Received: 9 March 2019
Accepted: 21 June 2019
First Online: 26 June 2019
Ethics approval and consent to participate
: Our institutional review board approved the study, and the requirement for the informed consent of all participants was waived. In this article, institutional review board was review board of obstetrics and gynecological hospital, Medical College, Fudan University.
: All authors read and approved the final manuscript. All authors agreed to submit this article.
: The authors declare that they have no competing interests.