Introduction

Since the advent of assisted reproductive technology (ART), clinicians and researchers have sought to enhance their procedures with a singular objective: to enhance the live birth rate (LBR). Endometrial thickness (EMT) is a pivotal factor affecting endometrial receptivity, which is essential for optimizing the LBR.1 A thin endometrium is typically defined as having an EMT of less than 7 mm, with its prevalence ranging from 2.4% to 8.5%.2,3 Within the realm of ART, frozen embryo transfer (FET) is regarded as a pivotal strategy for patients with thin endometrium.4,5 The methodologies for preparing frozen embryo transfer (FET) can be primarily classified into three categories: natural cycle (NC), hormone replacement cycle (HRT), and GnRH agonist (GnRH-a) downregulation in combination with HRT (GnRH-a -HRT).6,7 GnRH-a can downregulate pituitary function, and numerous studies have demonstrated that GnRH-a suppression prior to HRT significantly enhances the probability of achieving pregnancy.8,9

Concerning intramural fibroids, researchers contend that intramural myomas exert a secondary influence on the outcomes of ART.10,11 Recently, GnRH-a has been increasingly employed in patients with intramural fibroids to diminish the magnitude of the fibroids and to mitigate their recurrence following surgical intervention.12,13 Furthermore, given the positive impact of GnRH-a on endometrial receptivity and their therapeutic effects on intramural fibroids, we hypothesize that a GnRH-a-HRT protocol could enhance reproductive outcomes in individuals with thin endometrium and intramural fibroids undergoing FET. If validated, this individualized medical treatment plan would establish a foundation for clinical interventions aimed at maximizing patient benefits. However, there remains a paucity of reliable evidence to substantiate this hypothesis.

In the present study, we performed a retrospective cohort analysis to evaluate three endometrial preparation protocols and to determine whether a more suitable regimen exists for patients with thin endometrium and uterine fibroids during FET.

Materials and MethodsStudy Design and Participants

This study retrospectively included 360 FET cycles conducted at the People’s Hospital of Zhengzhou University between January 2017 and December 2023. The following inclusion criteria were applied: (1) individuals diagnosed with at least one intramural myoma via ultrasound; and (2) EMT ≤ 7 mm on the day of transfer.

The exclusion criteria were as follows: (1) cycles canceled for various reasons; (2) untreated individuals with hydrosalpinx, submucous fibroids, uterine adhesions, endometrial polyps, and uterine malformations that may affect pregnancy success rates; (3) Women with fibroids larger than 5 cm in diameter were excluded; (4) history of myomectomy; (5) individuals who encountered FET following preimplantation genetic testing (PGT) or those presenting with chromosomal disorders; (6) individuals diagnosed with endometriosis or adenomyosis; (7) individuals experiencing recurrent spontaneous abortion (RSA) and recurrent implantation failure (RIF); (8) individuals with comorbidities contraindicating pregnancy, such as severe cardiovascular history, liver disease, kidney disease, uncontrolled immune disorders, and gynecological malignancies; and (9) patients diagnosed as polycystic ovary syndrome (PCOS).

RSA is characterized by the occurrence of two or more pregnancy terminations before the 20th week of gestation.14 RIF is characterized by the inability to attain clinical pregnancy following the transfer of a minimum of three high-quality embryos across three frozen or fresh cycles.15 In this context, a high-quality embryo is defined as a day 3 embryo with at least 8 cells, exhibiting symmetry and less than 10% fragmentation,16 or a blastocyst graded ≥3BB.17

Women did not sign an informed consent since the study was retrospective. However, all women who were referred to our unit provided an informed consent for their data to be used for research purposes and those denying this consent were excluded. All patient information is anonymized and kept strictly confidential. The whole research protocol received review and approval from the Institutional Review Board and Ethics Committee of Henan Provincial People’s Hospital (approval number: SYSZ-LL-2021091501).

Endometrial Preparation ProtocolsNatural Cycles (NCs), Including Modified Cycles

Patients undergoing the natural cycle (NC) preparation protocol commenced monitoring follicular proliferation and endometrial development via transvaginal ultrasound from the tenth to twelfth day of the menstrual cycle until the occurrence of ovulation or luteinization; the detection of LH in urine or blood may be utilized to facilitate diagnosis when deemed necessary. Considering the individual variability in the temporal sequence of luteal peaks, a dosage of 10,000 IU of human chorionic gonadotropin (HCG) was administered to induce ovulation when serum luteinizing hormone level fell below 20 IU/L. FET was conducted three days post-ovulation for cleavage stage embryos or five days post-ovulation for blastocysts.

