Effect of Endometrial Thickness Change in Response to Progesterone Administration on Pregnancy Outcomes in Frozen-Thawed Embryo Transfer: Analysis of 4465 Cycles

Endometrial compaction to predict pregnancy outcomes in patients undergoing assisted reproductive technologies: a systematic review and meta-analysis

STUDY QUESTION

Does endometrial compaction (EC) help predict pregnancy outcomes in those undergoing ART?

SUMMARY ANSWER

EC is associated with a significantly higher clinical pregnancy rate (CPR) and ongoing pregnancy rate (OPR), but this does not translate to live birth rate (LBR).

WHAT IS KNOWN ALREADY

EC describes the progesterone-induced decrease in endometrial thickness, which may be observed following the end of the proliferative phase, prior to embryo transfer. EC is proposed as a non-invasive tool to help predict pregnancy outcome in those undergoing ART, however, published data is conflicting.

STUDY DESIGN SIZE DURATION

A literature search was carried out by two independent authors using PubMed, Cochrane Library, MEDLINE, Embase, Science Direct, Scopus, and Web of Science from inception of databases to May 2023. All peer-reviewed studies reporting EC and pregnancy outcomes in patients undergoing IVF/ICSI treatment were included.

PARTICIPANTS/MATERIALS SETTING METHODS

The primary outcome is LBR. Secondary outcomes included other pregnancy metrics (positive pregnancy test (PPT), CPR, OPR, miscarriage rate (MR)) and rate of EC. Comparative meta-analyses comparing EC and no EC were conducted for each outcome using a random-effects model if I 2 > 50%. The Mantel-Haenszel method was applied for pooling dichotomous data. Results are presented as odds ratios (OR) with 95% CI.

MAIN RESULTS AND THE ROLE OF CHANCE

Out of 4030 screened articles, 21 cohort studies were included in the final analysis (n = 27 857). No significant difference was found between LBR in the EC versus the no EC group (OR 0.95; 95% CI 0.87-1.04). OPR was significantly higher within the EC group (OR 1.61; 95% CI 1.09-2.38), particularly when EC ≥ 15% compared to no EC (OR 3.52; 95% CI 2.36-5.23). CPR was inconsistently defined across the studies, affecting the findings. When defined as a viable intrauterine pregnancy <12 weeks, the EC group had significantly higher CPR than no EC (OR 1.83; 95% CI 1.15-2.92). No significant differences were found between EC and no EC for PPT (OR 1.54; 95% CI 0.97-2.45) or MR (OR 1.06; 95% CI 0.92-1.56). The pooled weighted incidence of EC across all studies was 32% (95% CI 26-38%).

LIMITATIONS REASONS FOR CAUTION

Heterogeneity due to differences between reported pregnancy outcomes, definition of EC, method of ultrasound, and cycle protocol may account for the lack of translation between CPR/OPR and LBR findings; thus, all pooled data should be viewed with an element of caution.

WIDER IMPLICATIONS OF THE FINDINGS

In this dataset, the significantly higher CPR/OPR with EC does not translate to LBR. Although stratification of women according to EC cannot currently be recommended in clinical practice, a large and well-designed clinical trial to rigorously assess EC as a non-invasive predictor of a successful pregnancy is warranted. We urge for consistent outcome reporting to be mandated for ART trials so that data can be pooled, compared, and concluded on.

STUDY FUNDING/COMPETING INTERESTS

H.A. was supported by the Hewitt Fertility Centre. S.G.P. and J.W. were supported by the Liverpool University Hospital NHS Foundation Trust. D.K.H. was supported by a Wellbeing of Women project grant (RG2137) and MRC clinical research training fellowship (MR/V007238/1). N.T. was supported by the National Institute for Health and Care Research. D.K.H. had received honoraria for consultancy for Theramex and has received payment for presentations from Theramex and Gideon Richter. The remaining authors have no conflicts of interest to report.

REGISTRATION NUMBER

PROSPERO CRD42022378464.

