What is recurrent implantation failure?

Recurrent implantation failure (RIF) with in vitro fertilization (IVF) occurs when an embryo is transferred into the uterus and does not attach to or implant in the uterine wall, and this failure happens in multiple cycles.ⁱⁱⁱ However, specific criteria for RIF vary across practitioners and organizations. Here is one suggested set of criteria: two cycles in which the patient has a negative pregnancy test (which measures serum hCG) 14 days after embryo transfer of four or more cleavage stage embryos or two good-quality blastocyst embryos.^iv

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 three or more failed embryo transfers (including both fresh and frozen cycles).^v In 2023, an international consensus statement on RIF was published and experts agreed on only providing a RIF diagnosis when a patient has had at least three successive failed transfers of euploid (genetically tested for the correct number of chromosomes) embryos or an equivalent number of unscreened embryos adjusted for patient age and ploidy rate.^vi

A 2019 study analyzed data from 1,221 couples with good prognoses to determine the frequency of RIF. The study found that approximately 15 percent of the couples experienced RIF.^vii However, a large study by Pirtea et al (2021) showed that the true incidence of RIF is likely lower, at less 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 occurred in less than five percent of couples after three transfers. The authors concluded that most failures to implant are likely due to issues with the embryo.^viii 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.^ix

RIF vs. recurrent IVF failure

RIF is not the same as recurrent IVF failure. It is estimated that embryos fail to implant approximately 40 to 75 percent of the time in IVF.^x,xi While this percentage seems high, note that even in young fertile people trying to conceive naturally (not using assisted reproductive technology), the chance of an embryo implanting is only 20 to 25 percent each month.

IVF cycles can fail to result in pregnancy due to many factors (say, poor response to ovarian stimulation with few eggs retrieved, or embryos failing to develop after fertilization) resulting in no embryo transfer being performed. Thus, RIF is just one cause of recurrent IVF failure.^xii

What causes recurrent implantation failure?

Potential causes of recurrent implantation failure are commonly grouped into four categories:^xiii,xiv

• A problem with the embryo (abnormal structure or abnormal genetics)
• A problem with the uterus (structural or microscopic abnormalities)
• A problem with uterus-embryo interaction
• Unexplained

A problem with the embryo

In IVF, the quality of the embryo is believed to be responsible for over one third of implantation failures, but this number may be much higher depending on the patient’s age.^xv,xvi Higher-grade embryos (those with superior morphology, i.e. structure) are more likely to implant successfully.^xvii,xviii 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).^xix,xx

Increased maternal age increases the risk of having aneuploid (genetically abnormal) embryos and thus reduces the chances of implantation.^xxi 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 females under 35 years old compared to females over 35 years old.^xxii

A problem with the uterus

Uterine causes of RIF may come from abnormalities of the uterine lining caused by several factors:^xxiii

• ‍^‍Polyps (benign overgrowth of cells)
• Submucosal fibroids (benign overgrowth of muscular tissue)
• Fibrous adhesions or scar tissue inside the uterus (Asherman’s syndrome)
• Immune dysregulation in the uterus
• An undeveloped thin endometrium
• An unreceptive endometrium (meaning it does not provide the correct molecular environment to allow the embryo to implant

These issues are diagnosed using ultrasound or hysteroscopy (a procedure to look inside the uterus with a small camera).

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

Other factors

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 body mass index (greater than 25 kg/m2) also have a higher chance of implantation failure compared to patients of average weight.^xxviii

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

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:^xxix

• ‍Uterine interventions: Some examples include 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 chronic endometritis (intrauterine infection/inflammation) with antibiotics, or assessing the structure and quality of the endometrium through endometrial biopsy and microscopic imaging.
• ‍Laboratory and procedural technologies and interventions for the embryo: These treatments include 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 females over 40 years old, or preimplantation genetic testing for aneuploidy (PGT-A).
• ‍Immunomodulatory therapies and drugs: Some options include 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 prednisone 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): Some examples include injections of human growth hormone (hGH), 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 versus fresh embryo transfer leads to better IVF outcomes; however, the transfer of frozen embryos may be beneficial for patients who had many eggs retrieved. One study observed that frozen embryo transfers had the same live pregnancy rate as fresh transfers (ranging between 43 and 67 percent for both groups) if between one and 10 eggs were retrieved. However, the live pregnancy rate was better for frozen transfer compared to fresh transfer (71.5 percent vs. 57.8 percent) if 11 to 25 eggs were retrieved.^xxx

