Association between prenatal tobacco exposure and attention-deficit/hyperactivity disorder related characteristics at 6 and 8 years: a birth cohort in Japan.

Objectives: To examine the association between maternal plasma cotinine concentrations during pregnancy and attention-deficit/hyperactivity disorder (ADHD) related characteristics in children.

Design: Prospective birth cohort study from the Hokkaido Study on Environment and Children's Health.

Setting: Hokkaido, Japan.

Participants: A total of 7217 children were included in the analysis. Their mothers were recruited to this study between 2003 and 2012 during early pregnancy. Blood samples of mothers in their third trimester of pregnancy were obtained.

Main Outcome Measures: ADHD-related characteristics screened by the ADHD-Rating Scale (ADHD-RS) in children at 6 and 8 years of age.

Results: The mothers were classified into three groups according to plasma cotinine levels (median=0.22 ng/mL)-non-smokers (≤0.21 ng/mL), passive smokers (0.22-11.48 ng/mL) and active smokers (≥11.49 ng/mL). ADHD was suspected in 302 (8.6%), 334 (10.9%) and 90 (14.5%) children in the non-smoker, passive smoker and active smoker groups, respectively. Children born to mothers in the smoking group had a statistically significant increased risk of ADHD-related characteristics (adjusted RR=1.49, 95% CI=1.19-1.86 and adjusted RR=1.20, 95% CI=1.03-1.40 for the active and passive smoking groups, respectively).

Conclusion: This large Japanese cohort study suggests an association between smoking during pregnancy and the risk of ADHD in children 6 and 8 years of age.