1. Department of Obstetrics, Jiaozhou Central Hospital of Qingdao City, Qingdao, Shandong 266300, P.R. China
2. Community Health Service Center, Qingdao, Shandong 266300, P.R. China
3. Department of Obstetrics, Hainan General Hospital (Hainan Affiliated Hospital of Medical University), Haikou, Hainan 570311, P.R. China