Impact of a Thickened Endometrium on Pregnancy Outcomes in Frozen-Thawed Embryo Transfer: A Retrospective Cohort Study

Impact of a Thickened Endometrium on Pregnancy Outcomes in Frozen-Thawed Embryo Transfer: A Retrospective Cohort Study | Authorea try { document.documentElement.classList.add('js'); } catch (e) { } var _gaq = _gaq || []; _gaq.push(['_setAccount', 'G-8VDV14Y67G']); _gaq.push(['_trackPageview']); (function() { var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true; ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js'; var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s); })(); Skip to main content Preprints Collections Wiley Open Research IET Open Research Ecological Society of Japan All Collections About About Authorea FAQs Contact Us Quick Search anywhere Search for preprint articles, keywords, etc. Search Search ADVANCED SEARCH SCROLL This is a preprint and has not been peer reviewed. Data may be preliminary. 27 January 2026 V1 Latest version Share on Impact of a Thickened Endometrium on Pregnancy Outcomes in Frozen-Thawed Embryo Transfer: A Retrospective Cohort Study Authors : Xiaorao Wang , Jing Li , Xiaohan Zhang , Xi Zhang , Keke Wei , Hong Lv 0000-0003-4978-5120 , Yanbo Du , and lei yan 0000-0001-6548-1317 [email protected] Authors Info & Affiliations https://doi.org/10.22541/au.176954411.11873914/v1 122 views 70 downloads Contents Abstract Supplementary Material Information & Authors Metrics & Citations View Options References Figures Tables Media Share Abstract Objective: To assess the impact of an excessively thick endometriumon pregnancy outcomes in frozen-thawed embryo transfer (FET) cycles. Design: Retrospective cohort study. Setting: A university-affiliated reproductive medicine center. Population: A total of 6,222 FET cycles (involving 5,434 women) with an endometrial thickness (EMT) ≥11 mm performed between January 2018 and December 2023. Methods: The association between EMT and outcomes was visualized using dual-axis combination plots and conditional density plots (CDPs). To account for the correlation between multiple cycles within the same patient and potential confounders, generalized estimating equations (GEE) were employed. Main Outcome Measures: Clinical pregnancy rate (CPR) and live birth rate (LBR). Results: Baseline characteristics were comparable across five EMT groups (11 to <12, 12 to <13, 13 to <14, 14 to <15, and ≥15 mm), except for the distribution of preparation protocols. CDPs revealed that both CPR and LBR remained highly stable across the EMT spectrum of 11–15 mm, irrespective of cycle type (natural or hormone replacement therapy). Multivariable GEE models showed no significant interaction between EMT and preparation protocols. Compared to the reference group (EMT 11 to 0.05). Significant independent predictors of LBR were female age at transfer (adjusted odds ratio [aOR] 0.95), embryo quality, body mass index (aOR 0.98), and the use of preimplantation genetic testing (aOR 1.55). Sensitivity analyses, which included redefining the thickest category as EMT ≥14 mm and restricting the cohort to first-cycle patients only, confirmed the robustness of the observed ”plateau effect”. Conclusions: In FET cycles, increased thickness beyond 11 mm does not provide additional benefits for, nor detriment to, LBR. The practice of canceling or deferring transfers based solely on the presence of an ”overly thick” endometrium lacks empirical justification. Introduction Endometrial receptivity is widely recognized as one of the main limiting factors in the success of assisted reproductive technology (ART), particularly within frozen-thawed embryo transfer (FET) cycles. Among various markers, endometrial thickness (EMT) serves as the most commonly used prognostic indicator for evaluating receptivity in clinical practice. Although the relationship between EMT and FET outcomes remains debated, a thin endometrium is consistently associated with reduced pregnancy success 1,2 . In contrast,. the clinical implications of an excessively thick endometrium is unclear, with existing literature showing considerable heterogeneity. Current evidence suggests that an EMT of 9–11 mm represents an optimal ”comfort zone” for most patient datasets 3-5 ; beyond this range, the data diverge significantly. A threshold of ≥14 mm is often utilized to define a thick endometrium 6-9 . For instance, Yoeli et al. 10 identified 14 mm as the 95th percentile in a large cohort, while Zhang et al. 11 reported a curvilinear relationship between EMT and pregnancy outcomes, proposing an optimal interval of 8.7–14.5 mm for cleavage-stage FET cycles. Furthermore, Griesinger et al. noted that the majority of patients (85.65%) exhibit an EMT between 8 and 15 mm on transfer day 12 ; consequently, several studies have adopted 15 mm as a critical clinical cutoff 3,13 . Even higher thresholds, such as 17 mm 14,15 , have also been proposed. The effect of increased EMT on reproductive outcomes is far from settled. While some studies associate greater thickness with higher pregnancy rates 15-19 , others indicate potential negative effects. 