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 treatments and may be an optimal first step to improve IVF outcomes.^xxxi

Uterine imaging for anatomical issues

There are several uterine abnormalities that can lead to RIF by distorting the anatomy of the uterus. Some examples include polyps, fibroids, intrauterine adhesions/Asherman’s syndrome, and septate uterus (in which a membrane, or septum, splits the uterus into two parts). 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 patients who had a hysteroscopic polyp removal as compared to individuals who did not have any such evaluation or intervention (30.4 percent vs. 21.6 percent).^xxxii

Whether or not treating uterine septa improves pregnancy outcomes with IVF is unclear.^xxxiii For example, one study showed that hysteroscopic treatment of uterine septa in patients undergoing IVF decreased the pregnancy loss rate to the level of IVF patients without a uterine anomaly.^xxxiv However, the only randomized controlled trial to evaluate uterine septum resection did not find a benefit.^xxxv

Immune testing and immunosuppressive treatment

An immune imbalance or inflammatory imbalance in the uterus could 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.^xxxvi

Immunosuppressive drugs, such as tacrolimus, have been suggested as a potential treatment for RIF patients with elevated Th1/Th2 ratios (Th1 and Th2 are T-helper cells, a type of white blood cell important in the immune system).^{xxxvii,xxxviii} One small study observed that females with RIF and elevated Th1/Th2 ratios treated with tacrolimus had a live birth rate of 60 percent (compared to 0 percent for the controls).^xxxix The treatment group received tacrolimus starting two days prior to embryo transfer for a total of 16 days.^xl

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. They can be given intravenously, orally, or through the uterus.^xli Some studies have found that steroids do not improve IVF outcomes, whereas others have shown a positive impact. For example, 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.^xlii In contrast, a 2023 randomized clinical trial found prednisone did not significantly improve live birth rates in females who had previously experienced two or more unsuccessful embryo transfers in comparison to the placebo (control) group.^xliii More research is required to better understand the potential benefits of steroid treatment in RIF patients and to determine which patients are more likely to benefit from it.

For patients experiencing RIF due to chronic endometritis (inflammation of the endometrium, not to be confused with endometriosis), two small studies observed improved outcomes with doxycycline (an antibiotic)^xliv and prednisone^xlv treatment.

Higher proportions of a type of immune cell called natural killer (NK) cells in both the blood and uterus have also been detected in RIF patients when compared to control groups.^xlvi While elevated uterine NK cells have been associated with RIF, a 2022 meta-analysis of 60 studies found no association between NK cell levels and improved IVF outcomes, such as live birth.^xlvii Overall, there is no robust evidence supporting improved live birth rates with the treatment of “abnormal” uterine NK cell levels. It is also worth noting that based on the evidence, the authors concluded that measuring peripheral NK cells as a “proxy” for uterine NK cell activity is not clinically useful.^xlviii Finally, while uterine NK cells may provide more useful insight than peripheral blood NK cells, cut-off values for high NK cell levels still need to be standardized.^xlix

It is important to note that the American Society for Reproductive Medicine (ASRM) and European Society of Human Reproduction and Embryology (ESHRE) do not recommend immune testing or immunotherapy for patients using assisted reproductive technology (ART), even for individuals with RIF.^l,li Their reason is the current lack of research evidence showing its benefits.

Thrombophilia testing and treatment

Some studies suggest that genetically inherited thrombophilia (a blood disorder in which a person is at risk of developing blood clots in the veins or arteries) may be the cause of recurrent implantation failure in 44 percent of females with unexplained RIF. Therefore, testing for thrombophilia and treatment with antithrombotic agents (medication to reduce the formation of blood clots) may be an option for patients with RIF.^lii

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 IVF patients without RIF and 25 percent of patients with spontaneous pregnancies.^liii

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 (LMWH) had a higher pregnancy rate of 29.52 percent compared to 17.19 percent in individuals not treated with LMWH.^liv In contrast, a study by Berker et al of 219 females, also published in 2011, demonstrated no statistically significant improvement in pregnancy rate amongst patients with RIF treated with LMWH compared to those not treated.^lv This study concluded 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 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 recurrent implantation failure 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 endometrial receptivity analysis (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, thus improving the chance of implantation.^lvi

However, the success of the ERA test in improving implantation and live birth rates has been mixed.

