Article History
Received: 25 September 2019
Accepted: 30 December 2019
First Online: 8 January 2020
Ethics approval and consent to participate
: The Ethics Committee of Peking University First Hospital approved the study protocol (Number 2018–130). Written informed consent was obtained from each recruited parturient after providing them with adequate explanations regarding the aims of this study.
: Not applicable.
: The authors declare that they have no competing interests.