1. Prenatal Diagnosis Center, Shijiazhuang Obstetrics and Gynecology Hospital, Shijiazhuang, Hebei 050011, P.R. China
2. Hebei Key Laboratory of Maternal and Fetal Medicine, Shijiazhuang, Hebei 050011, P.R. China
3. Department of Pediatric Orthopaedics, The Third Hospital of Hebei Medical University, Shijiazhuang, Hebei 050051, P.R. China
4. Prenatal Diagnosis Center, Langfang Maternal and Child Health Care Hospital, Langfang, Hebei 065000, P.R. China