Endometrial elasticity in infertile women assessed by transvaginal shear wave elastography

The global burden of infertility is increasing, affecting approximately 17.5% of adults worldwide, with about 1 in 6 individuals experiencing infertility at some point in their lifetime ( 1 ). In China, recent epidemiological studies report infertility rates ranging from 12% to 18%, with a steady upward trend over the past decade ( 2 ). Impaired endometrial receptivity (ER) is recognized as one of the key pathological mechanisms underlying infertility, particularly in cases where ovulation and embryo quality are normal but implantation repeatedly fails ( 3 ). ER refers to the physiological and molecular state of the endometrium during the “window of implantation”, when it becomes conducive to blastocyst adhesion and invasion ( 4 ). Disruption of this finely regulated state compromises implantation and contributes substantially to both spontaneous infertility and suboptimal outcomes in assisted reproductive technologies ( 5 ). Current ER evaluation methods—including histopathology, molecular testing, and hysteroscopy ( 6 )—are limited by invasiveness and lack of standardization ( 5 ), hindering widespread clinical adoption. Transvaginal ultrasound, with its non-invasive and real-time dynamic advantages, has become the mainstream choice in infertility treatment and pregnancy monitoring ( 7 ). However, conventional parameters (such as endometrial thickness, echo pattern, volume, contraction frequency, and hemodynamic indices of uterine and spiral arteries) suffer from inconsistent quantitative standards, operator dependence, and the absence of unified criteria ( 8 , 9 ). Shear wave elastography (SWE) quantifies the biomechanical properties of the endometrium via Young’s modulus (kPa). Its depth-focusing technology enables precise detection of functional layer elasticity ( 10 ), allowing dynamic monitoring of stiffness variations throughout the menstrual cycle. Studies suggest that SWE can detect early signs of endometrial injury and quantify stiffness changes ( 11 , 12 ), yet existing research primarily focuses on uterine lesion differentiation ( 7 ), leaving its application in ER assessment underexplored. Several recent studies have explored the application of SWE in assessing ER, reporting significant differences in endometrial stiffness between women with unexplained infertility and fertile controls, and suggesting potential diagnostic value of elasticity parameters ( 13 - 15 ). These preliminary findings provided the methodological and theoretical basis for the present study. However, existing evidence remains limited and largely observational, underscoring the need for further validation. Therefore, this study aims to investigate the differences in endometrial elasticity between infertile women and healthy controls across the menstrual cycle using transvaginal SWE, and to explore whether SWE can detect differences in endometrial elasticity associated with infertility. We present this article in accordance with the STROBE reporting checklist (available at https://qims.amegroups.com/article/view/10.21037/qims-2026-0530/rc ).

This prospective study enrolled reproductive-age women, including infertile patients and healthy controls, who visited The Third Affiliated Hospital of Guangzhou Medical University between March and September 2023. Reproductive age was defined according to the World Health Organization (WHO) criteria as 15–49 years. Female infertility was defined as failure to achieve a clinical pregnancy after at least 12 months of regular, unprotected sexual intercourse, with male-factor infertility excluded ( 16 ). Additional inclusion criteria required normal ovarian function and normal uterine cavity morphology, as well as physiological variation in endometrial thickness without the presence of submucosal fibroids, intrauterine adhesions, or endometrial polyps. Healthy women were included as controls if they had regular menstrual cycles, defined as a cycle length between 21 and 35 days with a variation of no more than 7 days across three or more consecutive cycles, no history of infertility or gynecological disease, and no use of hormonal medication within the previous three months. Of note, among the healthy controls with a history of miscarriage, none had undergone uterine curettage. For all participants, menstrual phases and cycle timing were determined based on the first day of their last menstrual period (LMP). Using a standard 28-day cycle as a baseline, the proliferative phase was defined as days 5–14 after the LMP, and the secretory phase as days 15–28 after the LMP; proportional adjustments were applied for variations in cycle length. For both the normal control and unexplained infertility groups, participants were further divided into separate cohorts for each phase: one cohort underwent ultrasound examination during the proliferative phase and a separate cohort during the secretory phase, rather than following the same individuals across two menstrual cycles. Ultrasound findings in these participants confirmed normal endometrial morphology (homogeneous echogenicity without space-occupying lesions) and serum sex hormone