What is recurrent implantation failure?

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Implantation failure with IVF occurs when an embryo is transferred into the uterus, and it does not attach to or implant in the uterine wall.ⁱⁱⁱ There is no generally accepted definition of RIF. One suggested set of criteria includes a negative serum hCG test 14 days after embryo transfer of four or more cleavage stage embryos, or two good quality blastocyst embryos after two fresh or two frozen cycles.

A survey conducted by the European Society of Human Reproduction and Embryology (ESHRE) of 735 clinicians and 300 embryologists reported that the majority of providers defined RIF based on the number of failed embryo transfers, with the most common threshold for diagnosis being ≥3 failed embryo transfers (including both fresh and frozen cycles).^iv

A 2019 study analyzed data from 1 221 good prognosis couples to determine the frequency of RIF. The study found that approximately 15 percent of the couples experienced RIF.^v However, a large study by Pirtea et al (2021) showed that the true incidence of RIF is likely lower, at than five percent. This study investigated frozen single embryo transfers of only euploid embryos (embryos that have the correct number of chromosomes) and found that RIF was less than five percent after three transfers. The authors concluded that most failures to implant are therefore likely due to issues with the embryo.^vi Diagnosis of RIF by specific criteria remains difficult with a false-positive rate of at least 46 percent due to inconsistencies in the diagnostic criteria that clinics use.^vii

RIF is not the same as recurrent IVF failure. It is estimated that embryos fail to implant approximately 40-75 percent of the time in IVF.^viii,ix While this seems high, note that even in young fertile people trying to conceive naturally (not using ART), the chance of an embryo implanting is only 20-25 percent each month. IVF cycles can fail to result in pregnancy due to other factors such as poor response to ovarian stimulation with few eggs retrieved or if embryos fail to develop after fertilization, resulting in no embryo transfer being performed. Thus, RIF is just one cause of repeated IVF failure.^x

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What causes recurrent implantation failure?

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Potential causes of RIF are commonly grouped into four categories: 

• the embryo (abnormal quality – microscopic differences in morphology or abnormal genetics) 
• the uterus (structural or microscopic abnormalities)
• uterus-embryo interaction
• unexplained^xi,xii

In IVF, the quality of the embryo is believed to be responsible for over one-third of implantation failures^xiii, but this may be much higher depending on age. Higher grade embryos (those with superior morphology) are more likely to implant successfully.^xiv,xv Additionally, studies suggest that the developmental stage of the embryo at transfer may also be important for implantation success. For example, multiple studies have demonstrated that blastocyst embryo transfers have higher continuing pregnancy and live birth rates (51 and 56 percent) compared to cleavage stage embryos (27 and 34 percent).^{xvi,xvii,xviii}

Increased maternal age increases the risk of having aneuploid (genetically abnormal) embryos and thus reduces chances of implantation.^xix One clinical study evaluated outcomes for untested embryos by patient age. They found that when comparing outcomes for 1 169 blastocyst transfers, the implantation rates (41.1 percent vs. 24.5 percent), clinical pregnancy rates (54.3 percent vs. 34.7 percent), and live birth rates (50.7 percent vs. 28.5 percent) were significantly higher in women under 35 years of age compared to women over 35 years old.^xx

Uterine causes of RIF may originate from abnormalities with the uterine lining due to polyps (benign overgrowth of cells), submucosal fibroids (benign overgrowth of muscular tissue), fibrous adhesions/scar tissue inside the uterus (Asherman’s syndrome), an undeveloped thin endometrium, or an unreceptive endometrium (does not provide the correct molecular environment to allow the embryo to implant).^xxi These issues are diagnosed using ultrasound or hysteroscopy (looking inside the uterus with a small camera).  

Some women with RIF have also been diagnosed with chronic endometritis (CE), which is inflammation or irritation of the endometrium.^xxii CE is often asymptomatic and only detectable by endometrial biopsy or hysteroscopy.^xxiii CE has been observed in approximately 30 percent of women with a history of RIF.^xxiv One study found that live birth rates in the next cycle of IVF increased from 13 percent to 61 percent following treatment with antibiotics.^xxv

Other possible causes of RIF include lifestyle habits (i.e., smoking and obesity), low quality of sperm (especially in older individuals), or thrombophilia (blood clotting disorder). Fertility patients with increased BMI (> 25kg/m²) have a higher chance of implantation failure compared to patients of average weight.^xxvi

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What can be done for people with RIF to achieve a clinical pregnancy?

