More studies are needed to confirm the findings, but experts said that yesterday's research points to a new potential cause for the worldwide rise in cases of ADHD, a neuro-behavioural condition which has no known cause and affects as many as five per cent of US children.
Women who took acetaminophen, also known as paracetamol, while pregnant had a 37 per cent higher risk of having a child who would be later given a hospital diagnosis of hyperkinetic disorder, a particularly severe form of ADHD, said the study in February 24 edition of the Journal of the American Medical Association (JAMA) Pediatrics.
Compared to women who did not take acetaminophen while pregnant, women who did also had a 29 per cent higher chance of having children who were later prescribed medications for attention deficit hyperactivity disorder, and a 13 per cent higher chance of exhibiting ADHD-like behaviours by age seven.
Previous research has suggested that acetaminophen can interfere with normal hormone function and may affect the developing fetal brain. The painkiller has also been linked to a slightly increased risk in boys of cryptorchidism, a condition in which the testicles do not descend.
The latest research was based on survey data on more than 64,000 Danish women from 1996 to 2002.
More than half said they took acetaminophen at least once during pregnancy.
Outside experts cautioned that the observational findings do not prove that taking Tylenol-like pain relievers causes ADHD, only that a preliminary link between the two has appeared and would need to be confirmed by further research.
"Findings from this study should be interpreted cautiously and should not change practise," said an accompanying editorial in JAMA Pediatrics by Miriam Cooper and colleagues at the Cardiff University School of Medicine.
"However, they underline the importance of not taking a drug's safety during pregnancy for granted."
The reasons the women took the painkillers could have also had a confounding effect on the outcome, they added.
The study was led by Zeyan Liew, of the University of California, Los Angeles, and was co-authored by Jorn Olsen of the University of Aarhus in Denmark.