Timing of Delivery of Low-Risk Persons and the Risk of Attention-Deficit Hyperactivity Disorder in Offspring: Sweden and British Columbia, Canada.

Background: An evidence gap exists concerning the timing of delivery at 37-42 weeks and the risk of attention-deficit hyperactivity disorder (ADHD) in offspring.

Objective: To determine the association between timing of delivery in low-risk pregnancies at term (37-42 weeks) gestations and ADHD in offspring.

Methods: This population-based cohort study comprised 1,424,453 singletons in Sweden and 403,765 in British Columbia (BC), Canada, live-born at 37-42 completed weeks to low-risk pregnant women between 2000 and 2018. Children were followed up from age 1 until the date of death, emigration, their first diagnosis, or December 2020 (study's end date). The exposure was time of delivery assessed through gestational age, and the outcome was the diagnosis of ADHD. Cox regression models were used to examine the association between gestational age at delivery and ADHD.

Results: During the follow-up period, 59,989 children in Sweden were diagnosed with ADHD (4.5 per 1000 child-years). Correspondingly, in BC, during the same period, there were 27,445 children diagnosed with ADHD (7.4 per 1000 child-years). In Sweden, the adjusted hazard of ADHD was 10%, 6%, and 3% higher at 37, 38, and 39 weeks gestation compared with those born at ≥ 38, ≥ 39, and ≥ 40 weeks, respectively. In BC, the corresponding hazards were 9%, 6%, and 3%, respectively. Both regions showed no elevated ADHD risks for infants born at 40 weeks compared to those born at ≥ 41 weeks, with slightly lower rates at 40 weeks.

Conclusions: In low-risk pregnancies, births at 37 and 38 weeks were associated with a higher ADHD risk, while births at 40 weeks showed no increased risk compared with those born at later gestations.