There are two major nationwide birth cohort studies in Japan, namely, the Longitudinal Survey of Newborns in the 21st Century conducted by the Ministry of Health, Labor and Welfare (MHLW) and the Japan Environment and Children's Study (JECS) conducted by the Ministry of Environment. The former was a longitudinal questionnaire survey focusing on environmental and socioeconomic factors for descriptive epidemiology conducted every year since 2001 by mail. The latter was based on 15 unit centers nationwide with environmental measurements and collection of biological samples for environmental risk evaluation. Both are prospective birth cohort studies whose findings will be expected as the basis for establishing health policies. The data obtained in the former study can be used for research with permission from MHLW. To date, there have been more than ten published studies using those data. We have reviewed these studies and introduced our preliminary findings on factors affecting infant growth. Employment before delivery, educational background of parents, household income, and smoking habit of both parents have been suggested to affect infant growth. We will analyze the associations between socioeconomic factors and infant growth trajectory to elucidate the most adequate intervention for children.