7,20,21 . Moreover, a significant body of research suggests that EMT may have limited independent predictive value, particularly as advances in embryo culture and selection could mitigate its influence 8,12,22 . These inconsistencies may arise from differences in ultrasound timing (e.g., progesterone administration day vs. embryo transfer day), endometrial preparation protocols, embryo developmental stage (cleavage vs. blastocyst), and ploidy status. To clarify the specific influence of an excessively thick endometrium on clinical pregnancy rates (CPR) and live birth rates (LBR), and to identify a potential threshold beyond which outcomes decline, we conducted this large-scale retrospective cohort study. We aim to provide evidence-based guidance for managing patients with thickened endometrium in FET cycles. Materials and methods Inclusion and Exclusion Criteria This retrospective cohort study was conducted at the Reproductive Hospital Affiliated to Shandong University. We analyzed women undergoing single frozen-thawed blastocyst transfer between January 2018 and December 2023 with an EMT ≥11 mm. Exclusion criteria included age ≥40 years, body mass index (BMI) ≥30kg/m 2 , the presence of submucosal fibroids, adenomyosis or uterine malformations, and double embryo transfers. Cycles with a non-trilaminar endometrial pattern on the final ultrasound prior to progesterone exposure were also excluded. Modified natural cycles and ovarian stimulation FET cycles were not included due to their limited frequency and protocol variability. All eligible treatment cycles from each patient were included analyzed. IVF/ICSI Protocols Oocyte retrieval and embryo culture followed standard institutional protocols 23 . Blastocysts were morphologically graded as: excellent (AA), good (AB, BA), average (AC, BB), or poor (BC) 24 . Prior to FET, all patients underwent uterine assessment via routine transvaginal ultrasound (TVU), supplemented by saline sonohysterography and/or hysteroscopy to exclude intrauterine pathology. Two primary protocols were utilized for endometrial preparation.The natural cycle (NC) protocol was applied to patients with regular menstrual cycles and spontaneous ovulation. Follicular development and EMT were monitored via TVU and serial measurements of serum estradiol, progesterone, and luteinizing hormone (LH) starting on cycle days 10–12. Day 0 (D0) was defined as the day of spontaneous ovulation or of human chorionic gonadotropin (hCG) trigger (administered when the dominant follicle reached 16–22 mm). Luteal phase support was then initiated, with blastocyst transfer occurring on day 5 (D5). For hormone replacement therapy (HRT) cycles, oral estradiol valerate (4–8 mg/d) was started on cycle days 1–3, with dosage adjustments based on individual endometrial response. After 10–14 days of estrogen therapy, EMT was assessed via TVU. Once adequate thickness was achieved, progesterone administration began (designated as D0) for endometrial transformation, with blastocyst transfer on D5. All patients received luteal phase support starting from D0 using oral dydrogesterone (20–40 mg/d), oral micronized progesterone (200–300 mg/d), or vaginal micronized progesterone (100–200 mg/d). Support continued until 8–10 weeks of gestation if a clinical pregnancy was confirmed. In NC protocol, EMT was recorded either on the day of hCG administration or during the last TVU performed prior to ovulation. For HRT cycles, EMT from the final TVU conducted before progesterone initiation was used. All scans were performed by certified sonographers using TVU. EMT was measured in the midsagittal plane at its maximum thickness, determined as the distance between the opposing outer borders of the double-layer endometrium (endometrial-myometrial interface). Outcome measures The primary outcome measures were CPR and LBR. Clinical pregnancy was defined as the ultrasonographic visualization of at least one gestational sacs. Live birth was defined as the delivery of any viable infant at or beyond 28 weeks of gestation. Statistical analysis Statistical analyses were performed using RStudio (R version 4.3.1). Continuous variables are presented as median and interquartile range (IQR), and categorical variables as counts and percentages. EMT was analyzed both as a continuous variable and as a categorical variable divided into five groups: 11 to <12 mm, 12 to <13 mm, 13 to <14 mm, 14 to <15 mm, and ≥15 mm. The association between EMT and outcomes was visualized using dual-axis combination