Received: 5 December 2017
Accepted: 16 May 2018
First Online: 23 May 2018
Ethics approval and consent to participate
: All women participated in the study voluntarily and gave written informed consent prior to study begin. They were informed that they can withdraw their consent and stop participation at any time without disclosing the reasons and without negative consequences for their future medical care. Institutional Review Board of Peking Union Medical College Hospital had approved this retrospective study (reference number ZS-1428).
: The authors declare that they have no competing interests.
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