Received: 15 March 2020
Accepted: 5 August 2020
First Online: 14 August 2020
Ethics approval and consent to participate
: The need for ethics approval of our study was deemed unnecessary according to the national regulations – “The Regulations of Ethical Reviews of Biomedical Research Involving Human Subjects (2016)”Footnote removed issued by National Health and Family Planning Commission of the People’s Republic of China. Also, the need for ethics approval was waived by the Ethics Committee of Capital Medical University Beijing Obstetrics and Gynecology Hospital. The data used in this study were anonymized before analysis.
: Not applicable.
: The authors declare that they have no competing interests.