Association between proliferative-to-secretory endometrial compaction and pregnancy outcomes after embryo transfer: a systematic review and meta-analysis

STUDY QUESTION

Does the change in endometrial thickness (EMT) from the end of the follicular/estrogen phase to the day of embryo transfer (ET) determine subsequent pregnancy outcomes?

SUMMARY ANSWER

Endometrial compaction from the late-proliferative to secretory phase is not associated with live birth rate (LBR) and other pregnancy outcomes.

WHAT IS KNOWN ALREADY

Endometrial compaction has been suggested to be indicative of endometrial responsiveness to progesterone, and its association with ET outcome has been investigated but is controversial.

STUDY DESIGN, SIZE, DURATION

A systematic review with meta-analysis was carried out. PubMed, EMBASE, and Web of Science were searched to identify relevant studies from inception to 18 November 2022. The reference lists of included studies were also manually screened for any additional publications.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Cohort studies comparing ET pregnancy outcomes between patients with and without endometrial compaction were included. A review of the studies for inclusion, data extraction, and quality assessment was performed by two independent reviewers. The effect size was synthesized as odds ratio (OR) with 95% CI using a random-effects model. Heterogeneity and publication bias were assessed by the I2 statistic and Egger's test, respectively. The primary outcome was LBR. Secondary outcomes included biochemical pregnancy rate (BPR), clinical pregnancy rate (CPR), miscarriage rate (MR), ongoing pregnancy rate (OPR), and ectopic pregnancy rate (EPR).

MAIN RESULTS AND THE ROLE OF CHANCE

Seventeen cohort studies involving 18 973 ET cycles fulfilled the eligibility criteria. The pooled results revealed that there were no significant differences between endometrial compaction and non-compaction groups in LBR (crude OR (cOR) = 0.95, 95% CI 0.87-1.04; I2 = 0%; adjusted OR (aOR) = 1.02, 95% CI 0.87-1.19, I2 = 79%), BPR (cOR = 0.93, 95% CI 0.81-1.06; I2 = 0%; aOR = 0.88, 95% CI 0.75-1.03, I2 = 0%), CPR (cOR = 0.98, 95% CI 0.81-1.18; I2 = 70%; aOR = 0.86, 95% CI 0.72-1.02, I2 = 13%), MR (cOR = 1.09, 95% CI 0.90-1.32; I2 = 0%; aOR = 0.91, 95% CI 0.64-1.31; I2 = 0%), and EPR (cOR = 0.70, 95% CI 0.31-1.61; I2 = 61%). The OPR was marginally higher in crude analysis (cOR = 1.48, 95% CI 1.01-2.16; I2 = 81%) among women with compacted endometrium, but was not evident in adjusted results (aOR = 1.36, 95% CI 0.86-2.14; I2 = 84%). Consistently, the pooled estimate of LBR remained comparable in further subgroup and sensitivity analyses according to the degree of compaction (0%, 5%, 10%, 15%, or 20%), type of ET (fresh, frozen, or euploid only), and endometrial preparation protocol (natural or artificial). No publication bias was observed based on Egger's test.

LIMITATIONS, REASONS FOR CAUTION

Although the number of included studies is sufficient, data on certain measures, such as EPR, are limited. The inherent bias and residual confounding were also inevitable owing to the observational study design. Furthermore, inconsistent definitions of pregnancy outcomes may affect the accuracy of our pooled analysis.

WIDER IMPLICATIONS OF THE FINDINGS

Given the lack of prognostic value, assessing endometrial compaction or repeated EMT measurement on the day of ET may not be necessary or warranted.

STUDY FUNDING/COMPETING INTEREST(S)

This work was supported by Natural Science Foundation of Jiangxi Province (20224BAB216025), National Natural Science Foundation of China (82260315), and Central Funds Guiding the Local Science and Technology Development (20221ZDG020071). The authors have no conflicts of interest to declare.

REGISTRATION NUMBER

CRD42022384539 (PROSPERO).

Does endometrial compaction before embryo transfer affect pregnancy outcomes? a systematic review and meta-analysis

Objective

There is no clear evidence of clinical significance of endometrial compaction, which can be measured by a reduction in endometrial thickness (EMT) during the follicular-luteal transition before the day of embryo transfer. In this study, we aim to determine whether endometrial compaction has an effect on in vitro fertilization (IVF) success.

