*These authors contributed equally to this work.
Correspondence : Pin Liu; Chunlin Chen; Department of Obstetrics and Gynecology, Nanfang Hospital, Southern Medical University, No.1838, Guangzhou Avenue, Guangzhou 510515, People’s Republic of China
Tel/Fax +86 206 278 7562
Email : lpivy@126.com; ccl1 @ smu.edu.cn