Pregnant women should think twice before taking Tylenol, a new study warns. Researchers from the University of Illinois Urbana-Champaign find that using acetaminophen, a common over-the-counter pain reliever and fever reducer, during pregnancy could possibly lead to language development delays in early childhood.

Acetaminophen, better known by the brand name Tylenol, is widely regarded as the safest pain relief option during pregnancy. Between 50 and 65 percent of women in North America and Europe use the drug.

The study builds upon previous research that associated acetaminophen use during pregnancy with poorer child communication skills. However, this new study employed more precise methods of measuring language development.

“The previous studies had only asked pregnant people at most once a trimester about their acetaminophen use,” says study author Megan Woodbury, who conducted the research as a graduate student at the University of Illinois Urbana-Champaign, in a university release. “But with IKIDS, we talked to our participants every four to six weeks during pregnancy and then within 24 hours of the kid’s birth, so we had six time points during pregnancy.”

Researchers engaged with participants regularly throughout their pregnancy and shortly after birth, providing a detailed timeline of acetaminophen use. They focused on language development in children at ages two and three, utilizing the MacArthur-Bates Communicative Development Inventories for assessing vocabulary, language complexity, and the length of children’s utterances.

The study found that increased acetaminophen use, particularly during the third trimester, correlated with lower vocabulary scores and shorter average utterance lengths in two-year-olds. At age three, greater use during the third trimester was linked to parents perceiving their children’s language abilities as lower than their peers, especially in male children. One notable result was that each instance of acetaminophen use in the third trimester was associated with an almost two-word reduction in vocabulary in two-year-olds.

“This suggests that if a pregnant person took acetaminophen 13 times – or once per week – during the third trimester of that pregnancy, their child might express 26 fewer words at age 2 than other children that age,” notes Woodbury, now a postdoctoral researcher at Northeastern University in Boston.

Researchers stress the importance of this period for fetal brain development, noting the critical development of hearing in the second trimester and the initiation of language development in the third trimester.

“It’s thought that acetaminophen exerts its analgesic effect through the endocannabinoid system, which is also very important for fetal development,” explains Woodbury.

Despite these findings, researchers caution against outright avoidance of acetaminophen during pregnancy, especially when needed for serious pain or high fever, as conditions like very high fever can be dangerous.

“There aren’t other options for people to take when they really need them,” says Susan Schantz, professor emerita of comparative biosciences at University of Illinois Urbana-Champaign and IKIDS principal investigator. “But perhaps people should use more caution when turning to the drug to treat minor aches and pains.”