We obtained data on paternal age, sex of the child, and parity from birth records, maternal smoking status and household income from the first survey, and maternal and paternal education from the second survey. Paternal age was categorized in the same way as maternal age categories. Smoking status was dichotomized as smokers versus non-smokers. We aggregated educational attainment into three levels: completed high school or less, completed two years of college or vocational school, and completed four years of college or more. At Wave 1, questions about maternal employment status one year prior to delivery included seven options ranging from unemployed to full-time employment. We dichotomized these responses into “employed full-time” versus “not employed full-time”. Household income is the combined amount of annual earnings from the father, mother, and other sources, for which JPY ¥10,000 is equivalent to approximately USD $100. Based on gestational age, we created three groups following the WHO classification : term births (>37 weeks), moderately preterm births (32–36 weeks), and very preterm births (<32 weeks). We treated birthweight as continuous.