Fig. 1From: Prenatal tobacco exposure and ADHD symptoms at pre-school age: the Hokkaido Study on Environment and Children’s HealthSelection of study population. Among children born between April 2008 and April 2012 (n = 6804), 3081 returned follow-up questionnaires by May 2017. Among 3801, maternal cotinine levels at the third trimester was available for 3216. These 3216 were categorized into four groups based on the maternal cotinine levelsBack to article page