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. 2025 Jul 16;15(1):25798.
doi: 10.1038/s41598-025-11258-6.

Uterine angle changes during the in vitro fertilization treatment cycle

Tatsuya Yoshihara  1 , Maki Ogi  1 , Yosuke Ono  1 , Dai Miyashita  1 , Kota Tanaka  1 , Yuri Tada  1 , Tatsuyuki Ogawa  1 , Tomohiko Fukuda  1 , Yasuhiko Okuda  1 , Osamu Yoshino  2
Affiliations

Uterine angle changes during the in vitro fertilization treatment cycle

Tatsuya Yoshihara et al. Sci Rep. .

Abstract

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.

Keywords: Assisted reproductive technology; Estradiol; Menstrual cycle; Progesterone; Uterine angle.

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Conflict of interest statement

Declarations. Competing interests: The authors declare no competing interests. Ethics approval and consent to participate: The Ethics Committee of the University of Yamanashi (approval No. 2584; November 1, 2022) authorized the research. All procedures were carried out in accordance with relevant guidelines and regulations and with the 1964 Helsinki Declaration and its later amendments or comparable ethical standards.

Figures

Fig. 1
Fig. 1
Pelvic MRI images of the same patient at different phases of the menstrual cycle. Pelvic MRI scans of the same volunteer (28 years old, parity = 1, regular 28-d cycle) were obtained three times during the menstrual cycle. (a) Day 3: menstrual period; (b) Day 14: ovulatory period; (c) Day 21: implantation period. Note the small angle and strong anteversion during the menstrual and implantation periods [(a) red line; (c) green line] and the larger angle with an upright shift during the ovulatory period [(b) yellow line].
Fig. 2
Fig. 2
Definition of the uterine angle. The uterine angle was defined as the angle formed by the line connecting the external and internal cervical os (A) and line connecting the internal os and uterine fundus (B).
Fig. 3
Fig. 3
Graph of changes in the uterine angle. Graph showing uterine angle changes in cases with significant variation from the menstrual to the ovulatory period and from the ovulatory to the implantation period. The Y-axis represents the uterine angle; the X-axis represents the menstrual, ovulatory, and implantation periods.
Fig. 4
Fig. 4
Schedule of frozen-thawed embryo transfer. (a) Menstrual period: second or third day of menstruation; (b) Ovulatory period: the day before the start of progesterone preparations in hormone replacement cycles or day before ovulation in natural cycles; (c) Implantation period: the day of embryo transfer. Ultrasonography was performed at all three time points. Serum hormone measurements (estradiol and progesterone) were obtained during the ovulatory (b) and implantation (c) periods.
See this image and copyright information in PMC

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