Funding for this research was provided by:
National Health and Family Planning Commission of the People's Republic of China (50)
Text and Data Mining valid from 2019-11-26
Received: 10 December 2018
Accepted: 13 November 2019
First Online: 26 November 2019
Ethics approval and consent to participate
: The ethics were approved by the Ethics Committee of Fuwai Hospital of Chinese Academy of Medical Sciences (project identification code: 2014–574), and written informed consent was obtained from all of the participants before data collection.
: Not applicable.
: The authors declare that they have no competing interests.