Li, Tian-gang
Nie, Fang
Li, Zhen-dong
Wang, Yan-fang
Li, Qi
Article History
Received: 10 June 2019
Accepted: 16 July 2019
First Online: 20 July 2019
Ethics approval and consent to participate
: The study protocol was approved by the Medical Ethics Committee of Gansu Provincial Maternity and Child-care Hospital.
: Not applicable.
: The authors declare that they have no competing interests.