HRT Cycles

In hormone replacement therapy (HRT) cycles, beginning on cycle day three, estradiol valerate (Progynova; Bayer Schering Pharma AG, Berlin, Germany) was administered via the oral route at a daily dosage of 4 to 8 mg. Transvaginal ultrasound was conducted between the 10th and 12th days of medication to assess endometrial thickness. Progesterone was administered to facilitate endometrial transformation once the EMT reached 7 mm or approached the maximum thickness observed in all previous fresh and FET cycle. FET was subsequently conducted after four days of progesterone therapy for cleavage stage embryos or six days for blastocysts.

GnRH-A-HRT Cycles

In the GnRH-A–HRT cohort, participants underwent a single injection of 3.75 mg of long-acting triptorelin acetate (Diphereline; Bayer Schering Pharma AG, Germany) on days 1 through 4 of the menstrual cycle. The normative criteria for pituitary downregulation are delineated as follows: EMT < 5 mm, luteinizing hormone (LH) < 5 IU/L, estrogen (E2) < 50 pg/mL, and the absence of large cysts or follicles. After approximately thirty days, estrogenic stimulation was administered in accordance with the HRT cycles.

Luteal Phase Support

Luteal phase support protocols for the GnRH-A-HRT cohort and HRT cohort comprised a sustained-release vaginal gel administered at a dosage of 90 mg/day (Crinone; Merck, Germany) alongside oral delivery of progesterone tablets (Duphaston; Abbott Healthcare Products B.V., Netherlands) at a dosage of 40 mg/day. Estradiol valerate continued to be administered as previously, and the dosage of progesterone remained unchanged in the NC cohort. Following FET, all participants administered luteal phase support up to day 14, when serum β-HCG concentrations were assessed. In the event of a positive result, hormone administration was maintained until 12 weeks of gestation.

Outcome Measures

At 14 days post-embryo transfer, serum β-HCG concentrations were evaluated. Transvaginal ultrasonography was conducted in participants with positive β-HCG results at 28 and 35 days following embryo transfer. Obstetric and neonatal outcomes were assessed through telephone interviews. The primary outcome was the LBR, while secondary outcomes included the CPR, early miscarriage rate, and ectopic pregnancy rate. The LBR is characterized as the number of deliveries yielding at least one live birth, quantified per one hundred embryo transfer cycles.18 Clinical pregnancy is characterized as a confirmed pregnancy through ultrasonographic visualization of gestational sacs; this includes both intrauterine pregnancies and ectopic pregnancies. The CPR is characterized as the number of clinical pregnancies per one hundred embryo transfer cycles. Early miscarriage is defined as spontaneous miscarriage occurring during the first 12 weeks of gestation following verification of pregnancy, and the early miscarriage rate is determined by calculating the ratio of early miscarriages to the total number of clinical pregnancy cycles. The ectopic pregnancy rate is calculated as the ratio of ectopic pregnancies to the total number of clinical pregnancy cycles.

Statistical Methods

Statistical analyses were performed utilizing IBM SPSS Statistics version 25 (v. 25.0; International Business Machines Corporation, Armonk, NY, USA). Initially, we employed the imputation approach method to address missing data. Subsequently, a normality test was performed on continuous variables. If the data satisfied the normality assumption, a T-test was employed, and results were reported as mean ± standard deviation. In cases where normal distribution was not observed, the Kruskal–Wallis H-test was utilized, with results reported as median and interquartile range. The Chi-square test was applied to nominal variables, with percentages (n) used for statistical description. Multivariate regression examinations were conducted to elucidate factors correlated with the LBR. Odds ratios (ORs) and 95% confidence intervals (CIs) were computed to elucidate the association between endometrial preparation protocols and LBR, after adjusting for variables in the multivariate regression model. A p-value below 0.05 was regarded as statistically significant.

Finally, subgroup analyses were pre-planned for the number of fibroids (one versus two or more), and dimension (diameter of the larger fibroid <3 versus 3–5 cm). This cut-off was based on the results of previous studies.19,20

ResultsBaseline Characteristics

This retrospective research ultimately included 360 participants, who were categorized into three groups: the GnRH-A–HRT group (n = 84), the HRT group (n = 180), and the NC group (n = 96). The baseline characteristics of the study participants are presented in Table 1.

