Uterine angle changes during the in vitro fertilization treatment cycle.

Studies have focused on uterine peristalsis and endometrial development, whereas knowledge is limited on cyclical changes in the uterocervical angle (the angle formed between the uterine corpus and cervix) during the menstrual cycle. Here, these dynamic changes were investigated. We conducted a retrospective analysis of 99 freeze-thawed blastocyst transfer cycles in 2020 and a prospective analysis of 192 cycles between 2022 and 2023. Transvaginal ultrasonography was performed to measure the uterine angle during the menstrual, ovulatory, and implantation periods. In the prospective study, conditions were standardized by instructing patients to empty their bladders and avoid uterine compression during scanning. In the retrospective analysis, the uterine angle significantly differed between the menstrual and ovulatory periods (141.1 ± 33.5° vs. 147.7 ± 40.9°, p = 0.04), but no correlation with serum estradiol or progesterone levels was observed. The prospective analysis confirmed significant differences in uterine angle between the menstrual and ovulatory periods and between the ovulatory and implantation periods (135.1 ± 27.7°, 141.5 ± 30.9°, and 136.8 ± 28.7°, respectively; both p < 0.01), without hormonal correlation. These findings suggest that the uterine angle varies throughout the menstrual cycle, becoming more anteverted during the menstrual and implantation periods and straightening around ovulation.