plots and conditional density plots (CDPs). CDPs provided a smoothed, high-fidelity visualization of the raw probability of success across the continuous EMT spectrum. To account for the correlation between multiple cycles within the same patient and potential confounders, generalized estimating equations (GEE) were employed. The regression model adjusted for female age at transfer, BMI, endometrial preparation protocol, embryo quality, and fertilization method. Statistical significance was set at a two-sided P <0.05. Study Population and Baseline Characteristics A total of 6,222 cycles from 5,434 women were included in the study. Of these, 4,285 (68.9%) were HRT cycles and 1,937 (31.1%) were NCs. As summarized in Table 1, baseline characteristics were well-balanced across the five EMT groups, with no significant differences observed in female age at oocyte retrieval and at embryo transfer, BMI, infertility duration, infertility type, fertilization method, cause of infertility, day of embryo development at transfer, or embryo quality. However, significant intergroup differences were noted in the endometrial preparation protocols. Notably, the CPR and LBR remained comparable across these groups. EMT Distribution and Pregnancy Outcomes Stratification by cycle type revealed distinct patterns in EMT distribution and its relationship with clinical outcomes (Fig. 1). In NCs, the prevalence of cycles declined sharply as EMT exceeded 13 mm; however, CPR and LBR remained relatively stable across the observed range, with the highest rates seen in the 14 to <15 mm group (Fig. 1A). In hormone replacement cycles, CPR showed a localized peak within the 14 to <15 mm range, whereas the LBR exhibited a gradual, non-significant downward trend as EMT increased to ≥15 mm (Fig. 1B). The continuous relationship between EMT and pregnancy outcomes was further elucidated using CDPs (Fig. 2). For the total cohort and the NC subgroup, both CPR and LBR exhibited remarkable stability within the 11–15 mm EMT range. Similarly, A similar plateau was observed in HRT cycles between 11 mm and 14 mm. Although pronounced fluctuations appeared at the terminal ends of the plots, these coincided with substantially smaller sample sizes. These fluctuations included a transient decline in success rates around 15 mm, followed by a rebound toward 16 mm. Impact of EMT on Pregnancy Outcomes Statistical analysis revealed no significant interaction between EMT groups and endometrial preparation protocols regarding LBR (all P > 0.05). Consequently, the interaction term was omitted from the final GEE model. Multivariable GEE analysis, using the 11 to <12 mm category as reference, revealed no statistically significant differences in the adjusted odds ratios (aORs) for LBR in any of the thicker endometrial categories (12 to <13 mm, 13 to <14 mm, 14 to covariates significantly associated with LBR included female age at transfer (aOR 0.95, 95% CI 0.94–0.96, P < 0.001) and embryo quality, with ”Good”, ”Average”, and ”Poor” grade embryos showing progressively lower odds of LBR compared to ”Excellent” embryos (all P < 0.05). Additionally, the use of preimplantation genetic testing (PGT) was associated with significantly higher odds of live birth (aOR 1.55, 95% CI 1.30–1.85, P < 0.001). BMI also showed a small but significant negative association with LBR (aOR 0.98, CI 0.97–1.00, P = 0.047). No significant differences were observed regarding fertilization methods (excluding PGT) or endometrial preparation protocols (Table 2). Sensitivity Analyses A sensitivity analysis re-categorizing the thickest endometrial group as ≥14 mm was performed (Supplementary Table S1). The GEE model confirmed that this threshold was not significantly associated with increased odds of LBR compared to the reference group (aOR 1.10, 95% CI 0.86–1.40, P = 0.457). All other variables maintained their significance consistent with the primary analysis. A second sensitivity analysis restricted to each patient’s first cycle (n=5,250 cycles) yielded consistent results, with no significant association between EMT category and LBR (Supplementary Table S2). In this subgroup, the aOR for LBR in the ≥15 mm group was 1.27 (P = 0.249). Notably, within this first-cycle analysis, BMI and ”Good” embryo quality did not reach statistical significance as predictors of LBR. Discussion In the present study, our data showed that in patients undergoing FET with an EMT ≥11 mm, further increases in thickness are not significantly associated with improved LBRs. This suggests a ”plateau effect”, where endometrial receptivity remains stable despite continued morphological thickening beyond this threshold. Moreover, the relationship between EMT and LBR was consistent across different endometrial preparation protocols, implying that the method of preparation