 The Japan Environment and Children's Study, JECS, is a large-scale birth cohort study conducted in Japan between 2011 and 2014. It aimed to investigate the environmental factors that may affect children's health and development. Overall, 100,778 pregnancies resulted in 100,148 live births, with 51,402 of these pregnancies involving male participants. The study covered 95,248 unique mothers and 49,189 unique fathers, and had a coverage rate of 45% in 2013. The data collected from this study was similar to that found in Japan's 2013 Vital Statistics Survey. This article was authored by Takahiro Michikawa, Hiroshi Nitta, Shoji F. Nakayama, and others.