Methods

We searched PubMed, Cochrane, Embase, and Web of Science electronic databases for studies published in English up to March 2023. Heterogeneity between studies was assessed using the I2 statistic. The random effects model and fixed effects model was used to pool the risk ratio (RR) with a corresponding 95% confidence interval (CI). A subgroup analysis was performed based on different methods of ultrasonic measurement and different endometrial compaction rates (ECR). Stata 17.0 software was used for meta-analysis. Pregnancy outcomes, which included clinical pregnancy rate, ongoing pregnancy rate, live birth rate, and spontaneous abortion rate, were evaluated.

Results

In this study, 18 cohort studies were included, involving 16,164 embryo transfer cycles. Pooled results indicated that there was no significant difference between the endometrial compaction group and the non-compaction group in terms of clinical pregnancy rate (RR [95% CI]=0.98 [0.90,1.08]; I2 = 69.76%), ongoing pregnancy rate (RR [95% CI]=1.18 [0.95,1.47]; I2 = 78.77%), live birth rate (RR [95% CI]= 0.97 [0.92,1.02]; I2 = 0.00%) or spontaneous abortion rate (RR [95% CI]= 1.07[0.97,1.26]; I2 = 0.00%). According to the subgroup analysis of ultrasonic measurement methods, in the transvaginal ultrasound (TVUS) combined with abdominal ultrasonography (AUS) cycles of the endometrial compaction group, the rate of ongoing pregnancy (RR [95% CI] = 1.69 [1.26, 2.26]; I2 = 29.27%) and live birth (RR [95% CI] = 1.27 [1.00,1.61]; I2 = 62.28%) was significantly higher than that of the non-compaction group. Additionally, subgroup analysis based on ECR revealed a significantly higher rate of ongoing pregnancy when ECR ≥ 15% (RR [95% CI] = 1.99 [1.61, 2.47]; I2 = 0.00%).

Conclusion

Endometrial compaction has no adverse effect on clinical pregnancy rate, ongoing pregnancy rate, live birth rate, or spontaneous abortion rate. A possible explanation for the contradictory findings of previous studies lies in the method by which the EMT is measured.

Systematic review registration

https://www.crd.york.ac.uk/prospero/display_record.php?ID=CRD42023430511, identifier CRD42023430511.

Impact of endometrial compaction on reproductive outcomes after cryotransfer of euploid embryos in a modified natural cycle: protocol for a prospective cohort study

Introduction

Embryo implantation is a complex and poorly understood process. Most studies to date have focused on the analysis of the endometrium at the end of the estrogenic phase, while the available data on its importance after secretory transformation are limited and inconsistent. Current evidence does not allow for a conclusive interpretation of the changes observed in the pre-implantation endometrium, whether in the natural or replacement cycle, and their relevance in the development of a pregnancy or the implications for clinical practice.

Methods

Multicenter prospective observational cohort study. Based on our sample size calculation, the study group will consist of 206 women (exposed or "compaction" group: 103 women with a decrease of ≥ 5% in endometrial thickness between the estrogenic phase and the day of embryo transfer; non-exposed "non-compaction" group: 103 women with similar or greater endometrial thickness between these time points). The main objective of this study is to compare the ongoing pregnancy rates in natural cycles for euploid embryo transfer in patients who present endometrial compaction at the time of transfer versus those who with a stable or greater endometrial thickness with respect to the estrogenic phase. The estimated duration of the study is 30 months. Inclusion criteria are: 18 to 50 years of age, with primary or secondary infertility, subjected to endometrial preparation in a modified natural cycle for transfer of a genetically euploid blastocyst, from their own oocyte or oocyte donation, with a normal uterine cavity. Exclusion criteria are: uterine or endometrial disease (e.g., multiple myomatosis, severe adenomyosis, Asherman syndrome, refractory endometrium), conditions that prevent correct ultrasound assessment (tilted uterus), or a history of recurrent implantation failure or repeated miscarriages.

