Two-year-old girls whose mothers took acetaminophen during their pregnancies had higher rates of language delays compared to those whose mothers did not, finds a new study. Similar delays were not seen in boys, who generally develop language skills more slowly than girls.

The study, published in the journal European Psychiatry, included 754 Swedish women who were interviewed between weeks 8 and 13 of their pregnancies. The women answered questions about how often they had used acetaminophen—an over-the-counter pain and fever reliever, sold as Tylenol in the United States—since they had conceived. They also provided urine samples, which were tested for acetaminophen concentration.

The researchers then compared women’s acetaminophen use with their children’s scores on a language-development screening given to all children in Sweden at 30 months, along with the mothers’ personal assessments of their child’s language use. Children who used fewer than 50 words at this age were considered to have a language delay.

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Acetaminophen use during early pregnancy was common, the researchers found: 59% of women reported taking it at least once in their first trimester, and some reported taking up to 100 pills in that time. For their primary analysis, they compared women with high exposure—those who took acetaminophen more than six times since becoming pregnant—with women who reported taking none at all.

Overall, about 10% of children in the study had a language delay at 30 months, with boys having greater delays than girls. But girls born to mothers in the high-acetaminophen group were nearly six times more likely to have language delays than girls whose mothers had used none. The more tablets women reported taking, and the higher the levels detected in their urine, the more likely their daughters were to have language delays.

There was no significant difference in language delays between boys whose moms took acetaminophen and those whose moms did not. In fact, women with higher urine concentrations of acetaminophen during pregnancy were slightly less likely to have sons with language delays.

The researchers can’t say for sure why acetaminophen during pregnancy seems to affect male and female fetuses differently, and they stress that their study was only designed to find associations, not a cause-and-effect relationship.

But they do offer one possible theory. Girls around 30 months tend to have higher vocabularies than boys—a “well-recognized female advantage” in early-childhood language development, they write in their study. Taking acetaminophen during pregnancy, the authors say, may diminish that advantage.

Previous studies on acetaminophen during pregnancy have reported other instances in which prenatal exposure to the drug seems to narrow the gap in gender-specific developmental differences, says senior author Shanna Swan, professor of environmental medicine and public health at the Icahn School of Medicine at Mount Sinai. For example, the distance between the anus and genitals—a measure linked to fertility—should be greater in boys than in girls, she says, but acetaminophen exposure has been associated with shorter distances in boys.

“What it looks like, although we need many more studies to confirm this, is that male-female differences in development are decreased in the presence of acetaminophen in early pregnancy,” says Swan.

The new findings are also consistent with research that has shown a link between mothers who use acetaminophen in pregnancy and children with decreased IQs or increased rates of communication problems. Use of the drug during pregnancy has also been associated with higher rates of ADHD in children.

The Swedish study will continue following the children tested at age 2 and will re-examine their language development at age 7. “A number of studies have shown that children who show poor learning development at 30 months go on to have learning problems and developmental disorders later,” says Swan. “If our findings are confirmed, the implications could be very big—and we could have the opportunity to do something about it.”

Swan says the study results, if they can be replicated, suggest that pregnant women should limit their acetaminophen use. While the study only looked at women through week 13 of their pregnancy, Swan says that the critical window for language development is not known precisely. “It’s probably best to err on the side of caution and minimize exposure as much as possible, until it’s ruled safe in other stages of pregnancy,” she says.

According to the Centers for Disease Control and Prevention, an estimated 65% of pregnant women in the United States use acetaminophen while pregnant. It’s often considered a safe medicine to take during pregnancy, and it’s recommended by doctors for treating fever and pain. (It’s also worth noting that fever itself can be dangerous for pregnant women and unborn babies, and has been linked to birth defects.)

But acetaminophen is also an ingredient in over-the-counter medicines marketed for other ailments, says Swan, and people take it for a variety of other reasons—such as depression, anxiety and trouble sleeping.

“What we want to tell women is, first of all, to consult your physician before taking any medication—over-the-counter or not—during pregnancy,” says Swan. “And second, take as little of it as possible, and only when medically indicated.”

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