1. Medical College of Shandong University, Jinan, Shandong 250000, P.R. China
2. Reproductive Medicine Center, Affiliated Yantai Yuhuangding Hospital of Qingdao University, Yantai, Shandong 264000, P.R. China
3. Department of Obstetrics and Gynecology, The Technical University of Munich, D‑80333, Germany