Funding for this research was provided by:
Shenzhen health and family planning system research project (szxj2017003)
Text and Data Mining valid from 2019-09-02
Received: 16 March 2019
Accepted: 29 August 2019
First Online: 2 September 2019
Ethics approval and consent to participate
: This study was approved by the Ethics Committee of Peking University Shenzhen Hospital. All the participants provided written consent to participate in the study.
: Not applicable.
: The authors declare that they have no competing interests.