This study was supported by the National Natural Science Foundation of China (No. 81070221) and the Project of Hunan Provincial Health and Family Planning Commission (B2016139, B2016133); Project of Hunan Provincial Department of Education (16C1392).
Project of Hunan Provincial Health and Family Planning Commission, Grant/Award Numbers: B2016133, B2016139; Project of Hunan Provincial Department of Education, Grant/ Award Number: 16C1392; National Natural Science Foundation of China, Grant/Award Number: No. 81070221; Natural Science Foundation of China