1. Key Laboratory of Birth Regulation and Control Technology of National Health and Family Planning Commission of China, Maternal and Child Health Care Hospital of Shandong Province, Shandong University, Jinan 250001, China
2. School of Public Health and Management, Weifang Medical University, Weifang 261042, China
3. School of Public Health, Zhengzhou University, Zhengzhou 450001, China
4. Department of Obstetrics and Gynecology, Shandong Provincial Hospital, Jinan 250001, China