Author Affiliations:
1. Department of First Clinical Medical College, Shanxi Medical University, Taiyuan, China;
2. Department of Obstetrics, First Hospital of Shanxi Medical University, Taiyuan, China.
*Corresponding author: Prof. Dr. Hailan Yang, Taiyuan, Shanxi Province, China; Telephone: +86-13834048059; E-mail:feichihai@sina.com.