1. Clinical Medical College of Shandong University, Jinan, Shandong 250012, P.R. China
2. Department of Obstetrics, Jinan People's Hospital, Jinan, Shandong 271100, P.R. China
3. Reproduction Medical Center, The Affiliated Yantai Yuhuangding Hospital of Qingdao University, Yantai, Shandong 264000, P.R. China