NIPT and What It Can Tell You

What NIPT actually measures

Non-invasive prenatal testing works on DNA from the placenta circulating in your bloodstream. The NHS Fetal Anomaly Screening Programme handbook describes it as "a technique that uses placental deoxyribonucleic acid (DNA) in the maternal blood to assess the chance of the baby having T21, T18 or T13".

The proportions explain both its power and its limits: "The majority of cfDNA is maternal, usually consisting of approximately 90% DNA from the woman's cells and 10% DNA from the placenta. If the baby has T21, there should be slightly more chromosome 21 than expected from the cffDNA in the maternal blood."

Two facts follow from that. The DNA is placental, not fetal, which is why a placenta with a different chromosome pattern from the baby can produce a misleading result. And placental DNA "remains in the maternal blood for only a few hours after each pregnancy", which is what makes the test specific to the current pregnancy rather than picking up a previous one.

Who is offered it on the NHS, and who is not

This is the point most misunderstood, because NIPT is widely advertised as a first-line test. On the NHS it is not. The handbook states that NIPT "is offered following a higher chance result (between 1 in 2 and 1 in 150) from either the NHS combined or the quadruple test in both singleton and twin pregnancies".

Its eligibility list confirms NIPT can be offered "when a woman receives a higher chance result for T21 or a joint higher chance result for T18 and T13 from the NHS combined test", in twin pregnancies, in IVF or donor egg pregnancies, and "up to 21 weeks and 6 days (21+6) of pregnancy".

Its exclusions are equally explicit. NIPT cannot be offered "when a woman receives a lower chance result for T21, T18 or T13", "in higher multiple pregnancies (triplets or more)", or "after 21+6 weeks of pregnancy".

So a lower-chance combined test result closes the NHS NIPT door, however much you might want the extra reassurance. That is a programme decision, not a local rationing one.

What the NHS test covers, and what it deliberately does not

The handbook is unambiguous: "As part of the NHS FASP care pathway, NIPT screening is offered for all 3 conditions (T21, T18 and T13) only and will not screen for other chromosomal conditions or assess the baby's sex."

That last clause is the difference people notice most. NHS NIPT does not tell you whether you are having a boy or a girl. Private NIPT usually does, and also frequently offers sex chromosome conditions and microdeletion panels that the NHS programme does not include.

Those extra panels are where private testing gets complicated. The rarer a condition, the more likely a positive screening result is to be a false alarm, simply because the condition is uncommon. If you are considering a private test, ask specifically what happens after a positive result for each condition on the panel and what the confirmatory pathway is.

Why it is still screening, not diagnosis

NIPT is far more accurate than the combined test for trisomy 21, but it does not diagnose. The FASP handbook keeps it firmly inside the screening pathway: after a higher-chance combined or quadruple result, women can choose "no further testing", "NIPT screening for all 3 conditions", or "prenatal diagnosis (PND), such as chorionic villus sampling (CVS) or amniocentesis".

A higher-chance NIPT result therefore leads to the offer of a diagnostic test, and the programme sets a timing standard for it: the procedure "should be completed within 3 working days of women receiving their higher chance or 'no result' NIPT screening results".

ACOG's patient information draws the same line for the same reason: screening tests "can tell you the chances that your fetus has an aneuploidy", while diagnostic tests tell you "with as much certainty as possible" whether it is actually present.

No result, and why it happens

Sometimes the laboratory cannot produce a result, usually because there is not enough placental DNA in the sample. This is more common earlier in pregnancy and at higher maternal weight. The FASP pathway treats a "no result" the same way as a higher-chance result for the purposes of the three-working-day standard on prenatal diagnosis, which tells you how it is regarded clinically.

A repeat blood test is often possible, but the 21+6 cut-off does not move, so a no result late in the window can force a decision quickly.

The consent detail worth knowing

The handbook asks that "women should be informed that the laboratory may keep NIPT screening samples for quality assurance (for example, validation) and testing development purposes for up to 5 years. The healthcare professional should inform the laboratory if a woman does not want her sample to be kept for these purposes. These discussions (including the decision on the retention of samples) must be recorded in the woman's maternity notes."

If nobody raised sample retention with you and you have a view about it, you can say so, and it should be recorded.

Making the decision

The practical question is what you would do with each possible result. NIPT avoids the miscarriage risk of an invasive test, and RCOG puts that additional risk at below 0.5% in singleton pregnancies with a skilled operator. But NIPT adds time before any definitive answer, and where an ultrasound finding rather than a blood result prompted the referral, teams often discuss going straight to diagnosis.

There is no wrong answer here, and declining further testing is a listed option in the national pathway. Antenatal Results and Choices supports parents through exactly these decisions and is independent of the maternity service.