The researchers Ruiz-Alonso et al (2013) used the ERA to assess endometrial tissue biopsies from females with a history of RIF and showed that 25 percent of these RIF patients had a shifted WOI (meaning they may have had an unreceptive endometrium at the time of embryo transfer). They observed that changing the timing of their embryo transfer (called a 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.^lvii However, 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 pET based on ERA results.

Another study by Tan et al (2018) analyzed outcomes for patients who had previously had one failed euploid embryo transfer and then underwent ERA and pET with euploid embryos. Of note, 22.5 percent of patients with a failed euploid transfer had a displaced window of implantation, so they qualified for pET. They observed an approximately 20 percent higher implantation rate for embryo transfer following ERA and pET compared to prior implantation rates.^lviii This improvement suggested a promising clinical application of the ERA test for RIF patients; however, it was a retrospective study, and other studies have had inconsistent results.^{lix,lx,lxi,lxii}

The largest and most robust clinical study evaluating ERA was performed by Doyle et al in 2022.^lxiii It was a two-year multicenter randomized controlled trial involving 767 IVF patients at 30 clinics and investigated the live birth rate results following personalized ET guided by ERA compared to controls. The live birth rate in the group undergoing pET guided by ERA was 58.5 percent compared to 61.9 percent undergoing standard timing frozen embryo transfer (FET); there was no statistically significant difference between the two. Thus, ERA-guided pET did not improve live birth rate in this cohort.

Furthermore, two recent meta-analyses did not show improvements in success rates following ERA.^lxiv,lxv The authors investigated the impact of ERA testing on patients undergoing IVF and receiving frozen embryo transfers, with or without prior ERA testing. In both meta-analyses, the researchers reported that there was no statistically significant change in the rate of pregnancy after IVF cycles using ERA. It was noted, however, that more research is needed in individuals with RIF, as the evidence was not of good quality to assess the RIF sub-population.^lxvi

BCL-6 testing

BCL-6 testing is a newer screening tool that may help assess the risk of recurrent implantation failure 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.^lxvii It is thought that BL6 is a marker for uterine inflammation. Therefore, 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.

One study demonstrated that in patients undergoing IVF, pregnancy and live birth rates were lower in patients with high BL6 gene expression.^lxviii Another 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.^lxix A 2022 case-control study did not find any differences in IVF live birth rates between individuals with or without over-expression of BCL6.^lxx More research into BCL6 and ReceptivaDx™ is still pending.

Parental karyotyping

Chromosomal abnormalities in patients with recurrent implantation failure are rare at just 2 percent, but they are more frequent compared to the general population. Parental karyotyping (a test to examine chromosome numbers) can be considered for females with a history of RIF and for males with infertility.^lxxi,lxxii

The most common of these rare chromosome abnormalities is a balanced translocation, which is a rearrangement of genetic material from one chromosome to another. A balanced translocation causes no symptoms in the individual because all the required genetic material is still present.  However, eggs or embryos formed from these individuals may have an imbalance in genetic material, 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.^lxxiii This technology allows for the pre-transfer testing of embryos so that only embryos with a normal amount of genetic material are used.

Donor egg or embryo and gestational carriers

Females who experience RIF often need to make decisions about whether they should continue with more IVF attempts if multiple IVF cycles or treatments for RIF have been ineffective. If the source of implantation failure is the embryo (due to embryos being low-grade or aneuploid), embryo adoption, donor eggs, or sperm may be suggested. On the other hand, if the problem is the uterus or an unknown cause, using a gestational carrier may be discussed as an option.^lxxiv

Conclusion

Some females trying to achieve pregnancy will experience repeated implantation failures and the frustration that accompanies undesirable IVF outcomes. Why implantation fails is a complex question that fertility experts continue to research and understand. Recurrent implantation failure can result from one or several reproductive factors, including genetic abnormalities in the embryo or endometrial/uterine issues.^lxxv,lxxvi Attempting to find a cause and using evidence-based treatment 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, which have led to viable pregnancies and live births in females who have previously experienced RIF.