Strengths and limitations
To our knowledge this is the first cohort study to investigate the relationship between uterine corpus inclination and pregnancy outcomes of IVF-ET. To avoid the influence of COH on the endometrial receptivity and the interference of cumulative pregnancy rate, we only selected patients undergoing their first FET cycle and who used HRT for endometrial preparation. We excluded cases with adenomyosis, myoma, and “anteverted and retroflexed” and “retroverted and anteflexed” uteri, and limited size of uterus in the included patients, because these gynecological diseases and the associated cervical distortion would increase the distance between the midline of uterine cavity and the ultrasonic probe, which would not objectively reflect the inclination of the uterine corpus in our calculated method.
The main limitation of this study lies in the method of measuring the uterine corpus inclination. In order to get a clear image, we would rotate the ultrasound probe to make the ultrasound probe as close as possible to the uterine corpus, and we couldn’t guarantee that the ultrasonic probe kept horizontal when we did the ultrasound examination. Therefore, we couldn’t get the actual uterine inclination angle in our retrospective ultrasound images. A great quantity of clinical work in our experience we found that the more horizontal the uterus, the farther the distance from the uterine cavity to the ultrasound probe, so we used the average distance from the midline of uterine cavity to the ultrasound probe to indirectly reflect the uterine corpus inclination. We tried to use the average distance of three, five, and eleven lines to represent the distance, and we found that the average distance of five lines and eleven lines gave very similar results, while the average values of three lines differ. We therefore selected five lines to represent the distance to simplify our measurement. Although we did not give an accurate method to measure the tilt angle of uterine corpus, this measurement method could be applied to our retrospective images, and it could also truly reflect the uterine inclination degree. Other limitations of this study lies in its retrospective nature and the possibility of unmeasured confounding factors, such as smoking habits and alcohol consumption.