Funding for this research was provided by:
National Key Research and Development Program of China (No. 2016YFC1000307)
CAMS Innovation Fund for Medical sciences (2018-I2M-1-004)
Beijing Talents Foundation from organization department of Beijing municipal committee of the CPC (grant number 2016000021223ZK20)
Received: 8 February 2020
Accepted: 20 July 2020
First Online: 15 September 2020
Ethics approval and consent to participate
: The study was approved by the Institutional Research Review Board at the National Research Institute for Family Planning, Beijing, China. Written informed consent was obtained from all NFPCP participants.
: Not applicable.
: The authors declare no conflicts of interest.