does not meaningfully alter the impact of thickness on clinical success. The CDPs illustrate that for both NCs and HRT protocols, an EMT ranging from 11 mm to 16 mm does not result in a detrimental decline in clinical success. While minor fluctuations were observed at the upper extremes (≥15 mm), no biologically significant threshold was identified. These variations likely reflect statistical instability due to the limited sample sizes in the thickest categories rather than a true decline in endometrial receptivity. This observation aligns with a recent large international multicenter cohort study of 30,676 women, which similarly reported stable LBR across an EMT range of 11–15 mm25. Consistent with the graphical analyses, our GEE analyses confirmed that an extremely thick endometrium (≥15 mm) does not confer additional benefit. This conclusion is strongly supported by sensitivity analyses: even when redefining the upper threshold as ≥14 mm or restricting the cohort to first-cycle FET patients, EMT remained a non-significant predictor of LBR.This indicates that the plateau effect reflects a biological reality rather than an artifact of patient selection or repeated cycles. The analysis of first-cycle patients is particularly informative, as it minimizes the potential confounding from repeated implantation failure and reduces healthy patient bias often present in retrospective studies. Our results are concordant with several previous reports. Yuric et al.13 found no significant differences in clinical pregnancy or LBR between EMT ≥15 mm and <15 mm in ICSI cycles, with similar results using a 14 mm cutoff. Comparable findings were reported in first-cycle IVF-ET patients8 and across ovarian response subgroups18, although these studies mainly involved fresh embryo transfers. In the context of FET, Check et al.26 concluded that an EMT >14.5 mm does not compromise outcomes. Another study indicated that an EMT ≥14 mm, compared to a reference range of 10–11.9 mm, was not associated with significant alterations in birth weight categories or preterm birth risk27. However, some evidence suggests a positive correlation between an excessively thick endometrium and improved pregnancy outcomes; for instance, Bu et al.17 reported higher CPR and LBR in women over 35 years with EMT >14 mm, though this subgroup constituted only 2% of their cohort. Others have suggested improved outcomes in patients with EMT >14 mm, with peak LBR noted above 17 mm15,28. In contrast, several studies indicate a negative impact, such as El-Toukhy et al.29, who reported higher CPR with an EMT of 9–14 mm, and Zhang et al.11, who observed slightly lower implantation rates, CPRs and LBRs in cleavage-stage FET cycles when EMT exceeded 14.5 mm. One multicenter cohort even suggested a significant decline in LBR above 14 mm, reaching its lowest point in the 17–25 mm range30; however, that study did not exclude patients with potentially intrauterine pathologies. It has been hypothesized that an abnormally thickened endometrium may impair outcomes due to catheter-related trauma during transfer7 or underlying histological abnormalities such as hyperplasia, polyps, or fibroids31-33. Most prior studies measure EMT on the day of embryo transfer11,12,17,26,34, but since EMT is not routinely re-measured once deemed adequate at progesterone initiation, evaluating its predictive value at the time of progesterone administration holds greater clinical relevance. However, existing evidence focusing on thick endometrium at this precise timepoint remains limited and has not yielded clear or consistent conclusions25,27,29. Endometrial preparation protocols are often considered influential in FET outcomes. Our data suggest that the relationship between EMT and LBR is independent of the protocol used. While a recent large multicenter study observed an interaction between EMT and certain protocols (such as HRT and modified NC), that study did not specifically focus on the ”overly thick” endometrium25. After simplifying our GEE model by removing the interaction term, we found that the protocol type was not predictive of FET outcomes, consistent with the findings of Eleftheriadou et al30. Notably, some research has suggested that NC may offer superior LBR compared to HRT in single euploid blastocyst transfers35. Since our study did not account for embryo aneuploidy—a major determinant of implantation failure—these results should be interpreted with caution. Furthermore, our analysis highlights that once a functional EMT threshold of 11 mm is reached, embryonic factors, rather than further endometrial proliferation, become the primary determinants of live birth. The significant negative correlation between female age and LBR, coupled with the superior outcomes observed in PGT-verified euploid transfers, underscores