Study design and participants' profile in the Sub-Cohort Study in the Japan Environment and Children's Study (JECS).

Background The Japan Environment and Children's Study (JECS) is a nationwide birth cohort study investigating environmental effects on children's health and development. A Sub-Cohort Study has begun, conducting extended exposure and outcome measurements by targeting a subgroup randomly selected from the JECS Main Study. We report the Sub-Cohort Study methodology and participants' baseline profiles. Methods Of 100,148 children in the JECS Main Study, children born after 1 April 2013 who met eligibility criteria ([1] all questionnaire and medical record data from children and their mothers collected from the first trimester to 6 months of age, [2] biospecimens [except umbilical cord blood] from children and their mothers collected at first to second/third trimester and delivery) were randomly selected for each Regional Centre at regular intervals. Face-to-face assessment of neuropsychiatric development, body measurement, paediatrician's examination, blood/urine collection for clinical testing and chemical analysis, and home visits (ambient and indoor air measurement and dust collection) are conducted. Participants are followed up at 1.5 and 3 years old for home visits, and 2, 4, 6 and 8 years old for developmental/medical examination. The details of protocols after age 10 are under discussion. Results Of 10,302 selected children, 5,017 participated. The profiles of the participating mothers, fathers and children did not substantially differ between the Main Study and Sub-Cohort Study. Conclusion The JECS Sub-Cohort Study offers a platform for investigating associations between environmental exposure and outcomes.