1. Department of Obstetrics, Jinan City People's Hospital, Jinan, Shandong 271199, P.R. China
2. Department of Obstetrics and Gynecology, Laigang Hospital Affiliated to Shandong First Medical University, Jinan, Shandong 271126, P.R. China
3. Department of Obstetrics, Shandong Provincial Western Hospital, Shandong Provincial ENT Hospital, Jinan, Shandong 250000, P.R. China
4. Department of Obstetrics and Gynecology, Jinan Fourth People Hospital, Jinan, Shandong 250031, P.R. China