[1] Peking Univ Third Hosp, Dept Gynecol & Obstet, 49 North Garden Rd, Beijing 100191, Peoples R China
[2] Peking Univ Third Hosp, Ctr Clin Epidemiol, Beijing, Peoples R China
[3] Natl Ctr Healthcare Qual Management Obstet, Beijing, Peoples R China
[4] Peking Univ Third Hosp, Natl Clin Res Ctr Obstet & Gynecol, Beijing, Peoples R China