Correspondence to:
Pro Q. Luo, Department of Obstetrics, Women’s Hospital, Zhejiang University School of Medicine, Hangzhou, 310006, China; tel: E-mail: luoq@zju.edu.cn)
Dr. B. Zhao, Department of Obstetrics, the Fourth Affiliated Hospital, Zhejiang University School of Medicine, Yiwu, 322000, China; (E-mail:zhaobh@zju.edu.cn)
Pro J. Xu, Department of Reproductive Center, the Fourth Affiliated Hospital, Zhejiang University School of Medicine, Yiwu, 322000, China; (E-mail: xuj@zju.edu.cn)