Received: 14 October 2020
Accepted: 20 January 2021
First Online: 30 January 2021
Ethics approval and consent to participate
: All procedures were performed in accordance with the 1964 Helsinki Declaration and its later amendments. The study was approved by the Ethics Committee of Tianjin Central Hospital of Obstetrics and Gynecology(2019KY052) and written informed consent was obtained from each participant. No administrative permissions are required.
: Not applicable.
: The authors declare that they have no competing interests.