levels were within the normal reference ranges for the corresponding calculated follicular or luteal phases. Participants were excluded if they met any of the following conditions: (I) poor-quality endometrial images in the longitudinal plane; (II) confirmed uterine or tubal pathologies identified by ultrasound or hysterolaparoscopy; (III) severe organic diseases or cardiopulmonary dysfunction that could interfere with ultrasound examination; (IV) menstruation or abnormal uterine bleeding at the time of the scan; (V) a history of uterine surgery that could affect endometrial structure or fibrosis, including dilation and curettage or myomectomy; or (VI) diagnosed adenomyosis, endometriosis, or clinical features suggestive of chronic endometritis that might alter endometrial dynamic or structural characteristics. The patient selection and grouping process are summarized in Figure S1 . This study was approved by the Institutional Ethics Committee of The Third Affiliated Hospital of Guangzhou Medical University (Medical Ethics Review [2023] No. 351), and conducted in accordance with the principles of the Declaration of Helsinki and its subsequent amendments. Written informed consent was obtained from all participants prior to enrollment. A Resona R9 PRO color Doppler ultrasound system (Mindray Medical International Ltd., Shenzhen, China) equipped with a transvaginal DE 10-3WU probe was used for all ultrasound examinations. For transvaginal conventional two dimensional (2D) and color Doppler ultrasound, patients were instructed to empty their bladders and assume the lithotomy position. Endometrial thickness was measured in the midsagittal plane using 2D ultrasound, and endometrial patterns were classified according to the Gonen criteria ( 17 ): Type A: triple-line pattern (hyperechoic outer and central lines with a hypoechoic area in between); Type B: uniform intermediate echogenicity with an indistinct central line; and Type C: homogeneous hyperechogenicity without a visible central line. Color Doppler was then used to evaluate endometrial blood flow signals. Endometrial patterns were classified according to the Gonen criteria, which are routinely used in our clinical practice and widely recognized by gynecologists involved in infertility management. This approach ensured consistency with daily clinical workflow and facilitated reproducible assessment. After completing the 2D ultrasound examination, the mode was switched to SWE ( Figure S2 ). A probe frequency of 5 MHz was used, with the elasticity scale set to a maximum of 90 kPa, and elasticity depth limited to 3 cm. The probe was inserted into the vaginal fornix without applying pressure. Uterine position (anteverted or retroverted) was recorded for each participant, and the anatomical identification of the anterior versus posterior endometrial wall was based on this recorded position. The probe was then adjusted to clearly visualize both the anterior and posterior endometrial walls and uterine cavity line. The system incorporated dual quality control features: Reliability Index (RLB index): must exceed 90% and Motion Stability Index (M-STB index): must display 5 green stars ( Figure 1 ). To ensure image stability, patients were instructed to hold their breath for 3–5 seconds. Once the quality indicators were met, the image was frozen and saved. Two methods were used to measure endometrial elasticity: (I) six circular regions of interest (ROIs) with a diameter of 1 mm were placed at 5, 10, and 15 mm from the uterine fundus on both anterior and posterior endometrial walls ( Figure S3A-S3C ), and (II) two elliptical ROIs were positioned to cover the anterior and posterior endometrial walls ( Figure S3D ). Elasticity parameters recorded included mean elasticity (Emean), maximum elasticity (Emax), and minimum elasticity (Emin). Two experienced physicians independently performed all measurements, and interobserver agreement was assessed. ROC curves of shear wave elastography parameters for predicting impaired endometrial receptivity in infertile patients. (A) ROC curves of anterior and posterior wall Emean and Emax during the proliferative phase. (B) ROC curves of anterior and posterior wall Emean during the secretory phase. Emax, maximum elasticity; Emean, mean elasticity; ROC, receiver operating characteristic. Statistical analysis was conducted using IBM SPSS 25.0 (IBM Corp., Armonk, NY, USA). Continuous variables with a normal distribution were expressed as mean ± standard deviation and compared using the independent samples t-test. Non-normally distributed variables were presented as median [interquartile range (IQR)] and analyzed using the Kruskal-Wallis H test. Interobserver agreement for SWE measurements was evaluated using the intraclass correlation coefficient (ICC), calculated from a two-way random effects model (absolute agreement, single measures) with 95% confidence intervals (CIs). Receiver operating characteristic (ROC) curves were constructed to assess the predictive value of SWE-derived elasticity parameters for impaired ER, and the area under the curve (AUC) with 95% confidence intervals was calculated. A two-sided P value <0.05 was considered statistically significant.