Discussion

The findings from this study will provide valuable insights into the potential influence of the "endometrial compaction" phenomenon on reproductive outcomes during natural cycle endometrial preparation. By examining this aspect, we aim to contribute to a better understanding of the factors that may impact successful outcomes in fertility treatments.

Establishment of a prediction model for the impact of endometrial thickness on the day of embryo transfer on ectopic pregnancy in frozen-thawed embryo transfer cycle

Objective

This study aims to investigate the factors affecting the ectopic pregnancy (EP) rate in the frozen-thawed embryo transfer (FET) cycle.

Methods

This study retrospectively analyzed 5606 FET cycles, including 5496 cycles resulting in intrauterine pregnancy and 110 cycles resulting in EP. Smooth curve fitting and piece-wise linear regression were utilized to evaluate a non-linear association between endometrial thickness (EMT) and EP. Multiple logistic regression analysis was used to study the effect of EMT on the embryo transfer (ET) day and other indexes on EP rate after adjusting for confounding factors. A nomographic prediction model was employed to predict EP occurrence. The predictive efficacy of the model was assessed using the area under the receiver operating characteristic (ROC) curve (AUC), utilizing the bootstrap sampling method for internal validation.

Results

After accounting for the confounding factors, the segmented linear regression analysis indicated that the EMT inflection point was 9 mm; the EP rate significantly decreased by 28% with each additional millimeter of EMT up to 9 mm (odds ratio (OR) = 0.72; 95% confidence interval (CI), 0.53-0.99; P = 0.0412) while insignificantly decreased when the EMT was greater than 9 mm (OR = 0.91; 95% CI, 0.76-1.08; P = 0.2487). Multivariate logistic regression analysis revealed that after adjusting for confounders, EP risk significantly increased in the number of previous EPs ≥ 1 (OR = 2.29; 95% CI, 1.26-4.16; P = 0.0064) and tubal factor infertility (OR = 3.86; 95% CI, 2.06-7.24; P < 0.0001). Conversely, EP risk was significantly reduced by the increased EMT (OR = 0.84; 95% CI, 0.74-0.96; P = 0.0078) and the blastocyst transfer (OR = 0.45; 95% CI, 0.27-0.76; P = 0.0027). These variables were used as independent variables in a nomogram prediction model, resulting in an AUC of 0.685. The nomination models were internally verified using self-sampling (bootstrap sampling resampling times = 500). This validation yielded an AUC of 0.689, with a sensitivity of 0.6915 and a specificity of 0.5790. The internal validation indicated minimal fluctuations in the AUC, signifying a relatively stable model.

Conclusion

Undergoing EMT on the day of ET poses a separate EP risk in the FET cycle; to mitigate the EP incidence, the EMT should exceed 9 mm before ET. Furthermore, previous EPs and tubal factor infertility were additional factors independently increasing EP risk. Furthermore, implementing blastocyst transfer demonstrated that EP incidence was significantly reduced. Utilizing a nomogram predicting system enables EP risk evaluation before ET for individual patients, establishing a basis for devising clinical strategies for ET.

Can endometrial compaction predict live birth rates in assisted reproductive technology cycles? A systematic review and meta-analysis

PURPOSE

Endometrial compaction (EC) is defined as the difference in endometrial thickness from the end of the follicular phase to the day of embryo transfer (ET). We aimed to determine the role of EC in predicting assisted reproductive technology (ART) success by conducting a meta-analysis of studies reporting the association between EC and clinical outcomes of ART.

METHODS

MEDLINE via PubMed, Web of Science, Scopus, and the Cochrane Central Register of Controlled Trials (CENTRAL) were searched from the date of inception to May 19, 2023. The primary outcome was live birth rate (LBR) per ET. Secondary outcomes were live birth or ongoing pregnancy per ET, ongoing pregnancy per ET, clinical pregnancy per ET, and miscarriage per clinical pregnancy.