the critical role of embryonic competence. In patients with thick endometria, clinical optimization efforts might be more effectively directed toward rigorous embryo selection rather than further modulation of EMT. Strengths and limitations This study have several strengths. First, the inclusion of multiple treatment cycles per patient offer a realistic reflection of clinical practice. Second, the use of CDPs enabled continuous visualization of pregnancy trends and captured subtle fluctuations that categorical analysis might miss. Finally, sensitivity analyses with alternate thickness thresholds and restriction to first-cycle patients reinforce the robustness of our conclusions. Several limitations should also be acknowledged. First, although the overall cohort is large, the number of patients with EMT exceeding 16 mm remains relatively small. This reflects the natural biological distribution of EMT but results in wider confidence intervals and greater volatility at the upper extreme of our plots, necessitating cautious interpretation for ultra-thick endometria. Second, as this study exclusively enrolled patients with an EMT ≥11 mm, we did not explore the lower boundaries of the plateau effect, as our focus was specifically on addressing the clinical dilemma of the overly thick endometrium. Finally, inter-observer variability in ultrasound measurements, though minimized by experienced personnel, remains an inherent limitation of retrospective designs. Conclusion An EMT ≥15 mm is not associated with a decline in LBRs, nor does it confer additional reproductive benefit, irrespective of the endometrial preparation protocol used. Therefore, the clinical practice of delaying or canceling FET cycles solely due to an overly thick endometrium lacks empirical support and warrants reconsideration. References 1. Gao G, Cui X, Li S, Ding P, Zhang S, Zhang Y. Endometrial thickness and IVF cycle outcomes: a meta-analysis. Reprod Biomed Online. 2020;40(1):124-133.2. Momeni M, Rahbar MH, Kovanci E. A meta-analysis of the relationship between endometrial thickness and outcome of in vitro fertilization cycles. Journal of human reproductive sciences. 2011;4(3):130-137.3. Ma NZ, Chen L, Dai W, Bu ZQ, Hu LL, Sun YP. Influence of endometrial thickness on treatment outcomes following in vitro fertilization/intracytoplasmic sperm injection. Reproductive biology and endocrinology : RB&E. 2017;15(1):5.4. Gallos ID, Khairy M, Chu J, et al. Optimal endometrial thickness to maximize live births and minimize pregnancy losses: Analysis of 25,767 fresh embryo transfers. Reprod Biomed Online. 2018;37(5):542-548.5. 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Figure Legends： Fig. 1. Distribution of endometrial thickness and its association with clinical pregnancy and live birth rates. Fig.2. Conditional density plots showing the crude association of clinical pregnancy and live birth with endometrial thickness. Plots are truncated to 11–16 mm range which contains 99.7% of all observations in the population. Supplementary Material File (supplementary table s1.docx) Download 20.17 KB File (supplementary table s2.docx) Download 20.28 KB File (table 1.docx) Download 26.49 KB File (table 2.docx) Download 20.28 KB Information & Authors Information Version history V1 Version 1 27 January 2026 Copyright This work is licensed under a Non Exclusive No Reuse License. Keywords fertility and assisted reproduction infertility: assisted conception reproductive science: endometrial function Authors Affiliations Xiaorao Wang Shandong University View all articles by this author Jing Li Shandong University View all articles by this author Xiaohan Zhang Shandong University View all articles by this author Xi Zhang Puyang People's Hospital View all articles by this author Keke Wei Shandong University View all articles by this author Hong Lv 0000-0003-4978-5120 Shandong University View all articles by this author Yanbo Du Shandong University View all articles by this author lei yan 0000-0001-6548-1317 [email protected] Shandong University View all articles by this author Metrics & Citations Metrics Article Usage 122 views 70 downloads .FvxKWukQNSOunydq8rnd { width: 100px; } Citations Download citation Xiaorao Wang, Jing Li, Xiaohan Zhang, et al. Impact of a Thickened Endometrium on Pregnancy Outcomes in Frozen-Thawed Embryo Transfer: A Retrospective Cohort Study. Authorea . 27 January 2026. DOI: https://doi.org/10.22541/au.176954411.11873914/v1 If you have the appropriate software installed, you can download article citation data to the citation manager of your choice. Simply select your manager software from the list below and click Download. For more information or tips please see 'Downloading to a citation manager' in the Help menu . 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