Funding for this research was provided by:
Beijing Natural Science Foundation Program (7184210, S160001)
‘Capital Clinical Characteristic Research’ from Beijing Science Committee (Z161100000516160)
National key research and development program (2016YFC1000304)
Received: 23 June 2019
Accepted: 13 August 2019
First Online: 23 August 2019
Ethics approval and consent to participate
: The study was approved by the Ethics Committee of Beijing Obstetrics and Gynecology Hospital (2012-KY-012, 2016-KY-066). Written informed consent was obtained from all participants. All procedures were performed in compliance with the Declaration of Helsinki.
: Not applicable.
: The authors declare that they have no competing interests.