NIPT is sold with a number: 99 point something percent accurate. That number is real, it is the wrong one to make decisions with, and the gap between the two is where most of the distress around this test comes from.
The advertised figure is sensitivity — how reliably the test flags an affected pregnancy. The figure that decides what a positive result means for you is the positive predictive value, and it is considerably lower, varies by condition, and depends on how common the condition is in the first place. This guide covers what the test reads, what it can and cannot conclude, why false positives happen and what each kind of result leads to.
The short answer: NIPT analyses placental DNA circulating in your blood and can be taken from 10 weeks. It screens for trisomies 21, 18 and 13, and usually reports the sex. It is much more accurate than the combined test but is still a screening test: in a public screening programme the positive predictive value for trisomy 21 was 91.78%, and lower for the rarer trisomies. A positive result always needs confirming by CVS or amniocentesis before any decision is made on it.
What the test actually reads
From about 5 weeks of pregnancy, fragments of DNA from the placenta cross into your bloodstream and circulate alongside your own. NIPT sequences everything in a blood sample and counts how much comes from each chromosome. A pregnancy with three copies of chromosome 21 produces slightly more chromosome-21 fragments than expected, and the test detects that excess.
Two consequences follow from this, and both explain almost everything else on this page:
- The DNA is placental, not fetal. The placenta and the baby usually have the same chromosomes, but not always. When they differ, NIPT describes the placenta.
- The test needs enough of it. The proportion of circulating DNA coming from the placenta is called the fetal fraction, and below a threshold the count is not reliable enough to report at all.
Sensitivity, specificity and the number that matters
Three different questions get answered by three different numbers, and only one of them is about your result.
| Measure | The question it answers | Who it matters to |
|---|---|---|
| Sensitivity | Of affected pregnancies, how many does the test flag? | The programme designing the pathway |
| Specificity | Of unaffected pregnancies, how many does it correctly clear? | The programme, again |
| Positive predictive value | My result was positive. What is the chance it is right? | You, holding the result |
Positive predictive value is not a fixed property of the test. It moves with how common the condition is in the group being tested. That is why the same laboratory, with the same assay, produces a much higher PPV for trisomy 21 than for trisomy 13 — and why a NIPT taken after a higher-chance combined test result performs better than the same test taken by someone with no prior indication.
In a public screening programme, the PPV for trisomy 21 was measured at 91.78%. That is genuinely good. It also means roughly one positive result in twelve is a false alarm, which is not what "99% accurate" prepares anyone for.
Why false positives happen
Confined placental mosaicism
The most common biological cause. A chromosomal abnormality is present in the placenta and absent in the baby, so the test reports something real that does not apply to the pregnancy. After a positive NIPT for trisomy 21, the chance of finding mosaicism in a chorionic villus sample is around 2%. This is the single strongest argument for confirming with amniocentesis, which samples amniotic fluid containing fetal cells rather than placental tissue.
A vanishing twin
If a twin stopped developing early, its DNA can persist in your circulation for weeks and be counted alongside the surviving pregnancy's. In one analysis of high-risk NIPT results in pregnancies with a vanishing twin, only half were confirmed on diagnostic testing. If any scan ever showed two sacs, say so before the sample is taken — it changes how the result should be read.
Maternal factors
Some of the circulating DNA is yours. A maternal chromosomal variation, a benign tumour, a previous transplant or, rarely, an undiagnosed maternal cancer can all shift the counts. These are uncommon, and they are a reason that an unusual pattern of results is investigated rather than simply reported.
No positive NIPT result should lead to a decision on its own. ACOG, the NHS programme and every equivalent body say the same thing: it is a screening result, and confirmation by CVS or amniocentesis comes first. A private clinic that reports a positive without arranging that conversation has not finished the job.
When there is no result at all
Around 2.9% of samples in one series returned no result, almost always because the fetal fraction was too low. The factors that push it down are known: earlier gestation, a higher maternal BMI, and fertility treatment.
A repeat sample a week or two later often succeeds. What matters is that a persistent no-call is not simply a technical annoyance — a consistently low fetal fraction is itself associated with a slightly raised chance of an abnormality, so the right response is a conversation about alternatives rather than a third attempt.
NIPT after a higher-chance screening result
This is where NIPT does its best work, and where most people meet it. After a higher-chance result from first trimester screening, NIPT sits between doing nothing and an invasive test: it substantially refines the probability without any procedure risk.
The trade-off is time. A NIPT result takes one to two weeks, and if it is positive you are still facing the same decision about a diagnostic test, now later in the pregnancy. Some people prefer to go straight to CVS or amniocentesis for that reason. Neither choice is the careful one; they optimise for different things.
💬 Worth establishing before you consent
- Which conditions does this specific panel report? The three trisomies are evidence-based; the extras are not equally so.
- Will I be told the fetal fraction? It is on the report and it tells you how much weight the result carries.
- What is the PPV for my situation? A good laboratory will give a figure adjusted for your age and prior screening result, not a generic one.
- Who arranges the follow-up if it is positive? Ask this of a private provider before, not after.
What NIPT does not tell you
- It is not a general test of health. Most conditions found in pregnancy are structural, and those are seen at the 20-week anomaly scan, not in blood.
- It does not replace the scans. A low-risk NIPT alongside a raised nuchal translucency still needs investigating, because the nuchal measurement is associated with heart defects independently of chromosomes.