RESULTS

Fifteen studies were included. When data from all studies reporting live birth were pooled, overall LBR rates were comparable in cycles showing EC or not [RR = 0.97, 95%CI = 0.92 to 1.02; 10 studies, 11,710 transfer cycles]. In a subgroup of studies that included euploid ET cycles, a similar LBR for patients with and without EC was noted [RR = 0.99, 95%CI = 0.86 to 1.13, 4 studies, 1172 cycles]. The miscarriage rate did not seem to be affected by the presence or absence of EC [RR = 1.06, 95%CI = 0.90 to 1.24; 12 studies].

CONCLUSION

The predictive value of EC in determining LBR is limited, and assessment of EC may no longer be necessary, given these findings.

TRIAL REGISTRATION

PROSPERO CRD42023410389.

Preparation of the endometrium for frozen embryo transfer: an update on clinical practices

Over the past decade, the application of frozen-thawed embryo transfer treatment cycles has increased substantially. Hormone replacement therapy and the natural cycle are two popular methods for preparing the endometrium. Hormone replacement therapy is now used at the discretion of the doctors because it is easy to coordinate the timing of embryo thawing and transfer with the schedules of the in-vitro fertilization lab, the treating doctors, and the patient. However, current results suggest that establishing a pregnancy in the absence of a corpus luteum as a result of anovulation may pose significant maternal and fetal risks. Therefore, a 'back to nature' approach that advocates an expanded use of natural cycle FET in ovulatory women has been suggested. Currently, there is increasing interest in how the method of endometrial preparation may influence frozen embryo transfer outcomes specifically, especially when it comes to details such as different types of ovulation monitoring and different luteal support in natural cycles, and the ideal exogenous hormone administration route as well as the endocrine monitoring in hormone replacement cycles. In addition to improving implantation rates and ensuring the safety of the fetus, addressing these points will allow for individualized endometrial preparation, also as few cycles as possible would be canceled.

Investigating the impact of endometrial compaction on clinical pregnancy rate in artificial frozen-thawed embryo transfer cycles

Objective: The aim of our study was to evaluate sonographic endometrial thickness succeeding the estrogen-only stage and on the day when embryo transfer (ET) occurred in artificial frozen embryo transfer (FET) cycles to delve into the effect of endometrial compaction (EC) on clinical pregnancy rate (CPR). Patients and Methods: In the first group endometrial thickness diminished when ET occurred when compared to the end of the estrogen-only phase (n:37). Endometrial thickness increased/did not alter for the second group (n:70). Demographic characteristics were recorded and the following were studied: in vitro fertilization (IVF) treatment indications, hormone levels, total antral follicle count, duration of infertility, embryo quality, embryo-fundus distance, endometrial thickness at the end of estrogen-only phase and on ET day, luteal support, CPR. Results: No significant difference occurred in CPRs (n:107). ET, on day 5 was higher in the first group (p

The role of timing in frozen embryo transfer

The process of implantation is characterized by a complex cross-talk between the endometrium and the blastocyst, with the endometrium only being receptive to implantation during a transient window of implantation of approximately 2-3 days during the midsecretory phase. The timing of embryo transfer, including frozen embryo transfer, is therefore critical to the success of implantation. In this article, we discuss various elements that may guide the timing of frozen embryo transfer, including the role of endometrial characteristics such as thickness, days postovulation or length of progesterone administration, stage of the embryo, and the application of endometrial receptivity tests to guide personalized embryo transfer.

Endometrial compaction is associated with increased clinical and ongoing pregnancy rates in unstimulated natural cycle frozen embryo transfers: a prospective cohort study

PURPOSE

To investigate the association between endometrial compaction and pregnancy rates in unstimulated natural cycle frozen embryo transfers.

DESIGN

A single-center prospective cohort study. Endometrial thickness by transvaginal ultrasound and blood progesterone levels on the day of ovulation and the day of embryo transfer were evaluated in patients undergoing natural cycle frozen embryo transfer. Compaction was defined as > 5% decrease in endometrial thickness between ovulation day and day of transfer. Clinical and ongoing pregnancy rates in cycles with and without compaction were compared.

