Text and Data Mining valid from 2020-02-17
Received: 15 May 2019
Accepted: 4 February 2020
First Online: 17 February 2020
Ethics approval and consent to participate
: This work was approved by Ethics Committee of Obstetrics and Gynecology Hospital of Fudan University, reference number (Kyy2018–18).
: Not applicable.
: The authors declare that they have no competing interests.