Study subjects
This cross-sectional observational study was performed at Sun Yat-sen Memorial Hospital of Sun Yat-sen University. From January 2018 to March 2020, 172 consecutive premenopausal women from the gynecology clinic center who were referred for pelvic ultrasonography were included. The inclusion criteria were as follows: 1) childbearing age; 2) with regular menstrual cycle; 3) pelvic and ultrasonic examination revealed normal sized uterine body; 4) had a normal uterine cavity on two- and three-dimensional transvaginal ultrasonography; 5) no history of uterine cavity surgery before the ultrasonography; 6) the subsequent hysteroscopy confirmed normal uterine cavity (without fibroids, endometrial polyps, uterine synechiae and intrauterine device). The exclusion criteria included the following items: 1) thickened endometrium (≥15mm); 2) simple or atypical endometrial hyperplasia, or endometrial malignancies; 3) having used contraceptives or any other kind of hormones in the 3 months before participating in the study; 4) had a history of pregnancy in the 6 months preceding the ultrasound scan.