Table 1 Baseline Characteristics and Embryo Transfer Variables of Study Participants

Significant differences were observed in the maximum fibroid diameter, with measurements of 1.90 (1.13, 3.00) cm, 2.00 (1.50, 3.00) cm, and 2.85 (1.60, 4.20) cm respectively (P < 0.001). Nevertheless, no statistically significant variances were detected in female age, body mass index (BMI), duration of infertility, type of infertility, method of fertilization, etiology, anti-Müllerian hormone (AMH), basal follicle-stimulating hormone (FSH), basal luteinizing hormone (LH), antral follicle count (AFC), number of oocytes retrieved, endometrial thickness on the day of transfer, number of embryos transferred, type of embryo transferred, and the quantity of fibroids among the three cohorts.

Clinical Outcomes

As presented in Table 2, the differences in LBR (20.8% vs 6.7% vs 23.8%, p < 0.001) and CPR (29.2% vs 14.4% vs 35.7%, p < 0.001) were statistically significant among the three endometrial preparation protocols. Conversely, the early miscarriage rate (14.3% vs 38.5% vs 33.3%, p = 0.129) and ectopic pregnancy rate (0% vs 0.7 0.69% vs 0.0%, p = 0 0.087) did not demonstrate any significant differences among these preparations.

Table 2 Pregnancy Outcome After Embryo Transfer in Patients with Thin Endometrium and Intramural Fibroids

Multivariable Logistic Regression Analyses

A multivariable logistic regression examination was conducted with the LBR as the dependent factors, and female age, BMI, infertility type, aetiology, AMH, basal FSH, AFC, number of transferred embryos, endometrial thickness on the day of transfer, type of embryo transferred, maximum fibroid diameter, quantity of fibroids, and endometrial preparation regimens as independent factors. The logistic regression analysis indicated that, among participants with thin endometrium and intramural fibroids, the sole independent variable predictive of LBR was the endometrial preparation regimen. Specifically, when compared to the GnRH-A–HRT group, designated as the reference cohort, the LBR was significantly reduced in both the NC group (adjusted OR: 0.524, 95% CI: 0.457–0.956, p = 0.023) and the HRT group (adjusted OR: 0.269, 95% CI: 0.114–0.637, p = 0.003) (Table 3).

Table 3 Multivariate Logistic Regression Analysis of Factors Associated with LBR

Subgroup Analyses

The further subgroup analyses according to number (one versus two or more) and dimension fibroids (<3 versus 3–5 cm) are showed in Table 4. The baseline characteristics of the different subgroups are presented in Supplemental Table 1-4. Among three protocols, significant differences were observed in the number of eggs in the subgroup of dimension <3cm; basal FSH in the subgroup with a single fibroid (N=1), and the number of transferred embryos in the subgroup with two or more fibroids (N≥2). Nevertheless, no statistically significant variances were detected in other clinical indicators in the four subgroups among three protocols. In all four subgroups, participants adhering to the GnRH-A–HRT protocol demonstrated improved LBR and CPR, showing statistically significant differences (Table 4).

Table 4 Subgroup Analyses According to Number and Dimension of Intramural Fibroids

In all these four subgroups, a multivariable logistic regression examination was conducted respectively with the LBR as the dependent factors, and female age, BMI, infertility type, aetiology, AMH, basal FSH, AFC, number of transferred embryos, endometrial thickness on the day of transfer, type of embryo transferred, maximum fibroid diameter, quantity of fibroids, and endometrial preparation regimens as independent factors (Supplemental Table 5). The logistic regression analysis indicated that, among participants in all four subgroups, the endometrial preparation regimen was the independent variable predictive of LBR. Additionally, BMI was the independent variable predictive of LBR both in fibroid dimensions <3 cm subgroup and in the subgroup of two or more fibroids (N≥2). AFC was also the independent variable predictive of LBR in the subgroup of two or more fibroids (N≥2). In the fibroid dimensions of 3–5 cm subgroup, endometrial thickness on the day of transfer was also an independent predictor of LBR.

Discussion

In this extensive retrospective study, participants with thin endometrium and intramural fibroids demonstrated improved LBR and CPR following the GnRH-A–HRT protocol, showing statistically significant differences. However, no significant differences were observed in early miscarriage rates or ectopic pregnancy rates among the three endometrial preparation schemes. Furthermore, the application of the GnRH-A–HRT protocol resulted in thicker EMT, indicating that prior conditioning may be advantageous for individuals with a thin endometrium. Besides, when conducting subgroup analyses according to number and dimension of intramural fibroids, the efficacy of the GnRH-A–HRT regimen remains evident.