RESULTS

Seventy-one women were included, of which 44% had endometrial compaction, with similar rates when subdividing the patients by day of transfer (day 3 or day 5). Clinical and ongoing pregnancy rates were higher in the compaction group compared to the non-compaction group (0.58 vs. 0.16, P < 0.001; 0.52 vs. 0.13, P < 0.001 respectively). Subdividing by degree of compaction > 10% and > 15% revealed similar pregnancy rates as > 5%, with no added benefit to higher degrees of compaction.

CONCLUSIONS

About half the patients in our study undergoing unstimulated natural cycle frozen embryo transfer experienced compaction of the endometrium, occurring as early as day 3 post-ovulation. This was significantly correlated with increased clinical and ongoing pregnancy rates.

Effect of endometrial thickness changes on clinical pregnancy rates after progesterone administration in a single frozen-thawed euploid blastocyst transfer cycle using natural cycles with luteal support for PGT-SR- and PGT-M-assisted reproduction: a retrospective cohort study

BACKGROUND

To investigate whether the endometrial thickness change ratio from the progesterone administration day to the blastocyst transfer day is associated with pregnancy outcomes in a single frozen-thawed euploid blastocyst transfer cycle.

METHODS

All patients used natural cycles with luteal support for endometrial preparation and selected a single euploid blastocyst for transfer after a biopsy for preimplantation genetic testing. The endometrial thickness was measured by transvaginal ultrasound on the progesterone administration day and the transfer day, the change in endometrial thickness was measured, and the endometrial thickness change ratio was calculated. According to the change rate of endometrial thickness, the patients were divided into three groups: the endometrial thickness compaction group, endometrial thickness non-change group and endometrial thickness expansion group. Among them, the endometrial thickness non-change and expansion groups were combined into the endometrial thickness noncompaction group.

RESULTS

Ultrasound images of the endometrium in 219 frozen-thawed euploid blastocyst transfer cycles were evaluated. The clinical pregnancy rate increased with the increase in endometrial thickness change ratio, while the miscarriage rate and live birth rate were comparable among the groups. The multiple logistic regression results showed that in the fully adjusted model a higher endometrial thickness change ratio (per 10%) was associated with a higher clinical pregnancy rate (adjusted odds ratio [aOR] 1.29; 95% confidence interval [CI], 1.01-1.64; P = .040). Similarly, when the patients were divided into three groups according to the change rate of endometrial thickness, the endometrial thickness noncompaction group had a significant positive effect on the clinical pregnancy rate compared with the endometrial thickness compaction group after adjusting for all covariates.

CONCLUSIONS

In frozen-thawed euploid blastocyst transfer cycles in which the endometrium was prepared by natural cycles with luteal support, the clinical pregnancy rate was higher in cycles without endometrial compaction after progesterone administration.

Endometrial thickness changes after progesterone administration do not affect the pregnancy outcomes of frozen-thawed euploid blastocyst transfer: a retrospective cohort study

OBJECTIVE

To evaluate whether the change in endometrial thickness from progesterone administration day to transfer day is related to pregnancy outcomes in single frozen-thawed euploid blastocyst transfer cycles.

DESIGN

Observational cohort study.

SETTING

Single reproductive medical center.

PATIENT(S)

All patients were transferred with a single biopsied euploid blastocyst, and their endometrium was prepared with hormone replacement therapy (HRT).

INTERVENTION(S)

The endometrial thickness on the day of blastocyst transfer and progesterone administration was measured by transvaginal ultrasound, and the difference between them and the change ratio were calculated.

MAIN OUTCOME MEASURE(S)

Clinical pregnancy rates and live birth rates.

RESULT(S)

Endometrial ultrasound images of 508 euploid blastocyst transfer cycles using HRT were evaluated by transvaginal ultrasound. Overall, pregnancy outcomes were comparable in different groups of endometrial thickness changes. The results of multiple logistic regression showed that the clinical pregnancy rate and live birth rate did not significantly increase with the increase in endometrial thickness change ratios (per 10%) in the fully adjusted model as a continuous variable. In the adjustment model as a categorical variable, there was no statistical difference in pregnancy outcomes among the groups with changes in endometrial thickness. Interaction analysis showed that after adjusting for confounders, there was no statistically significant interaction between the endometrial thickness change ratio and pregnancy outcomes in all subgroups.

