NIPT results explained

Non-invasive prenatal testing analyses tiny fragments of your baby's DNA (called cell-free DNA) that circulate in your bloodstream during pregnancy. It screens for the most common chromosomal conditions — Down's syndrome (trisomy 21), Edwards' syndrome (trisomy 18) and Patau's syndrome (trisomy 13) — and, depending on the test you choose, the sex chromosomes and certain other conditions. You can compare the options on our NIPT comparison guide.

The single most important thing to understand about your report is this: NIPT is a screening test, not a diagnostic one. According to the Royal College of Obstetricians and Gynaecologists, a 'higher-chance' NIPT result is not a diagnosis of a chromosomal condition — it estimates the chance that a condition is present, and any higher-chance result should be confirmed with a diagnostic test before decisions are made. Every NIPT at the London Pregnancy Clinic includes an ultrasound scan and a specialist review, so your result is never interpreted in isolation.

Laboratories usually report your result in one of three ways. The exact wording differs between tests, but the meaning is consistent.

• Low chance (also called 'low risk' or 'no aneuploidy detected') — the test found no evidence of the conditions screened. This is the most common outcome and is very reassuring, although no screening test can rule a condition out completely.
• High chance (also called 'higher chance', 'high risk' or 'aneuploidy detected') — the test suggests an increased chance of one of the conditions. This is not a diagnosis and needs confirming before any decisions are taken.
• No-call (also called 'no result', 'inconclusive' or 'test failure') — the laboratory could not produce a reliable result, usually because there was not enough of your baby's DNA in the sample. This is not a warning sign in itself, and a repeat sample often gives a clear answer.

You will often see NIPT described as '99% accurate'. That headline usually refers to the test's detection rate (also called sensitivity) — how often it correctly picks up a baby who does have the condition. A large meta-analysis published in Ultrasound in Obstetrics & Gynecology found that NIPT detected 99.7 per cent of babies with Down's syndrome, with a false-positive rate of just 0.04 per cent. Those are genuinely excellent figures.

But detection rate is not the same as the chance that your particular result is correct. The figure that answers the real question — 'my result came back high chance, so what is the actual chance my baby is affected?' — is the positive predictive value, or PPV. PPV is the proportion of high-chance results that turn out to be true positives. Crucially, it is not a fixed number: it depends on how common the condition is in someone like you, which for the conditions NIPT screens rises with maternal age.

The reason is simple arithmetic. When a condition is rare, most people tested do not have it, so even a very small false-positive rate can generate false alarms that rival the number of true positives. When a condition is more common — for example in older mothers — the true positives outnumber the false alarms and the PPV rises. Both the RCOG and published laboratory data make the same point: a test's sensitivity and specificity stay constant, while PPV changes with prevalence.

The worked example below shows how one highly accurate test can give very different PPVs. You can estimate your own figure — based on your age and the specific condition — with our free NIPT PPV calculator, then talk it through with our team or arrange genetic counselling.

A high-chance result is understandably frightening, but it is not a diagnosis — and, as the worked example shows, a meaningful proportion of high-chance results are not confirmed. Take a breath, and do not make any decisions based on the screening result alone.

The only way to know for certain is a diagnostic test that examines your baby's chromosomes directly. In the UK this means chorionic villus sampling (CVS), usually offered from around 11 to 14 weeks, or amniocentesis, usually from around 15 weeks. Both are invasive tests that carry a small risk of miscarriage, so the decision to have one is personal and should be made with your specialist.

Whatever your result, genetic counselling can help you understand exactly what it means for you, weigh up your options without pressure, and decide what to do next. Our specialists also review your ultrasound alongside the blood result, which adds important context that a blood test cannot provide on its own.

If you would like to understand the numbers before your appointment, our PPV calculator and our NIPT comparison guide are good places to start.

Occasionally NIPT returns a 'no-call' — also described as a 'no result', 'inconclusive' or 'test failure'. It usually means the laboratory could not measure enough of your baby's DNA in the sample to give a confident answer. The proportion of the cell-free DNA in your blood that comes from the pregnancy is called the fetal fraction, and if it is too low the test cannot report reliably.

A low fetal fraction is more likely earlier in pregnancy, at a higher maternal weight, or simply by chance on a given day. It is important to know that a no-call is not, by itself, a sign that anything is wrong.

In most cases the next step is a repeat blood test — often called a redraw — sometimes after waiting a week or two so the fetal fraction can rise. Many pregnancies that first return a no-call go on to get a clear result second time around. Our team will advise on timing and, where helpful, may recommend a scan or discuss whether a different test or a diagnostic option is more appropriate for you. Our guide to low fetal fraction and NIPT no-call results explains the causes, how often a repeat sample works and every option in more detail.