With the ongoing advancements of ART and laboratory embryo culture techniques, the quality of embryos has been progressively enhanced, consequently, endometrial function and receptivity have emerged as critical determinants influencing embryo implantation. Several tools for endometrial evaluation have been investigated,21 among them, ultrasound assessment of EMT is widely used as a routine prognosis factor for pregnancy. An EMT of 7 mm has been widely recognized as the threshold for defining thin endometrium in FET cycles.22 Thin endometrium is not only associated with reduced pregnancy rates but also appears to correlate with adverse perinatal outcomes, miscarriages, and abnormal placentation.3,4,23,24 The cause of thin endometrium has not yet been understood, but adhesions caused by various intrauterine manipulation (such as induced abortion) is considered to be the primary cause.25 Recently, numbers of treatments have been proposed to enhance EMT and optimize pregnancy rates of thin endometrium, including the administration of additional estrogen and GnRH agonists, hysteroscopic adhesiolysis followed by various adjunctive therapies, vitamin E supplementation, low-dose aspirin therapy, stem cell regenerative approaches, among others. However, research on thin endometrium remains in the exploratory stages, and the therapeutic effects of these interventions have yet to be fully delineated.26 In ART, the preference for FET over fresh embryo transfer is predominantly observed among most reproductive centers and patients, primarily due to concerns regarding low endometrial thickness. As of now, numerous researchers have performed comprehensive analyses and comparative assessments of reproductive outcomes in patients with thin endometrium undergoing endometrial preparation for FET across different populations using various preparation protocols.27,28 However, the existing evidence remains controversial and insufficient.29

Uterine fibroids represent the most prevalent benign neoplasms among females of reproductive age, with estimates suggesting that the likelihood of developing uterine fibroids may increase to 75% by the age of 50.30 The potential mechanisms by which uterine fibroids adversely affect fertility, as studied to date, encompass altered uterine vascular perfusion dynamics, endometrial function, myometrial contractile function, gamete migration, and myometrial/endometrial genomic expression patterns.31 A considerable number of studies have investigated the impact of GnRH-A on the reduction of both uterine and fibroid volume prior to myomectomy, as well as its role in delaying the recurrence of multiple uterine fibroids postoperatively.12 Currently, GnRH-A downregulation protocols are frequently employed in individuals with endometriosis, adenomyosis, PCOS, reduced ovarian reserve, and those experiencing RIF. However, the therapeutic effect of GnRH-A pretreatment prior to HRT in patients with uterine fibroids remains contentious regarding pregnancy outcomes.8

To the best of our understanding, this represents the first investigation into whether pretreatment with GnRH agonists could improve pregnancy outcomes in patients with intramural fibroids and a thin endometrium, compared to the NC group and the HRT group. In our study, the GnRH-A–HRT protocol exhibited a significant benefit in enhancing both the LBR and CPR. This beneficial effect is consistently observed across the four subgroups divided according to number (one versus two or more) and dimension fibroids (<3 versus 3–5 cm). Additionally, following the administration of GnRH-A, endometrial thickness was improved compared to both the NC group and the HRT group; nevertheless, this difference did not achieve statistical significance. This observation aligns with a previous retrospective cohort analysis that indicated GnRH-A pre-treatment can improve EMT on the day of progesterone therapy initiation and enhance the LBR.8 In another extensive analysis, the GnRH-A–HRT protocol was additionally correlated with a higher LBR compared to the NC protocol in frozen blastocyst-stage transfer cycles.32 GnRH-A are synthetic peptides that are structurally analogous to natural GnRH, which is secreted in a pulsatile pattern by the hypothalamus. When administered chronically, they inhibit normal pituitary-gonadal function and are widely utilized in routine reproductive medicine practice.33 Numerous researchers believe that GnRH-A may confer beneficial effects related to enhanced endometrial receptivity during embryo implantation. Recent investigations into molecular mechanisms have demonstrated that the administration of GnRH-A can positively influence endometrial receptivity by modulating the expression of endometrial adhesion molecules, genetic modifications, and implantation biomarkers, including HOXA10, MEIS, and LIF, which are recognized for their roles in promoting uterine ontogenesis and enhancing endometrial receptivity.34,35 Moreover, a separate study in mice demonstrated that GnRH-A partially reinstated the expression of endometrial integrin β3 and enhanced endometrial receptivity.35

