Funding for this research was provided by:
The National Science and Technology R&D Programme part of the 12th Five-Year Plan of China (No. 2012BAI32B08)
Text and Data Mining valid from 2019-10-29
Received: 16 April 2019
Accepted: 13 September 2019
First Online: 29 October 2019
Ethics approval and consent to participate
: This study was approved by the Research Ethics Committee of the Shanghai Institute of Planned Parenthood Research, WHO Collaborating Center on Human Research (code: PJ2014–20). The study was performed according to the Declaration of Helsinki. All participants in the study provided written consent.
: Not applicable.
: The authors declare that they have no competing interests.