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Certain therapies may help patients with recurrent implantation failure. However, many of the possible treatment options are still experimental and have not yet been validated by rigorous scientific studies. Proposed treatments for repeated implantation failure can be grouped into four categories:^xxvii

• Uterine interventions – endometrial scratching to stimulate low-level inflammation and natural wound healing processes (by inserting a catheter or pipelle into the uterus), hysteroscopy (uterine surgery to remove polyps/fibroids/adhesions), treatment of endometritis (intrauterine infection/inflammation) with antibiotics, or assessing morphology and quality of the endometrium through endometrial biopsy and microscopic imaging 
• ‍Laboratory and procedural technologies and interventions for the embryo – sequential transfers of embryos on both day 2/3 and day 5 (two transfers in same cycle), embryo transfer with hyaluronic acid media, blastocyst stage embryo transfer, assisted hatching of the embryo (to remove the embryo’s hard shell, the zona pellucida) for those >40 years old, or preimplantation genetic testing for aneuploidy (PGT-A) 
• ‍Immunomodulatory therapies and drugs – injecting peripheral blood immune cells (PBMC infusion) into the uterus, granulocyte colony stimulating factor (G-CSF), autologous (from the same patient) platelet-rich plasma PRP, human chorionic gonadotropin (hCG), intravenous immunoglobulin (antibodies), immunosuppressive drugs (such as prednisolone or tacrolimus), or blood-thinning drugs (such as low-molecular-weight heparin or aspirin) ‍
• Treatments enhancing endometrial receptivity or technologies aimed at identifying the limited endometrial window of implantation (WOI) - injections of human growth hormone (GH), vaginal sildenafil, or sampling endometrial tissue to determine receptivity gene expression using the endometrial receptivity array (ERA) 

There is an ongoing debate about the optimal embryo stage for transfer to increase the success of IVF after repeated implantation failure. There is also a lack of consensus on whether frozen vs. fresh embryo transfer leads to better IVF outcomes, but transfer of frozen embryos may be beneficial for patients with high response. One study observed that frozen embryo transfers had the same live pregnancy rate as fresh transfers if between 1-10 oocytes were retrieved (between 43-67 percent for both groups) and a better live pregnancy rate for frozen transfer compared to fresh transfer if 11-25 oocytes were retrieved (71.5 percent vs 57.8 percent).^xxviii

Scientists have suggested that smoking, obesity, and high cortisol (stress hormone) levels also impact the rate of failed implantation and that lifestyle changes (quitting smoking, following a healthy diet, and regular exercise) can have positive effects for RIF patients. These behavioral changes do not require invasive or medical treatments and may be an optimal first step to improve IVF outcomes.^xxix

Imaging for congenital and acquired anatomical issues

There are several uterine abnormalities that can lead to RIF by distorting the anatomy of the uterus. These include polyps, fibroids, intrauterine adhesions/Asherman’s syndrome, and uterine septa. Sometimes these abnormalities are not detected by traditional transvaginal ultrasound, and more invasive methods such as saline infusion sonogram (SIS), hysterosalpingogram (HSG), or hysteroscopy may be required to better assess the uterus before transferring an embryo. One study found that clinical pregnancy rates were significantly higher in patients who had a hysteroscopic evaluation of the uterus and/or those who had a hysteroscopic polyp removal as compared to those who did not have any such evaluation or intervention (30.4 percent vs. 21.6 percent).^xxx Hysteroscopic resection of uterine septa has also been shown to improve pregnancy outcomes with IVF. For example, one study showed that hysteroscopic treatment of uterine septa in patients undergoing IVF decreased pregnancy loss rate to the level of IVF patients without a uterine anomaly.^xxxi

Immune testing and immunosuppressive treatment

An immune or inflammatory imbalance in the uterus can negatively affect implantation. Abnormal levels of specific immune proteins (cytokines) or the presence of white blood cells beyond a certain threshold have been observed in RIF patients. Tacrolimus, an immunosuppressive drug, has been suggested as a potential treatment for RIF patients with elevated Th1/Th2 (T-helper cells, a type of white blood cells) ratio.^xxxii One study observed that women with elevated Th1/Th2 ratios treated with Tacrolimus had an implantation rate of 45.7 ^xxxiii However, only one study has shown benefit, and this was a cohort study with few patients, not a randomized trial.