A total of 169 participants were included, comprising 114 infertile patients and 55 healthy controls aged 28–35 years. As summarized in Table 1 , there were no significant differences between the two groups in terms of age (P=0.128), menstrual cycle phase distribution (P=0.386), abortion history (P=0.194), or the number of prior procedures (P=0.936). However, the frequency of hysteroscopy was significantly higher in the infertile group compared with the control group (P=0.010). Data are presented as mean ± standard deviation or number. Multiple procedures: defined as the sum of hysteroscopies and abortion episodes ≥2. No significant differences in endometrial thickness were observed between the infertile and control groups in the proliferative phase (P=0.323) and in the secretory phase (P=0.346). Similarly, no significant differences in endometrial pattern distribution were observed between the groups in the proliferative phase (P=0.446) or in the secretory phase (P=0.202) ( Table 2 ). Within-group comparisons revealed significantly increased endometrial thickness during the secretory phase compared to the proliferative phase in both groups (both P<0.001) ( Table S1 ). Data are presented as median (interquartile range), mean ± standard deviation or number. Type A: triple-line pattern (hyperechoic outer and central lines with a hypoechoic area in between); Type B: uniform intermediate echogenicity with indistinct central line; Type C: homogeneous hyperechogenicity without visible central line. Interobserver agreement of endometrial elasticity measurements was evaluated using two methods ( Table 3 ). Both methods demonstrated excellent consistency, with ICC values exceeding 0.75 for all indices. Method 2 (elliptical ROI) showed superior reproducibility compared with Method 1 for Emean (0.996, 95% CI: 0.994–0.997 vs. 0.956, 95% CI: 0.932–0.971), Emax (0.989, 95% CI: 0.985–0.992 vs. 0.850, 95% CI: 0.785–0.894), and Emin (0.970, 95% CI: 0.958–0.979 vs. 0.934, 95% CI: 0.912–0.951). Emean consistently exhibited the highest interobserver agreement and was therefore selected as the primary index for subsequent analysis. CI, confidence interval; Emax, maximum elasticity; Emean, mean elasticity; Emin, minimum elasticity; SWE, shear wave elastography. During the proliferative phase, infertile patients showed significantly higher Emean and Emax values than healthy controls in both the anterior wall [Emean: 32.26 (IQR, 24.28–49.11) vs. 23.06 (IQR, 17.76–27.65), P<0.001; Emax: 63.47 (IQR, 44.20–84.46) vs. 41.47 (IQR, 30.07–50.82), P<0.001] and the posterior wall [Emean: 34.91±16.10 vs. 26.77±8.03, P=0.002; Emax: 62.01 (IQR, 43.26–76.03) vs. 46.65 (IQR, 39.38–53.51), P=0.021). No significant differences were observed for Emin in either wall (both P>0.05) ( Table 4 ). During the secretory phase, infertile patients showed significantly higher Emean compared with healthy controls in both the anterior wall (20.18±9.92 vs. 16.10±5.08, P=0.019) and the posterior wall (21.23±10.00 vs. 16.00±6.19, P=0.006). In contrast, no significant differences were observed between infertile patients and healthy controls for Emax or Emin in either the anterior wall or the posterior wall (all P>0.05) ( Table 5 ). Data are presented as median (interquartile range) or mean ± standard deviation. Emax, maximum elasticity; Emean, mean elasticity; Emin, minimum elasticity. Data are presented as median (interquartile range) or mean ± standard deviation. Emean, mean elasticity; Emax, maximum elasticity; Emin, minimum elasticity. In the healthy control group, Emean and Emin values differed significantly between proliferative and secretory phases (all P0.05) ( Table S2 ). In the infertile group, all three elasticity parameters (Emean, Emax, and Emin) showed significant reductions from proliferative to secretory phases (all P<0.01) ( Table S3 ). During the proliferative phase, anterior wall Emean and Emax showed good predictive value for impaired ER (Emean: AUC =0.775, P<0.01; Emax: AUC =0.784, P<0.01), whereas posterior wall Emean and Emax showed moderate performance (Emean: AUC =0.634, P=0.040; Emax: AUC =0.651, P=0.021) ( Figure 1A ). In the secretory phase, only posterior Emean showed modest predictive ability (AUC =0.659, P=0.023), while anterior Emean was not significant (AUC =0.629, P=0.064) ( Figure 1B ). These results suggest that SWE-derived anterior wall elasticity, particularly during the proliferative phase, may help assess ER in infertile patients.

