Xu, Jihong
Chen, Ping
Ma, Xu
Funding for this research was provided by:
Special Fund of the Chinese Central Government for Basic Scientific Research Operations (2016GJM03)
Article History
Received: 12 August 2017
Accepted: 6 February 2018
First Online: 14 February 2018
Compliance with ethical standards
: Ethical approval for this study was granted by the Ethics Committee of the National Research Institute for Family Planning, Beijing, China.
: The authors declare that they have no competing interests.