1. Obstetrics and Gynecology Department, Peking University First Hospital, Beijing 100034, China
2. State Key Laboratory of Transducer Technology, Aerospace Information Research Institute, Chinese Academy of Sciences, Beijing 100190, P. R. China
3. University of Chinese Academy of Sciences, Beijing 100049, P. R. China
4. Fourth People’s Hospital of Jinan, Jinan 250031, China