Statistical analysis
All statistical descriptions and analyses were conducted using SPSS Statistics 17.0 (IBM, Chicago, IL, USA). The one-sample Kolmogorov-Smirnov test was first applied to check normal distribution. Continuous measurements are presented as mean± standard deviation (SD), median (minimum-maximum) and 95% medical reference range. Mean±1.96SD and P2.5-P97.5 were defined as 95% reference range for the normally distributed data (lateral indentation angle, cornual angle, lateral indentation depth and uterine cavity surface area) and non-normally distributed data (endometrial thickness, uterine cavity length, uterine cavity width and internal os width), respectively. Regression analyses were used to screen the factors that influence the area of uterine cavity. Variables with univariate P≤0.1 were included in the multiple linear regression model for simple linear regression analysis. Statistical significance was set at P < 0.05.