1. Department of Women and Children's Medical Center, Gansu Provincial Maternity and Child Care Hospital, Lanzhou, Gansu 730050, P.R. China
2. Reproduction Medicine Center, Gansu Provincial Maternity and Child Care Hospital, Lanzhou, Gansu 730050, P.R. China
3. Discipline of Physiology, Gansu University of Chinese Medicine, Lanzhou, Gansu 730000, P.R. China
4. Perinatal Center, Gansu Provincial Maternity and Child Care Hospital, Lanzhou, Gansu 730050, P.R. China