Recurrent implantation failure (RIF) refers to the situation where IVF treatment has involved multiple embryo transfers that have not resulted in pregnancy. There is no universally agreed clinical definition — some centres use three or more failed transfers, others two — but the experience of repeated failure with apparently good embryos is a distinct clinical scenario that warrants investigation.
It is one of the most frustrating and emotionally difficult situations in IVF, partly because it often has no clear explanation, and partly because patients are frequently offered a range of expensive tests and treatments with variable evidence bases.
Why Implantation Fails
Successful implantation requires two things: a viable, chromosomally normal embryo, and a receptive uterine environment. RIF may result from problems with either, or from their interaction.
Embryo factors — the most common cause. Chromosomal abnormalities in embryos (aneuploidy) are the leading reason IVF cycles fail, even when embryos appear morphologically good. The rate of aneuploidy rises sharply with the egg provider's age. In a patient over 38, the majority of IVF embryos may be aneuploid even if they grade well. This is not visible on morphological assessment (see embryo grading explained) — only genetic testing identifies it.
Uterine factors — structural problems, endometrial issues, or immune phenomena that prevent implantation even with a normal embryo. These include:
- Intrauterine adhesions (Asherman's syndrome)
- Submucosal fibroids or polyps
- Adenomyosis
- Thin or poorly receptive endometrium
- Endometritis (chronic inflammation of the uterine lining)
Thrombophilic conditions — inherited or acquired clotting disorders (such as antiphospholipid syndrome) are associated with implantation failure and recurrent miscarriage in some studies, though causation is debated.
Investigation after Repeated Failure
After two or three failed transfers, most UK fertility consultants will recommend a review and potentially additional investigations. The evidence base for what to test — and what to do with results — varies considerably.
Investigations with reasonably strong evidence:
Hysteroscopy. Direct visualisation of the uterine cavity to check for polyps, fibroids, adhesions, or signs of endometritis. Most fertility specialists consider hysteroscopy appropriate after two or three failed transfers if a saline sonogram or ultrasound has not already been done. Treatable uterine pathology is found in a significant minority of RIF patients.
Preimplantation genetic testing for aneuploidy (PGT-A). Testing embryos for chromosomal abnormalities before transfer. Where multiple embryos are available, this allows selection of euploid (chromosomally normal) embryos and can improve the success rate per transfer by avoiding aneuploid embryos that would not implant regardless of quality appearance. The HFEA currently classifies PGT-A as an "add-on with limited evidence" for all patient groups, but there is reasonable consensus that it has utility in RIF investigation — particularly for patients over 37 or those with a history of failed transfers with good morphology.
Antiphospholipid antibody testing. Testing for antiphospholipid syndrome (APS), which is associated with recurrent pregnancy loss and some evidence of implantation failure. Detected APS is treatable with aspirin and low molecular weight heparin.
Investigations with less established evidence:
Endometrial receptivity testing (ERA). Biopsy of the endometrial lining to identify an individual's optimal "window of implantation" — the timing for embryo transfer. Some clinics offer ERA as a personalised transfer timing tool. The evidence that ERA-guided transfer improves outcomes in RIF is mixed; the HFEA classifies ERA as an add-on without sufficient evidence of benefit. It is typically priced at £700–£1,200 and adds a cycle to the investigation process.
NK cell testing. Tests of natural killer (NK) cell levels in the blood or uterine biopsy. Elevated NK cells have been proposed as a cause of implantation failure. The evidence for NK cell testing and NK-modulating treatments (intralipids, steroids, immunoglobulin infusions) is not established, and the HFEA rates these as add-ons lacking evidence. Most UK fertility specialists are cautious about recommending these outside research settings.
Sperm DNA fragmentation testing. Where sperm DNA fragmentation is high, some clinics believe this may contribute to early embryo arrest or implantation failure, and may recommend modified sperm selection techniques (IMSI, PICSI, microfluidic sorting). Evidence is suggestive but not definitive.
What to Do Next
The most important question in RIF investigation is: are the embryos themselves chromosomally normal?
For patients who have not had PGT-A, the most evidence-supported next step is often doing a cycle with PGT-A to confirm whether euploid embryos are being transferred. If euploid embryos are failing to implant, the investigation shifts more firmly toward uterine and immune factors.
A hysteroscopy, if not already done, is minimally invasive and can identify treatable pathology. The yield from NK cell testing and ERA is much lower — and the clinical benefit of interventions based on those results is uncertain.
It is also worth discussing with your consultant whether a different stimulation protocol, a different lab technique, or transfer at a different stage (Day 3 vs Day 5) might be appropriate based on your specific embryo development history.
The Emotional Dimension
RIF is not just a clinical challenge. Repeated failure — especially in patients who have done "everything right" — is profoundly distressing. The uncertainty about causes and the proliferation of expensive tests that may not lead anywhere adds to this.
Accessing specialist fertility counselling is particularly important in this context. Many UK NHS and private fertility clinics include counselling as part of treatment, and it is available separately through BICA-registered fertility counsellors.
Frequently Asked Questions
Q: How many failed transfers constitute recurrent implantation failure?
A: There is no consensus definition. Most clinical and research definitions use three or more failed transfers, though some centres use two. The ESHRE (European Society of Human Reproduction and Embryology) defines RIF as failure to achieve a pregnancy after transfer of at least three good-quality embryos in three or more IVF cycles. In practice, UK clinics often start investigation after two failed cycles.
Q: Should I have a natural killer cell test?
A: This is one of the more controversial areas of fertility medicine. There is currently insufficient robust clinical trial evidence that NK cell testing identifies a treatable cause of RIF, or that NK-modulating treatments improve outcomes. The HFEA and most mainstream UK fertility specialists do not recommend NK testing or NK-modulating treatments as standard care. If you are considering it, discuss the evidence base and cost carefully with your consultant.
Q: Will PGT-A definitely improve my chances?
A: PGT-A removes aneuploid embryos from the transfer queue — so it can improve the chances that any specific transfer succeeds (by avoiding transfers that were going to fail regardless). But it cannot create more euploid embryos than you have. In older patients with a high aneuploidy rate, it is possible that no euploid embryos are identified in a cohort of retrieved eggs, which is informative but not helpful for achieving a pregnancy with own eggs.
Q: What is the ERA test and should I have one?
A: The ERA (endometrial receptivity analysis) test biopsies the endometrium to identify when it is most receptive, and adjusts the embryo transfer timing accordingly. Its clinical benefit in RIF is not established by high-quality trial evidence. The HFEA lists it as an add-on without sufficient evidence to recommend routinely. It may be worth discussing with your consultant if you have had multiple failures and all other investigations are normal.
Q: If investigation finds nothing abnormal, what does that mean?
A: A normal investigation means no identified structural, immune, or genetic cause was found — not that everything is fine. Unexplained RIF is a recognised diagnosis. Some patients with unexplained RIF eventually achieve pregnancy; others benefit from reviewing the stimulation protocol, changing the clinic, or exploring donor egg treatment. Fertility specialist input — ideally from a centre with specific RIF experience — is valuable at this stage.
This article is for information only and does not constitute medical advice. Always discuss investigation and treatment decisions with a qualified fertility specialist.