Ultrasound measurement of the uterus
Transvaginal ultrasound measurements were performed on the day of ET, using a Voluson E8 (GE, Healthcare Austria GmbH) equipped with a 46-Hz endovaginal transducer, at 165°. The patients were guided to empty the bladder and lie supine on the examination table, which was inclined at 30°, in the lithotomy position. The entire uterine corpus needed to be seen on a longitudinal transvaginal grayscale image, and the ultrasound probe was placed as close to the uterus corpus as possible without abdominal compression to ensure a clear image. In order to obtain a clear ultrasound image, we do not guarantee that the ultrasound probe is horizontal, so the respective ultrasound images we obtained from ultrasonography can not objectively reflect the inclination angle of the uterine corpus. So we used the vertical distance from the midline of uterine cavity to the ultrasonic probe to indirectly reflect the inclination degree of uterine corpus. An example of the calculated average distance was shown in Figure 1b, where the midline of the uterine cavity (from the anatomic internal cervical os to the fundal endometrium) was divided into four equal parts and the vertical distances were measured from each split point to the ultrasound probe (five lines in total); the average distance was calculated according to the average of the sum of the five lines. The schematic in Figure 1 illustrated the smaller average distance from the midline of uterine corpus to the ultrasonic probe, the greater inclination of the uterine corpus was. Conversely, the greater average length from the midline of uterine cavity to the ultrasound probe, the more the uterine corpus tended toward the horizontal position. All measurements were performed by two experienced doctors.