Research out of Copenhagen found that infant girls exposed to Tylenol in the womb showed sharply smaller ovarian and uterine measurements, raising fresh questions about the most common painkiller recommended during pregnancy.
A peer-reviewed study published Wednesday in Human Reproduction Open examined 302 three-month-old female infants and found that those whose mothers took acetaminophen, sold in the U.S. as Tylenol, during pregnancy had, on average, 40 percent smaller ovarian volume, 13 percent smaller uterine volume, and 23 percent fewer ovarian follicles than infants born to mothers who did not take the drug. The findings, reported by The Independent, mark the first time researchers have documented such associations in humans.
Lead author Margit Bistrup Fischer of Rigshospitalet in Copenhagen said women who took Tylenol during pregnancy should not be alarmed, noting that many exposed mothers had daughters with ovarian dimensions similar to those of unexposed infants. But the numbers are hard to wave away, and a separate, longer-term cohort of more than 1,200 girls tracked from infancy through adolescence showed the same pattern holding years later.
Fischer's team divided the primary cohort into three groups: 92 infants first exposed to Tylenol before 17 weeks of gestation, 67 exposed after 17 weeks, and 143 born to mothers who never took the drug. Tylenol levels were measured through periodic urine samples collected from the mothers during pregnancy. None of the mothers exceeded the recommended daily maximum of 4,000 milligrams.
Infants exposed earlier, before 17 weeks, showed lower concentrations of Anti-Müllerian hormone, a marker that measures egg quantity and quality. That detail matters because it suggests the timing of exposure, not just the fact of it, may shape the severity of the effect.
The secondary cohort added weight. Among 1,210 girls followed from infancy into adolescence, prenatal Tylenol exposure was linked to smaller uteruses at puberty and smaller ovaries during adolescence. Animal studies had previously pointed in this direction, suggesting acetaminophen could alter egg reserves and later reproductive health. Until now, no comparable human data existed.
Dr. Christian De Geyter, a reproductive medicine specialist at the University Hospital of Basel in Switzerland, wrote an accompanying commentary to the study. He said the findings align well with existing animal data and called for long-term tracking of girls exposed to Tylenol in utero, ideally through menopause.
De Geyter went further. He said current recommendations regarding Tylenol use during pregnancy ought to be reconsidered. That is a notable statement. Medical guidelines today continue to list acetaminophen as a safe option for treating pain and fever during pregnancy. It remains the go-to recommendation from doctors when expectant mothers ask what they can take for a headache or a fever.
Fischer struck a more cautious tone, noting that the study cannot forecast outcomes for individual mothers or children. She also pointed out that leaving severe pain or high fever untreated during pregnancy presents its own health risks. Determining whether the physical variations her team observed will affect adult fertility or the timing of menopause, she said, will require extended long-term evaluation.
Tylenol is now manufactured by Kenvue, a company that spun off from Johnson & Johnson in 2023. The study did not name Kenvue or any manufacturer as a party, and no response from the company appeared in the reporting.
President Donald Trump has previously linked Tylenol use in pregnancy to autism. The medical community and leading health authorities have widely rejected that specific claim as lacking scientific proof. The Copenhagen study does not address autism. It focuses narrowly on reproductive organ development, a different question, grounded in measurable physical differences documented in two separate cohorts across two stages of life.
Several open questions remain. The study does not detail whether the 1,210 girls in the secondary cohort came from the same population as the primary group of 302 infants or a different one. It does not specify the conditions, headaches, musculoskeletal pain, fever, that prompted mothers to reach for Tylenol, nor does it report average doses. Whether the researchers controlled for confounding variables, such as the underlying conditions that led mothers to take the drug in the first place, is not addressed in the available reporting.
Those gaps matter. A woman who takes Tylenol for a high fever may differ in medically important ways from a woman who never needed it. Correlation is not causation, and Fischer herself acknowledged as much by urging calm.
But a 40 percent reduction in ovarian volume is not a rounding error. And when a second, larger group of girls shows the same trend persisting into adolescence, the word "coincidence" starts to lose its force.
Millions of pregnant women take Tylenol every year on the advice of their doctors. If the guidelines are wrong, the people who deserve answers first are the mothers who followed them.