This prospective study demonstrated that transvaginal SWE is a feasible and reproducible tool to assess endometrial elasticity in patients with unexplained infertility. The endometrium displayed lower stiffness (elastic modulus values) in the secretory phase compared to the proliferative phase, and infertile patients showed significantly higher elastic modulus values than healthy controls in both phases. These findings suggest that SWE may serve as a noninvasive biomechanical biomarker of ER, potentially aiding in clinical fertility evaluations. SWE has previously been applied in various clinical areas, such as liver fibrosis, breast tumor differentiation, and musculoskeletal evaluations ( 18 - 22 ). In gynecology, it has shown promise for assessing uterine lesions and cervical maturation ( 23 - 25 ). However, its application in evaluating ER remains exploratory. One challenge is the lack of standardized protocols for endometrial elasticity measurement; notably, the International Society of Ultrasound in Obstetrics and Gynecology has not defined reference depths for evaluating the functional and basal layers. Additionally, variability in ROI placement across studies may affect accuracy and reproducibility. In this study, we tested two methods: Method 1 used conventional multi-point ROI sampling, while Method 2 adopted a regional fusion analysis across the whole endometrium. ICC analysis confirmed both methods were consistent, but Method 2 showed higher reproducibility, likely due to more comprehensive sampling and reduced influence of localized heterogeneity. Thus, Method 2 was used for the final analysis. Our results align with Wang et al. ( 26 ), who reported reduced endometrial stiffness in the secretory phase, likely due to histological remodeling. During the proliferative phase, estrogen-driven glandular proliferation and stromal density increase collagen content, resulting in higher stiffness. In contrast, the secretory phase under progesterone influence is marked by stromal edema, angiogenesis, and extracellular matrix (ECM) loosening ( 27 ), softening the tissue to support embryo implantation. These physiological changes are consistent with the elasticity shifts observed in SWE measurements. Compared with a previous SWE study on ER ( 14 ) that pooled anterior and posterior endometrial wall elasticity values into a single global average, our study separately recorded and analyzed the two walls. This design allows detection of potential regional differences in endometrial stiffness, which may exist due to variations in glandular density or vascularization between the anterior and posterior endometrium. Moreover, uterine position can affect probe contact angle and pressure on each endometrial wall; separate analysis reduces the risk of pooling‑induced bias and provides a more nuanced assessment. Thus, the separate recording approach enhances measurement transparency and may offer a framework for identifying localized endometrial abnormalities in future research. Clinically, this approach could guide individualized embryo transfer strategies if regional stiffness differences are found to impact implantation. Compared with healthy controls, infertile patients demonstrated higher endometrial elasticity in both phases. For the proliferative phase, anterior wall Emax ≥54.27 showed the best discriminative performance, while posterior wall Emean ≥18.71 was optimal during the secretory phase. These increased stiffness values may reflect underlying pathological alterations such as chronic inflammation, hormonal imbalances, or previous intrauterine procedures. Such conditions may activate transforming growth factor-β (TGF-β) signaling, promote collagen deposition, disrupt matrix metalloproteinase/tissue inhibitor of metalloproteinase (MMP/TIMP) balance, and cause ECM fibrosis—mechanisms linked to poor ER ( 28 ). Impaired vascularization and hypoxia may also contribute to stiffness and reduce tissue remodeling capacity ( 29 ). Additionally, progesterone resistance—a feature in some infertile patients—could impair stromal response and diminish cyclic softness in the secretory phase. Compared with conventional 2D and color Doppler ultrasound, which rely on morphological and hemodynamic indicators and are limited by operator dependence and inconsistent criteria ( 30 ), SWE provides quantitative biomechanical measurements that may more objectively reflect ER. Moreover, although endometrial receptivity arrays (ERA) are considered the gold standard for molecular ER assessment, they are invasive and costly, limiting their routine clinical use ( 30 ). SWE, as a noninvasive technique, offers a potential alternative for early discriminative evaluation of ER in infertile patients. While these findings are promising, several limitations exist. First, this was a single-center study with a relatively small and unevenly distributed sample size, which may limit generalizability. Second, the study focused on SWE parameters only and did not integrate other sonographic or biochemical predictors of ER. Third, there was no comparison with a gold standard such as ERA testing, nor were other ER‑related factors (e.g., endometrial microbiome) assessed. Besides, this study did not include implantation outcomes, clinical pregnancy follow-up, or live birth data. Therefore, impaired ER was inferred from infertility status rather than validated biological or clinical endpoints. In the absence of outcome-based validation or comparison with established molecular markers, the assumption that altered elasticity parameters directly reflect true ER remains speculative. Accordingly, the biological and clinical significance of these findings should be interpreted with caution. Moreover, when comparing endometrial elasticity between the proliferative and secretory phases, this study used a cross-sectional design with independent cohorts rather than a longitudinal within-subject design that follows the same individuals across two menstrual cycles. This approach may introduce inter-individual variability as a confounder, potentially affecting the accurate assessment of phase-dependent changes. Furthermore, menstrual phase assignment was based on LMP calculation with proportional adjustments. This may introduce bias in phase classification and affect the interpretation of SWE measurements. Several potential confounders were not fully controlled. We acknowledge that the potential impact of uterine position on SWE measurements was not formally evaluated, which constitutes a limitation of this study. Specifically, we did not collect data on gravidity, parity, body mass index (BMI), smoking and alcohol history, or classification of infertility; consequently, we were unable to adjust for these factors or retrieve the missing data retrospectively. Although recent hormonal use was excluded in the control group, cumulative hormonal exposure beyond three months was not assessed. These factors may have influenced endometrial elasticity measurements and should be considered when interpreting the findings. Future multicenter studies combining SWE with histopathology or molecular markers [e.g., collagen type, vascular endothelial growth factor (VEGF) expression] are needed to validate the biological significance of elasticity changes. We also recommend that future studies adopt a within-subject longitudinal design to more reliably reveal cycle-dependent endometrial elasticity changes.

This study suggests that SWE can noninvasively reflect cyclical and pathological changes in endometrial elasticity. With standardized protocols and further validation, SWE might offer an objective, biomechanical method for assessing ER in infertility management.

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