Funding for this research was provided by:
Natural Science Foundation of Hebei Province (No. C201606055)
National Natural Science Foundation of China (No. 31871391)
Text and Data Mining valid from 2019-09-13
Received: 19 July 2019
Accepted: 30 August 2019
First Online: 13 September 2019
Ethics approval and consent to participate
: The study was approved by the ethics committee of the Shijiazhuang Obstetrics and Gynecology Hospital. Written informed consent was obtained from all enrolled subjects.
: Written informed consent was obtained from all enrolled subjects.
: The authors declare that they have no competing interests.