CONCLUSION(S)

In the euploid blastocyst transfer cycle of preparing the endometrium with HRT, the endometrial thickness change ratio after progesterone administration was not related to pregnancy outcomes.

Preparation of the Endometrium for Frozen Embryo Transfer: A Systematic Review

Despite the worldwide increase in frozen embryo transfer, the search for the best protocol to prime endometrium continues. Well-designed trials comparing various frozen embryo transfer protocols in terms of live birth rates, maternal, obstetric and neonatal outcome are urgently required. Currently, low-quality evidence indicates that, natural cycle, either true natural cycle or modified natural cycle, is superior to hormone replacement treatment protocol. Regarding warmed blastocyst transfer and frozen embryo transfer timing, the evidence suggests the 6th day of progesterone start, LH surge+6 day and hCG+7 day in hormone replacement treatment, true natural cycle and modified natural cycle protocols, respectively. Time corrections, due to inter-personal differences in the window of implantation or day of vitrification (day 5 or 6), should be explored further. Recently available evidence clearly indicates that, in hormone replacement treatment and natural cycles, there might be marked inter-personal variation in serum progesterone levels with an impact on reproductive outcomes, despite the use of the same dose and route of progesterone administration. The place of progesterone rescue protocols in patients with low serum progesterone levels one day prior to warmed blastocyst transfer in hormone replacement treatment and natural cycles is likely to be intensively explored in near future.

The Effects of Endometrial Thickness on Pregnancy Outcomes of Fresh IVF/ICSI Embryo Transfer Cycles: An Analysis of Over 40,000 Cycles Among Five Reproductive Centers in China

OBJECTIVE

To investigate the effects of endometrial thickness (EMT) on pregnancy outcomes on hCG trigger day in fresh in vitro fertilization (IVF) and intracytoplasmic sperm injection (ICSI) cycles.

METHODS

A total of 42,132 fresh cycles between 1 January 2013 and 31 December 2019 were included in this retrospective cohort study. Data were collected from five reproductive centers of large academic or university hospitals in China. All patients were divided into different groups according to their endometrial thickness on hCG trigger day. Multivariate regression analysis, curve fitting and threshold effect analysis were performed.

RESULTS

After adjusting for age, body mass index, infertility type, number of embryos transferred, number of retrieved oocytes and COS (controlled ovarian stimulation) protocols, significant associations were found between endometrial thickness and clinical pregnancy rate (adjusted odds ratio [aOR]: 1.05; 95% confidence interval [CI]: 1.06-1.08, P < 0.0001), live birth rate (aOR: 1.04; 95% CI: 1.03-1.05, P < 0.0001) as well as miscarriage rate(aOR: 0.96; 95% CI: 0.94 - 0.98, P < 0.0001). When the endometrial thickness was less than 12mm, the clinical pregnancy rate and live birth rate were increased significantly by 10% and 9%(OR:1.10; 95%CI: 1.08-1.12, OR:1.09; 95%CI: 1.07-1.11), respectively, along with the increase of each millimeter increment of endometrial thickness. However, when the EMT ranged from 12-15 mm, were stable at the ideal level, that were not significantly associated with EMT growth. Additionally, clinical pregnancy rate and live birth rate were slightly reduced by 6% and 4% when EMT was ≥15mm. Meanwhile, the miscarriage rate was significantly declined by 8% (OR:0.92; 95%CI: 0.90-0.95)with each millimeter increment of EMT. And when EMT was thicker than 12mm, the miscarriage rate didn't change any more significantly.

CONCLUSIONS

Endometrial thickness exhibits a curvilinear relationship with pregnancy outcomes in fresh embryo transfer cycles. Clinical pregnancy rate, live birth rate and miscarriage rate may achieve their optimal level when EMT ≥ 12 mm, but some adverse pregnancy outcomes would be observed when EMT ≥15 mm especially for clinical pregnancy.