Results
The last participant was enrolled in March 2020 and data collection was completed in April 2020. There were 172 women eligible for the study. The mean age was 29.8 years with SD 5.4 years and range 18–49 years. The median endometrial thickness was 8mm with range 4-15 mm (uteri with endometrial thickness greater than15mm were excluded). The rest seven measurements were carried out on a 3D coronal image of the uterus, and among them three symmetrical measurements (cornual angle, lateral indentation angle and lateral indentation depth) from left and right side were used for calculation the average value of these measurements. Table 1 shows the mean, SD, median, range and 95% reference range of the 3D measurements. Based on normal distribution, two-sided 95% reference range of uterine cavity surface area, cornual angle, lateral indentation depth and angle of normal adult women in our clinic center were estimated as 3.0-8.0 cm², 39-88°, 0.8-6.4 mm and 131-171°, respectively. Due to the non-normal distribution, 95% reference range of uterine cavity length, uterine cavity width and internal os width were estimated as 26-38 mm, 25-40 mm and 4-11 mm, respectively, which were calculated by 2.5th and 97.5th percentile.
Focusing on the uterine cavity surface area , we found that 25.6% of uterine cavities were less than 4.5 cm2; 10.5% less than 4 cm2; and 2.3% less than 3.5 cm2, respectively. Only 7 participants or 4% had surface areas greater than 8 cm2. The interquartile range shows that 50% of surface areas in the group were between 4.5 cm2 and 6.2 cm2, respectively. 80% of subjects had a surface area between 4.0 cm2 and 7.2 cm2, respectively. In our group 95% of surface areas were between 3.5 cm2 and 8.2 cm2, respectively.
Univariate linear regression analyses revealed that the uterine cavity surface area was affected by age, endometrial thickness, cornual angle, uterine cavity width, uterine cavity depth, internal os width, lateral indentation depth and lateral indentation angle (all P < 0.05). We included all above variables in the multiple linear regression model using stepwise method. Table 2 showed the main statistics of the final model. The results indicated that uterine cavity width, uterine cavity length, lateral indentation depth, internal os width, endometrial thickness and cornual angle were significant independent influence factors of uterine cavity surface area. The variable of uterine cavity width had the biggest effect on uterine cavity surface area according to the standardized coefficient. The coefficient of determination R2= 90% suggested that final model fitted the data well.