Funding for this research was provided by:
China Family Planning Association (NA)
Received: 10 October 2019
Accepted: 9 August 2020
First Online: 17 August 2020
Ethics approval and consent to participate
: Ethical approval was obtained from the Peking University Health Science Centre Institutional Review Boards. All participants provide written consents.
: Not applicable.
: The authors declare that they have no competing interests.