Treatment with corticosteroids such as prednisone or dexamethasone has also been suggested to treat patients with RIF. Steroids are immunosuppressive and have been shown to decrease inflammatory markers in the uterus and can be given intravenously, orally, or through the uterus.^xxxiv Some studies have shown that steroids do not improve IVF outcomes. Whereas, a recent small study of 64 patients found improved IVF pregnancy outcomes in patients with idiopathic RIF (RIF with no known cause) treated with steroids.^xxxv More research is required to better understand how best to administer steroid treatment in these patients and who if anyone is likely to benefit from therapy.

Peripheral blood natural killer (NK) cells comprise 5-10 percent of peripheral blood cells, while uterine NK cells comprise 70-90 percent; higher NK-cell proportion (in both the blood and uterus) has been detected in RIF patients when compared to healthy patients. Cut-off values for NK cells still need to be standardized.^xxxvi However, a meta-analysis found no association between NK cell levels and IVF outcomes and no studies have demonstrated improved implantation with immune treatment of “abnormal” uterine NK cell levels.

Thrombophilia testing

Some research studies suggest that genetically inherited thrombophilia (a blood disorder where a person is at risk of developing blood clots in the veins or arteries) may be the cause of RIF in 44 percent of women with unexplained RIF. Therefore, testing for thrombophilia and treatment with thrombotic agents (blood clotting medication) may be an option for patients with RIF.^xxxvii

One study by Quablan et al (2020) detected at least one inherited or acquired thrombophilic factor in 68.9 percent of RIF cases compared to 25.6 percent of non-RIF IVF patients and 25 percent of patients with spontaneous pregnancies.^xxxviii Treatment with blood thinning medications such as heparin or low molecular weight heparin (LMWH or Lovenox) may improve implantation rates in patients with RIF, though not all research has demonstrated a benefit. One study of RIF patients by Lodigiani et al (2011) observed that patients treated with a blood thinning drug (low molecular weight heparin or LMWH) had a higher pregnancy rate of 29.52 percent compared to 17.19 percent not treated with LMWH.^xxxix In contrast, a study by Berker et al of 219 women also published in 2011 demonstrated no statistically significant improvement in pregnancy rate amongst patients with RIF treated with LMWH compared to those not treated.^xl The conclusion of this study was that the population size was likely too small to demonstrate a benefit and that additional larger randomized controlled trials are required to better demonstrate whether blood thinners are likely to beneficial for patients with RIF without known thrombophilia.

Endometrial receptivity analysis (ERA)

The window of implantation (WOI) depends on an optimal uterine environment or endometrial receptivity, with a balance of several factors such as growth and immune factors and adhesion proteins. This window lasts about 48 hours and typically occurs around six days after ovulation. One of the suggested causes of RIF is a change in endometrial receptivity caused by dysregulation of different genes, making the uterine environment no longer optimal to allow a transferred embryo to implant. The ERA test was first introduced by Diaz-Gimeno et al (2011), who described a test to identify changes in 238 genes known to be expressed during the WOI. They hypothesized that this test could determine a patient’s personalized WOI.^xli

The researchers Ruiz-Alonso et al (2013) used the ERA to assess endometrial tissue biopsies from women with a history of RIF and showed that 25 percent of these RIF patients had a shifted WOI (unreceptive endometrium). They observed that changing the timing of their embryo transfer (personalized embryo transfer or pET) resulted in a 38.5 percent implantation rate, similar to that of patients without RIF, and suggested that implantation may be possible in RIF patients with an unreceptive endometrium.^xlii It is important to note, however, that 74 percent of RIF patients had a receptive endometrium on the ERA, meaning no change was recommended in embryo transfer timing. Additionally, only nine patients underwent personalized embryo transfer based on ERA results.  

Another study by Tan et al (2018) analyzed outcomes for patients who had previously failed one euploid embryo transfer and then underwent ERA and pET with euploid embryos. Of note, 22.5% of patients with a failed euploid transfer had a displaced window of implantation so qualified for pET. They observed an approximate 20 percent higher implantation rate for the embryo transfer following ERA and pET compared to prior implantation rates.^xliii This suggests the promising clinical application of the ERA test for patients with an endometrial cause of RIF where embryo aneuploidy has been ruled out by PGT-A, however, it was a retrospective study. Some small prospective studies failed to show a benefit of pET following ERA.^xliv,xlv

Observational studies have reported that abnormal ERA testing results, i.e., those that detected a shifted WOI, occur in about 25 percent of RIF patients but also in 12-15 percent of non-RIF patients, suggesting the existence of a false-positive result. A recent clinical study examining the usefulness of ERA observed that live birth rates for patients undergoing frozen embryo transfers were the same with or without ERA.^xlvi There is still a lack of evidence to determine if ERA can consistently improve IVF outcomes.^xlvii  

BCL-6 Testing  

BCL-6 testing is a newer screening tool that may help assess for risk of RIF in patients undergoing assisted reproduction. The gene BL6, and the protein it codes for, have been found to be elevated in patients with endometriosis and unexplained infertility.^xlviii It is thought that BL6 is a marker for uterine inflammation. Because of this, BL6 testing, which is now available commercially as ReceptivaDx, can screen patients with infertility who are planning IVF to determine if endometriosis or inflammation may be present and inhibit embryo implantation. A recent study demonstrated that in patients undergoing IVF, pregnancy and live birth rates were lower in patients with high BL6 gene expression.^xlix Furthermore, one study of patients with elevated BL6 and unexplained infertility found that treatment with laparoscopic excision of endometriosis or hormonal suppression was found to improve live birth rates after fresh or frozen IVF transfer.^l Extensive data on ReceptivaDx is still pending.

Parental karyotyping

Chromosomal abnormalities in patients with RIF are rare at just 2 percent, but are more frequent compared to the general population. Parental karyotyping (a test to examine chromosome numbers) should be performed for females with a history of RIF and males with severe infertility.^li

The most common of these rare chromosome abnormalities is a balanced translocation, which is a rearrangement of genetic material from one chromosome to another. While the individual has no symptoms from a balanced translocation because all of the required genetic material is still present, eggs or embryos formed from these individuals may have an imbalance in genetic material. This would result in genetic abnormalities in resultant embryos, which can lead to RIF. If a balanced translocation is diagnosed in either partner, then preimplantation genetic testing for structural rearrangements (PGT-SR) can be considered.^lii This technology allows for testing of embryos prior to transfer, so only embryos with a normal amount of genetic material are transferred.  

Donor Egg or Embryo and Gestational Carriers

Women who experience RIF need guidance to make decisions about whether they should continue with more IVF attempts. Using donated gametes (egg and sperm) or looking into a gestational carrier may be necessary if multiple IVF cycles or treatments for RIF have been ineffective. If the source of implantation failure is the embryo (low quality or aneuploidy), embryo adoption or donor egg/sperm may be suggested. On the other hand, if the problem is the uterus, using a gestational carrier should be discussed as an option.^liii

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Conclusion

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Some women trying to achieve pregnancy will experience repeated implantation failures and the frustration that accompanies that undesirable IVF outcome. Why implantation fails is a complex question and one that fertility experts continue to attempt to understand and address. RIF can result from one or several reproductive failure factors, including genetic abnormalities in the embryo or endometrial/uterine issues.^liv,lv Attempting to find a cause and treating any abnormality is essential.  

Patients with repeated implantation issues face additional challenges to family building. However, they should know that assisted reproductive medicine has seen many advances in treatment and that several options exist for achieving successful implantation, leading to a viable pregnancy and live birth in women who have previously experienced RIF.