Study Links Prenatal Paracetamol Exposure to Shorter Anogenital Distance in Male Infants

A Cambridge study found that boys exposed to paracetamol during early pregnancy had shorter anogenital distances, a measurement associated with male reproductive development.

The difference was observed among infants whose mothers reported taking the medication between weeks 8 and 14 of pregnancy. In the United States, paracetamol is known as acetaminophen, the active ingredient in Tylenol.

The findings have renewed relevance amid wider discussions about acetaminophen use during pregnancy, although the study did not establish that the medication caused the observed difference.

Researchers identified a measurable association in male infants, but its implications for reproductive health later in life remain unknown.

What the Cambridge Study Found

The main finding concerned anogenital distance, not penis size or other visible genital abnormalities.

Published online in September 2016 and subsequently included in the November issue of Human Reproduction, the study examined whether paracetamol use during pregnancy was associated with differences in male genital development.

Anogenital distance, or AGD, measures the space between the anus and genitals. Researchers use it as a marker of androgen activity during fetal development.

The study’s authors summarized their findings:

“Intrauterine paracetamol exposure during 8–14 weeks of gestation is associated with shorter AGD from birth to 24 months of age.”

The researchers also examined penile length and testicular descent but found no significant association between paracetamol exposure and either measurement.

Related: Does Penis Size Really Matter? Asking for 50 Million Men

How Researchers Reached Their Findings

The investigation involved hundreds of male infants, though a smaller group drove its main finding.

The Cambridge Baby Growth Study recruited 2,229 pregnant women in the United Kingdom between 2001 and 2009.

Researchers analyzed genital development in 681 boys whose mothers completed questionnaires about medications taken during pregnancy.

Of those boys, 225 had reported prenatal paracetamol exposure, representing approximately 33% of the group.

Researchers collected anogenital distance measurements for 434 boys because they introduced this assessment after the study had already begun.

Among the broader group of 681 boys, 68 had reported exposure during weeks 8 through 14. Only 47 had AGD measurements available for the key analysis.

Researchers collected developmental measurements from birth through approximately 24 months, allowing them to examine whether the observed difference remained consistent during infancy.

Why the Timing of Exposure Mattered

The association was strongest during weeks 8 through 14, a period involved in male reproductive development.

Scientists describe this stage as the masculinization programming window, when androgen activity helps guide the development of male reproductive structures.

In the Cambridge study, boys with reported exposure during this period had anogenital distances approximately 0.27 standard deviations shorter than the comparison group.

The result was statistically significant, with a reported P value of 0.014.

Researchers did not identify a statistically significant association with exposure before week 8 or after week 14.

The finding does not mean the infants experienced a 27% reduction in anogenital distance. The figure describes a standardized statistical difference between groups.

What Remains Unanswered

The study could not determine whether the difference would affect reproductive health in adulthood.

Researchers followed the children through their first two years, but they did not examine adult fertility, sperm production, or sexual function.

The findings also depended on mothers recalling their medication use during pregnancy, sometimes months after taking it.

Other exposures could have influenced the results. As the researchers acknowledged:

“Confounding by other drugs or endocrine-disrupting chemicals cannot be discounted.”

The study did not identify a paracetamol dose proven to cause developmental harm.

These limitations mean the reported association cannot be treated as evidence that acetaminophen causes reproductive problems.

Where the Findings Fit Into the Tylenol Debate

The Cambridge study investigated male reproductive development, a separate issue from the autism and ADHD concerns that have dominated recent acetaminophen discussions.

In September 2025, the U.S. Food and Drug Administration announced an acetaminophen labeling action concerning a possible association between prenatal exposure and neurodevelopmental conditions.

The American College of Obstetricians and Gynecologists has continued to identify acetaminophen as an appropriate option for managing pain and fever during pregnancy when used judiciously in consultation with a medical professional.

The Cambridge findings do not establish that pregnant women should stop taking acetaminophen. They add another scientific question to a discussion already shaped by uncertainty about prenatal medication exposure.

What concerns you most about taking common medications during pregnancy?

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