidered.