Traditionally, the WOI period is considered quite wide, but the optimal window has been proved to be much narrower, which may last for only 12 ~ 48h [16]. The embryo transplantation outside the window will lead to poorer outcomes of assisted reproductive treatment [6, 7]. Therefore, it is very important to detect the normal WOI objectively for personalized embryo transfer (pET). It is reported that nearly 1/3 to 2/3 of implantation failures are caused by the endometrial non-receptivity status [17, 18]. It has been reported that the pregnancy outcomes of patients who received ERA test significantly increased compared with control group, especially in patients with RIF [19, 20]. However, there are also recent studies hold the opposite opinions that ERA can not improve the pregnancy outcome in RIF patients [21, 22]. Other investigators considered that the value was rather limited when applied ERA in the first embryo transfer cycles or on the patient with good prognosis, ERA were not recommended as a routine test for unselected patient populations [23, 24]. Because some factors can affect endometrial gene expression, including chronic endometritis, severe intrauterine adhesions, endometrial tuberculosis, endometrial hyperplasia, etc. [25–27], so we excluded some factors that may affect endometrial receptivity to eliminate these biases. In this study, we divide the population into two categories: non-RIF and RIF, and we found that non-RIF with pET group had higher clinical pregnancy rates (64.5% vs 58.3%, P = 0.025) and live birth rate (57.1% vs 48.3%, P = 0.003) than non-RIF with npET group. The pregnancy rate in RIF with pET group (62.7% vs 49.3%, P < 0.001) and the live birth rate (52.5% vs 40.4%, P < 0.001) are significantly higher than RIF with non-pET group after PSM matching, and the gap of pregnancy outcomes between the RIF with pET group and npET group is larger than that of non- RIF with pET and npET group. This study further confirmed the clinical value of ERA in infertile patients with previous failed ET cycle, especially in RIF patients. Not only that, we also found that for patients with non-RIF, pET guided by ERA has a certain effect on reducing the early spontaneous abortion rate. This is a new discovery, indicating that the displacement of the implantation window may be related to early abortion. However, for RIF patients, pET did not have such an effect, we believe the possible reasons are that patients with RIF have numerous other factors that can lead to miscarriage, including factors such as aneuploidy [28–31]. But we need to pay attention to another point of endometrial scraping, which is an intentional damage to the endometrium that is carried out with the aim of improving outcomes from fertility treatment or the chance of natural conception [32, 33]. So, the improvement in pregnancy outcomes may be related to endometrial scraping caused by endometrial biopsy during ERA test, but we cannot rule out the influence of this confounding factor, which is also the limitation of this article.
Embryo implantation is a process in which the endometrium in receptive state interacted with functional embryos, then the trophoblast established close links with the decidualized endometrium, which is controlled by estrogen, progesterone and a large number of transcription factors [1]. In this complex physiological process, numerous of molecular mediators participate in the initial maternal-fetal interaction under the coordination of ovarian steroid hormones [34]. Although it is known that ovaries produce many steroids throughout the menstrual cycle, only E2 and P have been proven to be essential for inducing endometrial receptivity [5, 35]. Studies have shown that the disorder of the sex hormone will affect the endometrial receptivity [36, 37], and low serum progesterone on day of transfer adversely impacts ongoing pregnancy rates in hormonally prepared single blastocyst frozen embryo transfer cycles [38]. Progesterone in advance can reduce pregnancy rate [39]. Research has confirmed that exposure of human endometrial epithelial cells to high levels of estrogen can lead to abnormal expression of many key implantation molecules [40]. Estradiol reduction adversely affect the embryo quality and clinical outcomes of IVF-ET [41]. However, these studies did not combine estrogen and progesterone with gene expression profile. There are currently no reports linking endometrial receptivity testing with hormone analysis for research. In this study, we measured the serum estrogen and progesterone levels on P + 5d, and compared the estrogen and progesterone levels between the normal and displaced WOI groups, but there is no difference between the two groups, and the logistic regression analysis also showed that the estrogen and progesterone levels were not linearly correlated with the displaced WOI. Therefore, we divided the E2/P values into tertiles for analysis, we found that the displaced WOI rate of the median group 4.46 < E2/P ≤ 10.39 pg/ng was significantly lower than that of the other two groups, which indicated that an appropriate E2/P ratio value was important for maintaining the endometrial receptivity state.
Age is an important influencing factor on pregnancy rate of assisted reproduction. There have some studies reported the relationship between advanced maternal age and decline of endometrial receptivity [42–44], but all these reports studied the endometrial receptivity from the standpoint of pregnancy outcomes. So far, there has no studies combined age with the gene expression test of endometrial receptivity. Our study showed that there has a significant difference in age between the normal WOI and displaced WOI patients, and the results of logistic regression analysis also showed that age was positively correlated with displaced WOI. We divided all patients who received ERA test into three groups according to their age, we found that the displaced WOI rate increased gradually with the increase of age group, which indicated that as age increases, the occurrence probability of displaced WOI also gradually increases.
We also found that there was a significant difference in the number of previous failed ET cycle between the normal WOI and displaced WOI patients, and the results of logistic regression analysis also showed that the number of previous failed ET cycle was positively correlated with the displaced WOI. According to the number of previous failed ET cycle, we divided the patients into three groups for detailed analysis, and we found that the displaced WOI rate increased gradually with the increase of the number of previous failed ET cycle. Our result showed that the higher the number of previous failed cycle, the greater the likelihood of displaced WOI in patients.
Psychological pressure and loss of hope for success are factors that must be taken seriously in the progress of assisted reproduction [45]. According to reports, a surprising number of couples stop continuing treatment after at least one failed IVF cycle [46]. The cause of implantation failure is due partly to individual differences of WOI. According to our study, we suggest that positive actions including endometrial receptivity assessments are recommended to improve the outcomes of assisted reproduction for elderly patients and patients who have experienced one or more failed cycles.
As our study suggests that the endometrial receptivity is influenced by some maternal factors, including age-time changes, hormonal changes, and there maybe has some other unknown factors. Due to the invasive examination of endometrial biopsy, embryo transfer cannot be performed during the biopsy cycle. So, the endometrial receptivity status during the biopsy cycle may differ from the actual embryo transfer cycle, which may also lead to different outcomes. We believe this may also be a reason for controversy regarding the application of ERA. Therefore, it is necessary to search for new non-invasive methods for ERA.