Funding for this research was provided by:
National Natural Science Foundation of China (71804036, 71403073, 71874045, 71333003)
China Postdoctoral Science Foundation (2016M590296)
Heilongjiang Health and Family Planning Commission Project (2014-427)
Received: 6 March 2020
Accepted: 13 July 2020
First Online: 14 September 2020
Ethics approval and consent to participate
: Ethics approval and consent to participate is not applicable due to the retrospective nature of this study.
: All authors agree to publish.
: The authors declare that they have no competing interests.