To eliminate confounding factors that may influence the endometrium, we omitted participants diagnosed with adenomyosis, endometriosis, PCOS, RIF, RSA, and uterine malformations. As postulated, maternal age, infertility duration, AMH, basal FSH, AFC, BMI, infertility type, and aetiology were commensurable throughout the three groups. The application of multivariate logistic regression examination and adjusted marginal means (95% confidence intervals) concerning the LBR and CPR further facilitated the elimination of confounding factors. The multivariate logistic regression model indicates that the modality of endometrial preparation is a significant factor affecting both the LBR and CPR. Furthermore, the intima thickness is significantly improved in the GnRH-A–HRT group. Although the precise mechanisms remain unclear, they may be attributed to endometrial thickening, which can prolong the pregnancy duration and increase the likelihood of conception and live birth.36 Furthermore, the perturbation of the consecutive menstrual cycle engendered by the extended down-regulation of the pituitary gland might facilitate the full functionality of the hormone-sensitive system.37 However, this conclusion needs to be confirmed.

Estrogen plays a crucial role in endometrial growth.38 Consequently, various hormonal strategies have been developed to address thin endometrium. Similarly, the findings of our study demonstrated that HRT treatment enhanced endometrial thickness. However, for individuals with thin endometrium and intramural fibroids, the pregnancy outcomes in the HRT group were not only significantly worse than those in the GnRH-A group but also inferior to those observed in the NC group. In this study, although the LBR and CPR were lower in the NC group compared to the GnRH-A-HRT group, the early miscarriage rate was also reduced in the NC group, albeit without reaching statistical significance. The NC protocol is among the most commonly employed and widely utilized FET protocols in ART.39 A meta-analysis demonstrates that the clinical CPR in patients undergoing the NC protocol is significantly superior to that of those in the HRT group. The selection of an appropriate endometrial preparation strategy should be customized to align with each patient’s unique circumstances.40 In the NC protocol, the formation of a corpus luteum following natural ovulation results in an endometrium that more closely resembles the conditions of natural pregnancy, thereby enhancing endometrial receptivity.41 The corpus luteum synthesizes progesterone and estrogen, in addition to vasoactive agents such as relaxin, angiogenic metabolites, and vascular endothelial growth factor (VEGF) derived from estrogen. The HRT protocol is correlated with an insufficiency of these vasoactive products compared to the NC protocol, which involves the presence of a corpus luteum.41,42 Furthermore, within the HRT protocol, once the EMT reaches the specified criterion of approximately 0.8 centimeters, luteal supplementation is typically initiated rather than prolonging the proliferative phase to attain increased thickness. In contrast, during a natural protocol, the endometrium generally attains optimal thickness in conjunction with the development and rupture of the dominant follicle.

This investigation presents three notable advantages. Firstly, to the best of our understanding, this research is the inaugural investigation to ascertain which endometrial preparation regimen may yield superior reproductive outcomes for patients of thin endometrium with uterine fibroid. This study possesses significant clinical implications and offers valuable insights for the selection of individualized frozen embryo transfer protocols. Secondly, the substantial sample size of this single-center investigation, where clinical and IVF laboratory procedures were consistently standardized, alleviated potential biases and potentiated statistical power. Thirdly, we delineated rigorous exclusion and inclusion criteria throughout the screening of the study cohort, thereby eliminating disorders that might potentially impinge upon reproductive outcomes and the integrity of the endometrium and uterine cavity, such as adenomyosis, endometriosis, and uterine malformations. Moreover, patients diagnosed with RSA and RIF were also precluded.

This study also presents certain limitations. Primarily, this investigation is a retrospective cohort study. As with any retrospective analysis, certain inherent limitations are unavoidable. It is also noteworthy that some participants with a thin endometrium underwent fresh embryo transfer, whereas others opted to cancel the transfer. This discrepancy may have introduced bias and consequently might not accurately reflect the true impact of varying endometrial preparation protocols. Furthermore, the data regarding the maximum fibroid diameter were obtained prior to the initiation of a treatment cycle. No re-assessment of uterine fibroid size was conducted on the day of transfer, thereby precluding the observation of the effects of the three endometrial preparation regimens. Nevertheless, this does not affect the conclusion regarding the positive impact of the GnRH-A-HRT protocol on patients’ LBR, as the maximum fibroid diameter was significantly larger in the GnRH-A group compared to the other two groups; however, this finding underscores the need for additional multicenter, large-scale prospective studies to further validate and corroborate our results.