- It cannot exclude everything. It reports on the chromosomes it counts and says nothing about the rest of the genome.
- The sex result is a by-product. Our guide to finding out the baby's sex covers how that part performs and where it is restricted.
When to call your midwife
Call the same day if: you have vaginal bleeding, severe or one-sided abdominal pain, shoulder-tip pain, faintness or a fever. A blood test causes none of these, and they are the reasons to ring during the weeks this page covers regardless of what any screening result said.
A positive NIPT result is never an emergency and nothing about it needs deciding the same week. If a result arrives without a follow-up appointment attached, chase the appointment — the thing you need next is a conversation with someone who reads these results daily, and that conversation is part of the pathway rather than a favour.
Frequently Asked Questions
At how many weeks can NIPT be done?
From 10 weeks. The limit is not arbitrary: the test needs enough placental DNA circulating in your blood to analyse, and before 10 weeks that fraction is often too low, which produces a failed test rather than a wrong one. Later is generally better for the fetal fraction, so a test at 11 or 12 weeks is more likely to return a result first time than one taken the day you become eligible.
Is NIPT the same as a diagnostic test?
No. NIPT analyses DNA shed by the placenta, not by the baby, and the two are not always genetically identical. It is far more accurate than the combined test, but it is still screening: a positive result needs confirming by chorionic villus sampling or amniocentesis before any decision is made on it, and every professional body says so.
How accurate is NIPT really?
Its sensitivity — the proportion of affected pregnancies it flags — is very high, which is the number marketing quotes. The number that matters when you receive a positive result is the positive predictive value: the chance the result is correct. One study in a public screening programme found a PPV of 91.78% for trisomy 21, and lower values for trisomies 18 and 13, which are rarer. Roughly one positive T21 result in twelve is a false alarm.
Why is PPV lower for Edwards' and Patau' syndromes?
Because they are much rarer. When a condition is uncommon, even a very specific test produces more false positives than true ones relative to the number of cases — the arithmetic of screening, not a flaw in the assay. Trisomies 13 and 18 are also more often confined to the placenta, which adds a second source of false positives on top of the statistical one.
What is confined placental mosaicism?
It is when a chromosomal abnormality exists in the placenta but not in the baby. Since NIPT reads placental DNA, this produces a positive result for a pregnancy that is unaffected. Following a positive NIPT for trisomy 21, the risk of finding mosaicism in a chorionic villus sample is around 2%. It is the main biological reason a confirmatory test is always required.
What does a "no result" or failed NIPT mean?
Usually that the fetal fraction — the proportion of circulating DNA coming from the placenta — was too low to analyse. In one series this happened in about 2.9% of samples. It is more common early in pregnancy, at a higher maternal BMI and after fertility treatment. A repeat sample often succeeds, though a persistently low fetal fraction is itself associated with a slightly higher chance of an abnormality and is usually discussed rather than simply repeated.
Can NIPT be wrong if I had a twin that did not develop?
Yes, and this is one of the clearest causes of a false positive. DNA from a twin that stopped developing early — a vanishing twin — can persist in your blood for weeks and be read as an abnormality in the surviving pregnancy. In one analysis of high-risk results in that situation, only half were confirmed. Tell whoever orders the test if an early scan ever showed two sacs.
Should I pay for a NIPT panel that screens for extra conditions?
Expanded panels add microdeletions and rare syndromes, and this is where the PPV problem is at its worst: the rarer the condition, the more of the positive results are false. A positive for a very rare microdeletion is more likely to be wrong than right, while still triggering an invasive test and weeks of anxiety. The three common trisomies are what the evidence supports; everything beyond that is worth a specific conversation before you consent, not after.
NIPT — at a glance
NIPT reads placental DNA circulating in your blood from 10 weeks and screens for trisomies 21, 18 and 13. Its sensitivity is very high, which is the figure used in advertising, but the figure that matters when a result is positive is the positive predictive value: 91.78% for trisomy 21 in one public programme, and lower for the rarer trisomies. False positives come mainly from confined placental mosaicism, present in around 2% of chorionic villus samples following a positive T21 result, and from a vanishing twin, where only half of high-risk results were confirmed in one analysis. About 2.9% of samples return no result at all because the fetal fraction is too low, which is commoner early, at a higher BMI and after fertility treatment. It is a better screening test than anything before it and it is still a screening test — a positive result is confirmed by CVS or amniocentesis before any decision follows from it.
Inside Baby Novum: the lab results journal keeps each result with its date, gestational week and a photo of the report, so the screening result, the NIPT result and any diagnostic result sit in one dated series rather than three emails. Diary entries and results are encrypted on the device with a hardware-backed key, and there is no account and no server holding a copy.
Read next
Sources
- NHS — Screening for Down's, Edwards' and Patau's syndromes
- ACOG — Prenatal Genetic Testing Chart
- Eur J Obstet Gynecol Reprod Biol 2018 — NIPT in pregnancies with trisomy 21, 18 and 13 performed in a public setting: factors of importance for correct interpretation of results
- Reilly, The Obstetrician & Gynaecologist 2023 — Pitfalls of prenatal diagnosis associated with mosaicism
- Frontiers in Pediatrics 2022 — Factors affecting the fetal fraction in noninvasive prenatal screening: a review