A new study was just published in JAMA Psychiatry providing prospective evidence of an an association between tylenol (acetaminophen) use during pregnancy and about doubling of later risk of ADHD or autism in children. The use of acetaminophen was measured in the cord blood at delivery, which is a very accurate assessment of a predictor. The evidence was gathered in a study called the Boston Birth Cohort, which collected data on deliveries in the last 20 years (1998-2018) at Boston Medical Center (n=3163 mothers participated, cord blood was available in 996).
The odds ratio for ADD in the higher tertiles (second and third tertiles) of cord blood acetaminophen were 2.26-2.86, and for autism were 2.14-3.62.
Obviously any epidemiological study has a risk of confounding factors, and cannot be taken at face value. These associations held up when stratified by certain specific confounding factors, like substance use and preterm birth, among others. Yet other confounders still can exist.
The study is strengthened by being prospective, though, and by measuring cord blood levels of acetaminophen.
Its plausibility is strengthened by animal studies which suggest neurotoxicity with acetaminophen in rodents and disruption of fetal brain development in rodents.
Other clinical association studies also exist, but still one cannot be definitive. It would seem reasonable though to recommend avoidance of acetaminophen during pregnancy when other pain treatment options exist. Many over-the-counter medications can have harmful effects on the fetus. Less is more is a wise policy during pregnancy.