Conclusions

Our research indicates that, compared to NC and HRT protocol, significantly improved reproductive outcomes were observed in the GnRH-A–HRT group. These findings also provide valuable insights for further exploration of the underlying mechanisms by which the GnRH-A-HRT protocol enhances reproductive outcomes in patients of thin endometrium with intramural fibroids.

Data Sharing Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.

Informed Consent Statement

Not applicable.

Consent for Publication

Not Applicable.

Institutional Review Board Statement

This study was conducted in accordance with the Declaration of Helsinki, as revised in 1983. This study was reviewed and approved by the Institutional Review Board and Ethics Committee of Henan Provincial People’s Hospital, China.

Author Contributions

All authors made a significant contribution to the work reported, whether that is in the conception, study design, execution, acquisition of data, analysis and interpretation, or in all these areas; took part in drafting, revising or critically reviewing the article; gave final approval of the version to be published; have agreed on the journal to which the article has been submitted; and agree to be accountable for all aspects of the work.

Funding

This research did not receive any specific grants from any funding agency in the public, commercial, or not-for-profit sector.

Disclosure

The authors declare no conflicts of interest.

References

1. Casper RF. Frozen embryo transfer: evidence-based markers for successful endometrial preparation. Fertil Steril. 2020;113(2):248–251. doi:10.1016/j.fertnstert.2019.12.008

2. Kasius A, Smit JG, Torrance HL, et al. Endometrial thickness and pregnancy rates after IVF: a systematic review and meta-analysis. Hum Reprod Update. 2014;20(4):530–541. doi:10.1093/humupd/dmu011

3. Ribeiro VC, Santos-Ribeiro S, De Munck N, et al. Should we continue to measure endometrial thickness in modern-day medicine? The effect on live birth rates and birth weight. Reprod Biomed Online. 2018;36(4):416–426. doi:10.1016/j.rbmo.2017.12.016

4. Blockeel C, Drakopoulos P, Santos-Ribeiro S, Polyzos NP, Tournaye H. A fresh look at the freeze-all protocol: a SWOT analysis. Hum Reprod. 2016;31(3):491–497. doi:10.1093/humrep/dev339

5. Ozgur K, Berkkanoglu M, Bulut H, Humaidan P, Coetzee K. Perinatal outcomes after fresh versus vitrified-warmed blastocyst transfer: retrospective analysis. Fertil Steril. 2015;104(4):899–907.e3. doi:10.1016/j.fertnstert.2015.06.031

6. Mackens S, Santos-Ribeiro S, van de Vijver A, et al. Frozen embryo transfer: a review on the optimal endometrial preparation and timing. Hum Reprod. 2017;32(11):2234–2242. doi:10.1093/humrep/dex285

7. Park CW, Choi MH, Yang KM, Song IO. Pregnancy rate in women with adenomyosis undergoing fresh or frozen embryo transfer cycles following gonadotropin-releasing hormone agonist treatment. Clin Exp Reprod Med. 2016;43(3):169–173. doi:10.5653/cerm.2016.43.3.169

8. Xia L, Tian L, Zhang S, Huang J, Wu Q. Hormonal replacement treatment for frozen-thawed embryo transfer wi th or without GnRH agonist pretreatment: a retrospective cohort study stratified by times of embryo implantation failures. Front Endocrinol (Lausanne). 2022;13:803471. doi:10.3389/fendo.2022.803471

9. Cao X, Chang HY, Xu JY, et al. The effectiveness of different down-regulating protocols on in vitro fertilization-embryo transfer in endometriosis: a meta-analysis. Reprod Biol Endocrinol. 2020;18(1):16. doi:10.1186/s12958-020-00571-6

10. Bai X, Lin Y, Chen Y, Ma C. The impact of FIGO type 3 fibroids on in-vitro fertilization outcomes: a nested retrospective case-control study. Eur J Obstet Gynecol Reprod Biol. 2020;247:176–180. doi:10.1016/j.ejogrb.2019.12.018

11. Yan L, Yu Q, Zhang YN, et al. Effect of type 3 intramural fibroids on in vitro fertilization-intracytoplasmic sperm injection outcomes: a retrospective cohort study. Fertil Steril. 2018;109(5):817–822.e2. doi:10.1016/j.fertnstert.2018.01.007

