Zheng, Yunting
Ji, Ying
Dong, Hongbo
Chang, Chun
Article History
Received: 15 August 2017
Accepted: 7 May 2018
First Online: 24 May 2018
Ethics approval and consent to participate
: Since the study utilized national data from the National Commission of Health and Family Planning, the ethics approval has been exempted by Peking University Institution Review Board.
: The authors declare that they have no competing interests.
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