Long-term acetaminophen use by women during pregnancy was associated with an increased risk of attention-deficit/hyperactivity disorder (ADHD) in offspring,
...according to findings from a large cohort study published in the journal Pediatrics.
Researchers used data from the Norwegian Mother and Child Cohort Study (MoBa) to investigate the association between acetaminophen use during pregnancy and ADHD; the final sample included 112,973 offspring. Questionnaires were used to assess medication use during pregnancy. Mothers filled out two questionnaires, one during and one after birth; fathers completed surveys on medication use 6 months before pregnancy.
The study authors noted that, "In contrast to previous studies we were able to adjust for indications of acetaminophen use and parental symptoms of ADHD. We were furthermore able to analyze maternal use of acetaminophen prior to pregnancy as a specificity control and to estimate the effect of paternal use prior to pregnancy."
Results showed that after adjustments were made for maternal use of acetaminophen pre-pregnancy, familial risk and indications of acetaminophen use, a modest association between any prenatal maternal use of acetaminophen in one (hazard ratio [HR], 1.07; 95% CI, 0.96–1.19), two (HR, 1.22; 95% CI, 1.07–1.38), and three trimesters (HR, 1.27; 95% CI, 0.99–1.63) was observed. 
Use of acetaminophen for >29 days in both mothers and father was associated with a two-fold increased risk of ADHD in offspring.
Prenatal use of acetaminophen for fever and infections for 22 to 28 days was associated with an even greater risk for ADHD (HR 6.15 [95% CI 1.71–22.05]). However, use for <8 days was negatively associated with offspring ADHD. This, the authors write, indicates “that the antipyretic effect could be beneficial with regard to fetal development."
In total 52,707 women (almost 47%) took acetaminophen during pregnancy, and 2,246 children were diagnosed with ADHD. Given that paternal use was also associated with ADHD, the authors concluded that they "do not provide definitive evidence for or against a causal relation between maternal use of acetaminophen and ADHD.
