1. Department of Obstetrics, Affiliated Hospital of Chengde Medical University, Chengde, Hebei 067000, P.R. China
2. Department of Geriatrics, Affiliated Hospital of Chengde Medical University, Chengde, Hebei 067000, P.R. China
3. Department of Obstetrics, Yan'an Hospital Affiliated to Kunming Medical University, Kunming, Yunnan 650051, P.R. China