1. Department of Obstetrics, The Fourth Hospital of Shijiazhuang, Shijiazhuang, China
2. Department of Gynecology, The Fourth Hospital of Shijiazhuang, Shijiazhuang, China
3. Electrocardiogram Room, The Fourth Hospital of Shijiazhuang, Shijiazhuang, China
4. Department of Obstetrics, The Gaoyi County Hospital of Shijiazhuang, Shijiazhuang, China