Department of Epidemiology and Health Statistics, Xiangya School of Public Health, Central South University, No. 110 Xiangya Road, Kaifu District, Changsha, 410078, Hunan, China. Department of Neurology, The Third Xiangya Hospital, Central South University, Changsha, China. Department of Gastrointestinal Surgery, Xiangya Hospital, Central South University, Changsha, China. Department of Obstetrics and Gynaecology, University of Ottawa, Ottawa, ON, Canada. Ottawa Hospital Research Institute, Ottawa, ON, Canada. Department of Epidemiology and Health Statistics, Xiangya School of Public Health, Central South University, No. 110 Xiangya Road, Kaifu District, Changsha, 410078, Hunan, China. qweruli@sina.com.