1.Department of Obstetrics and Gynecology,Provincial Hospital Affiliated to Shandong University,Jinan 250021,China;2.Department of Obstetrics and Gynecology,Provincial Hospital Affiliated to Shandong University.Department of Obstetrics,Maternal and Child Health Care of Shandong Province.Key Laboratory of Birth Regulation and Control Technology of National Health and Family Planning Commission of China,Jinan 250021,China