Funding for this research was provided by:
Ministry of Health, Labour and Welfare (Health and Labour Sciences Research Grant (H30-Sukoyaka-Ippan-002))
Received: 15 March 2021
Accepted: 19 July 2021
First Online: 3 August 2021
: The study protocol was approved by the institutional review board of the Graduate School of Medicine at The University of Tokyo (approval no: 2019270NI, approved on January 23, 2020) and by the ethics committee of Akita University Graduate School of Medicine (approval no: 2300, approved on September 20, 2019). The Ministry of Health, Labor and Welfare approved the secondary use of data from the National Fertility Survey, and informed consent was not required because of anonymous data.
: Not applicable.
: The authors declare that they have no competing interests with respect to this research study and paper.