Families from all over Japan were considered eligible to participate in the survey if their child was born between the 10th and 17th of January or July in 2001 or between the 10th and 24th of May in 2010. Baseline questionnaires were sent to all families when infants reached 6 months of age (cf. Wave 1). For both cohorts, the response rates were 88%. Follow-up questionnaires were sent to participating families every 12 months, at 18 months (Wave 2), 30 months (Wave 3), and so on. Birth record information included birthweight, gestational age, sex of the child, and parental age. We obtained information on Waves 1–12 (the 2001 cohort) and Waves 1–3 (the 2010 cohort) from the MHLW. Participant numbers and loss to follow-up are summarized in Figs 1 and 2. This study was approved by the Japanese National Center for Child Health and Development Institutional Review Board (No. 1007).