1. Nanjing Drum Tower Hospital, The Affiliated Hospital of Nanjing University Medical School, Nanjing, P. R. China
2. Department of Prenatal Diagnosis, Guangdong Women and Children Hospital, Guangzhou, P. R. China
3. Subei People’s Hospital of Jiangsu Province, Clinical Medical School of Yangzhou University, Yangzhou, P. R. China
4. Department of Ultrasonography, Guangdong Women and Children Hospital, Guangzhou, P. R. China