12. Wei J, Ma X, Wang W, et al. Gonadotropin-releasing hormone ago nist versus expectant management for treating multiple leiomyomas after myomectomy: the study protocol for a multicentre, prospective, randomised controlled clinical trial. BMJ Open. 2021;11(10):e044347. doi:10.1136/bmjopen-2020-044347

13. Donnez J, Dolmans MM. Uterine fibroid management: from the present to the future. Hum Reprod Update. 2016;22(6):665–686. doi:10.1093/humupd/dmw023

14. Practice Committee of the American Society for Reproductive Medicine. Evaluation and treatment of recurrent pregnancy loss: a committee opinion. Fertil Steril. 2012;98(5):1103–1111. doi:10.1016/j.fertnstert.2012.06.048.

15. Cimadomo D, Craciunas L, Vermeulen N, Vomstein K, Toth B. Definition, diagnostic and therapeutic options in recurrent implantation failure: an international survey of clinicians and embryologists. Hum Reprod. 2021;36(2):305–317. doi:10.1093/humrep/deaa317

16. Alpha Scientists in Reproductive Medicine and ESHRE Special Interest Group of Embryology. The Istanbul consensus workshop on embryo assessment: proceedings of an expert meeting. Hum Reprod. 2011;26(6):1270–1283. doi:10.1093/humrep/der037

17. Gardner DK, Surrey E, Minjarez D, Leitz A, Stevens J, Schoolcraft WB. Single blastocyst transfer: a prospective randomized trial. Fertil Steril. 2004;81(3):551–555. doi:10.1016/j.fertnstert.2003.07.023

18. Zegers-Hochschild F, Adamson GD, Dyer S, et al. The international glossary on infertility and fertility care, 2017. Hum Reprod. 2017;32(9):1786–1801. doi:10.1093/humrep/dex234

19. Christopoulos G, Vlismas A, Salim R, Islam R, Trew G, Lavery S. Fibroids that do not distort the uterine cavity and IVF success rates: an observational study using extensive matching criteria. BJOG. 2017;124(4):615–621. doi:10.1111/1471-0528.14362

20. Yan L, Ding L, Li C, Wang Y, Tang R, Chen ZJ. Effect of fibroids not distorting the endometrial cavity on the outcome of in vitro fertilization treatment: a retrospective cohort study. Fertil Steril. 2014;101(3):716–721. doi:10.1016/j.fertnstert.2013.11.023

21. von Grothusen C, Lalitkumar S, Boggavarapu NR, Gemzell-Danielsson K, Lalitkumar PG. Recent advances in understanding endometrial receptivity: molecular basis and clinical applications. Am J Reprod Immunol. 2014;72(2):148–157. doi:10.1111/aji.12226

22. Gingold JA, Lee JA, Rodriguez-Purata J, et al. Endometrial pattern, but not endometrial thickness, affects implantation rates in euploid embryo transfers. Fertil Steril. 2015;104(3):620–8.e5. doi:10.1016/j.fertnstert.2015.05.036

23. Liu KE, Hartman M, Hartman A, Luo ZC, Mahutte N. The impact of a thin endometrial lining on fresh and frozen-thaw IVF outcomes: an analysis of over 40 000 embryo transfers. Hum Reprod. 2018;33(10):1883–1888. doi:10.1093/humrep/dey281

24. Yuan X, Saravelos SH, Wang Q, Xu Y, Li TC, Zhou C. Endometrial thickness as a predictor of pregnancy outcomes in 10787 fresh IVF-ICSI cycles. Reprod Biomed Online. 2016;33(2):197–205. doi:10.1016/j.rbmo.2016.05.002

25. Mahajan N, Sharma S. The endometrium in assisted reproductive technology: how thin is thin? J Hum Reprod Sci. 2016;9(1):3–8. doi:10.4103/0974-1208.178632

26. Liu KE, Hartman M, Hartman A. Management of thin endometrium in assisted reproduction: a clinical practice guideline from the Canadian Fertility and Andrology Society. Reprod Biomed Online. 2019;39(1):49–62. doi:10.1016/j.rbmo.2019.02.013

27. Liu X, Shi W, Shi J. Natural cycle frozen-thawed embryo transfer in young women with regular menstrual cycles increases the live-birth rates compared with hormone replacement treatment: a retrospective cohort study. Fertil Steril. 2020;113(4):811–817. doi:10.1016/j.fertnstert.2019.11.023

