The Longitudinal Survey of Babies in 21st Century (LSB21) is an ongoing study in Japan comprising two birth cohorts, in which the first started in 2001 and the second in 2010. The main purpose of the LSB21 is for the Ministry of Health, Labour and Welfare (MHLW) to develop strategies against the decline of fertility among Japanese families . Details of the LSB21 have been described elsewhere, but the 2010 cohort has not been analyzed [12–14].