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.