Introduction
Repeated implantation failure (RIF) is a form of refractory infertility that refers to the absence of a gestational sac at 5 or more weeks; this is followed by embryo transfer (ET)—with three transfers of high-quality embryos or after the transfer of ≥ 10 embryos in multiple transfers [
1]. The etiology of RIF can be attributed to the embryonic quality, endometrial dysfunction, uterine anomaly, immunological factors, thrombophilia, hormonal or metabolic disorders, and/or genetic predisposition [
2‐
5]. However, the cause of RIF remains obscure in most cases, and unexplained RIF has become a frequently encountered and difficult clinical challenge [
6].
Insufficient endometrial receptivity leading to altered embryo-endometrial synchrony is a leading cause of RIF [
7]. Endometrial receptivity is defined as a transient period in which the endometrium is prepared to receive the implanting embryo, and is also known as the “window of implantation” (WOI)[
8]. The characteristics of the WOI have achieved a favored status in reproductive research over many decades, and relevant assessments of the WOI include ultrasonographic imaging, endometrial biopsy, endometrial-fluid aspirate, or hysteroscopy, but little progress has been made on clinically meaningful prognostic tests and treatments [
9].
In the 1950s, Noyes et al. created the classical histological criteria used to evaluate the endometrium on each specific day after ovulation, based on morphological changes of the epithelium and stroma [
10]. According to the Noyes criteria, histological dating of a timed endometrial biopsy specimen is made available to assess the WOI, and then embryonic transfer time is adjusted in a subsequent cycle to restore the synchrony between embryonic and endometrial development [
11]. However, the subjective nature of such assessments [
12] and the apparently substantial variation in results of different studies [
13‐
15] have greatly limited the clinical application of such histological dating.
In our previous study, we first verified the Noyes criteria in pregnant patients [
16], and “out-of-phase” designated a difference of more than 2 days between the histological date and the actual day after ovulation [
17]. We then executed personalized frozen-thawed embryo transfer (pFET) in the RIF patients that were “out-of-phase”, and found that histological endometrial dating improved the clinical outcomes of those patients with unexplained RIF. However, our results were limited by a small sample size, which may have biased our conclusions. The clinical feasibility, reliability, and repeatability of histological endometrial dating for RIF patients would thus require the clinical validation of a larger sample size.
Discussion
A successful pregnancy relies primarily upon successful implantation, which is itself a complex reproductive process that involves reciprocal interactions between the endometrium and the embryo [
18]. The WOI refers to a narrow time frame of maximal endometrial receptivity, and an endometrial status outside this window is incompatible with pregnancy [
9]. The endometrial receptivity array (ERA) test that is based upon a transcriptomic signature has recently emerged as a novel diagnostic tool and has been employed for endometrial dating to guide pFET in RIF patients [
19,
20]. The ERA test is, however, in its early stages of exploration, and its extrapolability to the general adapted population remains controversial [
21]. In addition, the clinical application of this novel methodology requires support by a corresponding transcriptomics-analysis platform, limiting its promotion and use, and both time and financial costs need to be considered. While the Noyes criteria have been the “gold standard” for endometrial dating since its inception in 1950, some skeptics have recently raised concerns over their accuracy [
22,
23]. In our previous pilot study, we verified the endometrial dating criteria in patients with favorable prognoses by performing endometrial biopsies at different time points [
16], and we also demonstrated that these criteria were easily mastered and executed with reference to the Noyes criteria.
We had tentatively explored the clinical availability of histological endometrial dating on patients with RIF prior to our pilot study (Study I)[
16]. We therein recruited 155 RIF patients: a total of 49 patients were evaluated as being out-of-phase; ultimately, 47 out-of-phase patients underwent pFET once and five did so twice. To verify the reliability and repeatability of histological endometrial dating, we enrolled more than eight times as many patients in the current study (Study II), and results revealed that the out-of-phase rate in RIF patients stabilized at ~ 30% in both our studies. We further reevaluated the endometrial dating of five patients by re-staging the biopsies in Study I, and all five showed the expected endometrial dating results. While the expected dating rate for reevaluation was as high as 94.8% in Study II, the two studies exhibited comparable implantation (47.8% vs. 53.9%,
P = 0.341) and clinical pregnancy rates (63.8% vs. 65.3%,
P = 0.846) in the first pFET. Therefore, from the standpoint of WOI asynchrony and the clinical outcomes of pFET, we had reason to believe that the platform of histological endometrial dating was stable with respect to RIF patients.
The clinical pregnancy rate of first, second, and third pFETs in the present study were 65.3%, 50.0%, and 44.4%, respectively; and the cumulative clinical pregnancy rate reached 74.9% after three transfers. The Simón team in Spain reported in a small number of subjects the clinical value of ERA in guiding pFET for RIF patients [
19], and they also corroborated this in a large-sample multicenter study in which they targeted a population undergoing IVF with blastocyst transfer at their first appointment [
21]. There is currently a lack of other such platforms that employ a large sample size of RIF patients for use in stability verification. In a large retrospective study at our center, the clinical pregnancy rate for the first FET cycle after a previous fresh cycle was 60.25% with thawed cleavage-stage ET and 64.98% with blastocysts cultured from thawed cleavage-stage embryos transferred to young women [
24]. Our study revealed that when pFET was applied, the clinical pregnancy rate for RIF patients with asynchrony of their WOI increased to the level of routine patients who underwent FET. Moreover, our data suggested that if there were enough embryos for transfer, the problem of failure to develop to clinical pregnancy could be addressed after 2–3 pFETs in nearly 75% of RIF patients showing WOI displacement.
We observed that the clinical pregnancy rate tended to diminish as the number of pFETs increased. This might have been due to the reduced quality of the transferred embryos in previous pFET cycles, although the number of high-quality embryos transferred was not statistically different. A first pFET may select the highest-quality embryos to be transferred, whereas the quality of the embryos following transplantation may be reduced, even when all of the transferred embryos are of sufficiently high quality. The increasing pressure on patients who experienced several previously failed cycles might be another cause of worse clinical outcomes. The potential causes of RIF include low quality of gametes, and molecular displacements (asynchrony) and molecular disruptions (pathology) within the endometrium [
25]. Thus, exploring the pathology of the endometrium before implementing the second/third pFET is of paramount importance. Accordingly, we recommend that other modalities be executed to improve pregnancy outcomes once a patient experiences two pFET failures.
The cases we presented further illustrated that our histological endometrial dating results reflected good reproducibility and verifiability in the same patients, even with a more than the 2-year duration. Histological examination exhibited several other strengths such as simple methodology, low cost, and short reporting periods; and was thus worthy of clinical promotion and application. However, there were some limitations to the present study. Restricted by subjects, we could not design this study as a randomized controlled trial; the results thus still need to be treated with caution. Intriguingly, our data revealed that the WOI for approximately 70% of the RIF patients was not displaced; therefore, the mechanisms underlying implantation failure in this population require further exploration.
In conclusion, by expanding the sample size we further validated the high stability of our histological endometrial dating platform, including the out-of-phase rate and the expected dating rate for the reevaluation of RIF patients. Furthermore, pFET—as an embryo-transfer strategy constructed according to histological endometrial dating— manifested a satisfactory clinical value and was worthy of promotion for patients with unexplained RIF.
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