A new study found smaller ovaries and uteruses, and fewer ovarian follicles, in infant girls exposed to Tylenol before birth, but researchers stress that the findings do not prove causation.
By MediaBites News Desk
A commonly used painkiller taken during pregnancy may be associated with differences in reproductive organ development in baby girls, according to a new study that researchers say raises questions requiring years of follow-up.
The study examined 302 three-month-old girls whose mothers’ use of Tylenol (acetaminophen or paracetamol) had been monitored during pregnancy.
Researchers found that girls exposed to the drug in the womb had, on average, 40% smaller ovarian volume, 13% smaller uterine volume and 23% fewer ovarian follicles than girls whose mothers had not reported using the medication.
The findings were published Wednesday in Human Reproduction Open.
Researchers emphasized that the observational study does not establish that Tylenol caused the differences. It also cannot determine whether the measurements will translate into reduced fertility, earlier menopause or other reproductive problems 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 of Rigshospitalet in Copenhagen said.
The researchers assessed Tylenol exposure using urine samples collected from pregnant women at regular intervals. Most women in the study took relatively low amounts of the drug to treat headaches or musculoskeletal pain, and none exceeded the recommended daily maximum of 4,000 milligrams.
The study divided the infants into three groups: 92 whose mothers first used Tylenol before 17 weeks of pregnancy, 67 whose mothers first used it after 17 weeks, and 143 whose mothers did not use the drug.
Girls exposed earlier in pregnancy also had lower levels of anti-Müllerian hormone, a marker associated with the ovarian egg reserve.
The researchers noted that female reproductive development begins before birth and that girls are born with the eggs they will carry throughout their lives. Earlier animal research has similarly suggested that fetal exposure to acetaminophen could affect ovarian egg reserves and later reproductive function.
A separate group of 1,210 girls followed from infancy into adolescence provided additional evidence. In that group, prenatal Tylenol exposure was associated with smaller uteruses at puberty and smaller ovaries during adolescence.
But researchers cautioned against interpreting the results as a prediction for any individual child. Many women who took Tylenol during pregnancy had daughters whose reproductive-organ measurements were similar to those of girls whose mothers did not use the drug.
Current guidelines continue to consider Tylenol an option for treating pain and fever during pregnancy. Researchers also noted that untreated high fever or severe pain can themselves pose risks to pregnant women and fetuses.
The findings come amid wider debate over Tylenol and pregnancy. U.S. President Donald Trump has previously linked prenatal Tylenol use to autism, a claim that has been widely rejected by medical experts and major health organizations as lacking sufficient scientific evidence.
In an accompanying commentary, reproductive medicine specialist Dr. Christian De Geyter said the findings are consistent with animal research and argued that girls exposed to Tylenol before birth should be followed into adulthood — potentially through menopause — to determine whether the early differences have lasting consequences.
For now, the biggest unanswered question is not whether Tylenol caused the changes, but whether the differences observed in infancy matter decades later.
