Received: 11 June 2020
Accepted: 29 October 2020
First Online: 7 November 2020
Ethics approval and consent to participate
: The study was approved by the Ethics Committee of the Obstetrics and Gynecology Hospital of Fudan University (No. 2017–35, 2017–35-C1) and was conducted in accordance with Chinese law and the Guidelines of the National Human Biomedical Research Policies. No informed consent was obtained from the patients because the study was retrospective. Administrative permissions of the data was acquired by the authors for research purposes.
: Not applicable.
: The authors declare that they have no competing interests.