PRELI® endometrial assessment

If you've been through a miscarriage, a failed transfer, or fertility treatment that hasn't yet given you an answer, it's natural to focus on the embryo. But an embryo can only do half the job. The other half belongs to the womb lining, known as the endometrium, and how ready it is to receive that embryo.

PRELI® is a molecular test that examines a small sample of your endometrium to build a picture of how it's functioning: whether it's transforming at the right pace to support implantation, how it's responding to progesterone, and what's happening within its immune environment. It draws on more than 20 years of research into implantation and recurrent miscarriage carried out at the University of Warwick Implantation Clinic.

Please note: PRELI® is only available from the 3rd of August 2026.

Why you might consider PRELI®

PRELI® is usually raised as an option once other explanations have been explored, or alongside them. It tends to be most relevant if:

You've had one or more miscarriages and want to understand whether the womb lining played a part
You've transferred good quality embryos more than once without a positive result
You'd simply like the reassurance of ruling out an endometrial factor before your next transfer

Some patients find that a result, whatever it shows, gives them something concrete to plan around. Others use it to rule a factor in or out before deciding on next steps. There's no obligation to have the test, and choosing not to won't affect your care.

PRELI® gives us a much clearer picture of what's happening in the womb lining, not just the embryo. For patients who've had a miscarriage or a transfer that hasn't worked despite a good quality embryo, that can be the missing piece. It doesn't replace the rest of someone's fertility investigations, but it often tells us something we wouldn't otherwise know.

The endometrium is the soil. Progesterone is the weather that prepares it. PRELI® is designed to examine the soil itself, and how it's responding to that weather, rather than the seed.

Timing of the implantation window

By looking at the balance of specific cell types in your biopsy alongside its timing, PRELI® can indicate whether your window of implantation is opening and closing as expected, running late, or moving too fast, which may affect your chance of a successful transfer.

How you respond to progesterone

Your biopsy is read alongside a progesterone blood test taken on the same day. This can reveal progesterone resistance, where tissue doesn't respond as expected despite normal hormone levels, as well as low progesterone, heightened responsiveness, or a typical pattern throughout.

Your endometrium's immune balance

Implantation depends on your immune system adapting to allow it to happen, something that's often misunderstood. PRELI® looks at the balance between immune-tolerant cells and reactive, cytolytic ones in your tissue, adding insight into your endometrial environment.

When and how the test is carried out

Your clinical team will time the biopsy to match how your body would behave during an actual transfer, either within a mock transfer cycle, using the same medication schedule you'd follow for a frozen embryo transfer, or during a natural cycle. In practice, this usually means the sample is taken six to 10 days after a positive ovulation test, or the same window after starting progesterone in a medicated cycle, with a progesterone blood test on the day of the biopsy itself.

The procedure is quick: a thin catheter is passed through the cervix to collect a small tissue sample, usually in a matter of seconds. Most patients don't need anaesthesia, though taking paracetamol around an hour beforehand can help with the period-like cramping some people experience. The sample is then sent away for specialist laboratory analysis.

What to expect afterwards: period-like cramping, light spotting for a day or two, mild pelvic discomfort, or brief dizziness are all possible. Complications from endometrial biopsy are uncommon.

What your results could mean

Roughly 40% of patients get a result showing normal endometrial function. For everyone else, results tend to fall into one of a few patterns: a window of implantation that's running early or late, progesterone responsiveness that's lower or higher than typical, or immune signals that suggest the environment has shifted.

Results generally take about three weeks, after which your fertility specialist will talk you through what they mean and whether they change anything about your treatment plan.

What might happen next

What your clinician recommends depends entirely on what PRELI® finds. For many patients, this means adjusting progesterone support, either increasing or decreasing it. In some cases, a repeat biopsy is suggested, either to confirm treatment has worked or because the original result showed a significant change. It's worth knowing, too, that a normal Preli® result doesn't rule out miscarriage or implantation failure happening for other, unrelated reasons.

Sitagliptin

If your result points to progesterone resistance, your team may raise sitagliptin, a medication more familiar as a type 2 diabetes treatment. Early research suggests it may help recruit the stem cells involved in repairing the endometrium, potentially improving how it responds to progesterone. Its use here is off label, the evidence is still limited to one small trial alongside clinical experience at Warwick, and it isn't considered established care outside a PRELI result.

You'd typically take it for eight weeks before your next transfer or repeat biopsy, stopping as soon as you get a positive pregnancy test, since safety in early pregnancy hasn't yet been established. Most people tolerate it well; a small number experience nausea or abdominal discomfort, and pancreatitis is a rare risk, at around 1 in 2,000. If you develop abdominal pain, nausea, or vomiting while taking it, stop and call your clinic straight away.

Prednisolone

Prednisolone comes up less often. It's well established in pregnancy and fertility care more broadly, though using it specifically after a PRELI result relies more on clinical experience than large-scale trial evidence.

Possible side effects include disrupted sleep, mood changes, a higher chance of infection, and weight gain.

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What PRELI® can't tell you

PRELI® adds a piece to the picture, not the whole picture. It won't guarantee a future pregnancy, doesn't look at embryo quality at all, and can't account for every possible cause of miscarriage or infertility. Some patients whose results look unusual will still have had treatment failures for reasons the test can't explain, and some of the treatments it points towards are still being evaluated clinically rather than fully established.

Where PRELI® currently sits with regulators

PRELI® was developed academically, with its results validated across more than 2,000 samples at Warwick, and it holds the European regulatory approvals needed for clinical use. Its laboratory is still completing the additional ISO accreditation required for routine use within HFEA-licensed clinics, which is why Care Fertility currently offers it through a CQC-regulated clinical service rather than as HFEA-licensed activity. Your consultant can answer any questions about this before you go ahead.

Frequently Asked Questions

Does PRELI® look at the embryo? No, PRELI® doesn't assess embryo quality. It looks only at the womb lining, the environment the embryo would implant into, not the embryo itself.

When is the PRELI® biopsy carried out? The biopsy is usually timed within a mock transfer cycle or a natural cycle, six to 10 days after a positive ovulation test or after starting progesterone, alongside a progesterone blood test on the same day.

What might a PRELI® result show? Around 40% of patients get a normal result. Others may show a delayed or accelerated window of implantation, altered progesterone responsiveness, or patterns suggesting a shift in the immune environment.

Is PRELI® HFEA licensed? PRELI® holds the relevant European regulatory approvals and is offered through a Care Fertility CQC-regulated clinical service. The laboratory's ISO accreditation for routine provision within HFEA-licensed clinics is still in progress.

What happens if my result is low risk? A low risk result means a chromosomal abnormality is unlikely, within the limits of the test. Your clinician may discuss whether further investigations could help look for other possible causes of miscarriage.

Ready to talk about PRELI®?

If you think PRELI® might be right for you, or you'd simply like to ask a few questions first, fill in the form below and a member of our patient enquiry team will call you to talk it through. They'll answer any questions you have and, if you'd like to go ahead, arrange your biopsy at one of our four CQC-licensed clinics.

There's no obligation once you've submitted the form, and no pressure to decide on the call.

PRELI® ENDOMETRIAL ASSESSMENT