Article History
Received: 9 September 2018
Accepted: 2 February 2019
First Online: 12 February 2019
Compliance with ethical standards
:
: Author YJ Hu has received research grants from Tianjin Health Bureau. Author T Wang has received a speaker honorarium from Tianjin Central Obstetrics and Gynecology Hospital. All authors declare that they have no conflict of interest. Ethical approval this article does not contain any studies with human participants performed by any of the authors.
: All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards.
: Informed consent was obtained from all individual participants included in the study.