28. Vinsonneau L, Labrosse J, Porcu-Buisson G, et al. Impact of endometrial preparation on early pregnancy loss and live birth rate after frozen embryo transfer: a large multicenter cohort study (14 421 frozen cycles). Hum Reprod Open. 2022;2022(2):hoac007. doi:10.1093/hropen/hoac007

29. Ranisavljevic N, Raad J, Anahory T, Grynberg M, Sonigo C. Embryo transfer strategy and therapeutic options in infertile patients with thin endometrium: a systematic review. J Assist Reprod Genet. 2019;36(11):2217–2231. doi:10.1007/s10815-019-01576-w

30. Conforti A, Mollo A, Alviggi C, et al. Techniques to reduce blood loss during open myomectomy: a qualitative review of literature. Eur J Obstet Gynecol Reprod Biol. 2015;192:90–95. doi:10.1016/j.ejogrb.2015.05.027

31. Pier BD, Bates GW. Potential causes of subfertility in patients with intramural fibroids. Fertil Res Pract. 2015;1(1):12. doi:10.1186/s40738-015-0005-2

32. Hill MJ, Miller KA, Frattarelli JL. A GnRH agonist and exogenous hormone stimulation protocol has a higher live-birth rate than a natural endogenous hormone protocol for frozen-thawed blastocyst-stage embryo transfer cycles: an analysis of 1391 cycles. Fertil Steril. 2010;93(2):416–422. doi:10.1016/j.fertnstert.2008.11.027

33. van der Linden M, Buckingham K, Farquhar C, Kremer JA, Metwally M. Luteal phase support for assisted reproduction cycles. Cochrane Database Syst Rev. 2015;2015(7):CD009154. doi:10.1002/14651858.CD009154.pub3

34. Xu B, Geerts D, Hu S, et al. The depot GnRH agonist protocol improves the live birth rate per fresh embryo transfer cycle, but not the cumulative live birth rate in normal responders: a randomized controlled trial and molecular mechanism study. Hum Reprod. 2020;35(6):1306–1318. doi:10.1093/humrep/deaa086

35. Ruan HC, Zhu XM, Luo Q, et al. Ovarian stimulation with GnRH agonist, but not GnRH antagonist, partially restores the expression of endometrial integrin beta3 and leukaemia-inhibitory factor and improves uterine receptivity in mice. Hum Reprod. 2006;21(10):2521–2529. doi:10.1093/humrep/del215

36. El-Toukhy T, Coomarasamy A, Khairy M, et al. The relationship between endometrial thickness and outcome of medicated frozen embryo replacement cycles. Fertil Steril. 2008;89(4):832–839. doi:10.1016/j.fertnstert.2007.04.031

37. Edwards RG. Clinical approaches to increasing uterine receptivity during human implantation. Hum Reprod. 1995;10(Suppl 2):60–66. doi:10.1093/humrep/10.suppl_2.60

38. Hapangama DK, Kamal AM, Bulmer JN. Estrogen receptor β: the Guardian of the endometrium. Hum Reprod Update. 2015;21(2):174–193. doi:10.1093/humupd/dmu053

39. Groenewoud ER, Cantineau AE, Kollen BJ, Macklon NS, Cohlen BJ. What is the optimal means of preparing the endometrium in frozen-thawed embryo transfer cycles? A systematic review and meta-analysis. Hum Reprod Update. 2013;19(5):458–470. Erratum in: Hum Reprod Update. 2017;23(2):255-261. doi: 10.1093/humupd/dmw046.doi:10.1093/humupd/dmt030

40. Yarali H, Polat M, Mumusoglu S, Yarali I, Bozdag G. Preparation of endometrium for frozen embryo replacement cycles: a systematic review and meta-analysis. J Assist Reprod Genet. 2016;33(10):1287–1304. doi:10.1007/s10815-016-0787-0

41. Singh B, Reschke L, Segars J, Baker VL. Frozen-thawed embryo transfer: the potential importance of the corpus luteum in preventing obstetrical complications. Fertil Steril. 2020;113(2):252–257. doi:10.1016/j.fertnstert.2019.12.007

42. von Versen-Höynck F, Strauch NK, Liu J, et al. Effect of mode of conception on maternal serum relaxin, creatinine, and sodium concentrations in an infertile population. Reprod Sci. 2019;26(3):412–419. doi:10.1177/1933719118776792