The short answer
Low fetal fraction means the blood sample did not contain enough DNA from the placenta for the laboratory to give its usual answer. Common reasons include testing early in pregnancy, higher body weight, twins, IVF, and problems with the sample itself — and occasionally a chromosome difference is involved. A no-result test is not a low-risk result and should not be filed as one. Depending on the report, the scan and how many weeks pregnant you are, the options are usually a repeat blood test later or moving on to a diagnostic test.
What “fetal fraction” actually means
Most of the free-floating DNA in a pregnant person’s blood comes from their own cells. A smaller share comes mainly from the placenta. Fetal fraction is simply the estimated percentage that comes from the placenta.
The laboratory needs enough of that placental signal to pick out the chromosome pattern against the much larger background of your own DNA. If it cannot, it reports something like “low fetal fraction”, “insufficient fetal DNA”, “no call”, “non-reportable” or “no result”.
All of those describe the test’s ability to answer the question. None of them means there is no fetal DNA, and none of them is a diagnosis.
Why it happens
| Possible reason | Why it matters |
|---|---|
| Blood taken early in pregnancy | The placental share rises as pregnancy goes on, so waiting often helps |
| Higher body weight | More of your own DNA in circulation, diluting the placental share |
| Twins, or a twin that stopped developing | More than one placental source complicates the reading |
| IVF, and some health conditions and medicines | These can affect how the test performs |
| A problem with collection or transport | The sample may simply not meet the laboratory’s quality checks |
| A chromosome difference in the placenta or baby | Some differences are linked with a higher no-result rate |
| No clear reason at all | Often there isn’t one |
The reason printed on your report matters far more than the general phrase “the NIPT failed”.
Why a no-result is not a low risk
The laboratory did not get the evidence needed to lower your chance of anything — so nothing has been lowered.
There is also a real, if modest, signal in the data: chromosome differences are found somewhat more often among pregnancies with a no-result than among everyone tested. That does not mean most no-result pregnancies are affected. It means the result needs an active plan rather than being quietly filed as reassuring.
Repeat the blood test, or move on?
A repeat test is often reasonable when the first sample was early, the scan looks fine, and there is enough time left for another result. Waiting a week or two raises the fetal fraction for many people.
Going to ultrasound, genetic counselling and CVS or amniocentesis may be more useful when:
- the laboratory does not recommend a repeat;
- the test has already failed more than once;
- something has been seen on a scan;
- you are far enough along that another wait would cost you options;
- a definite answer matters more to you than another estimate.
Current guidance is that a no-result cell-free DNA test should be followed up rather than ignored, because of that slightly raised average chance. Which route is right still depends on your own report.
Five things to find out before deciding
- How many weeks pregnant you were on the day the blood was taken.
- The fetal fraction, if it was reported.
- The laboratory’s stated reason for not issuing a result.
- Whether they recommend — and pay for — a repeat sample.
- What your latest scan showed, and which diagnostic tests are still available at this stage.
Another blood test is the simple option, but simple is not the only goal. The useful next test is the one that can give you an answer while that answer still changes something.
Sources and further reading
Common questions
- Does low fetal fraction mean something is wrong with the baby?
- Usually it tells you nothing of the kind — most no-result tests happen in perfectly healthy pregnancies. But chromosome conditions are somewhat more common in this group than in everyone tested, which is why the result deserves a proper follow-up rather than either reassurance or panic.
- Will repeating the test give a result?
- Often it will, especially if the first sample was taken early. Not always, though — it depends on the reason, the timing and the laboratory.
- What counts as a normal fetal fraction?
- There is no single normal number, and laboratories use different cut-offs. One laboratory may issue a result at a fraction that another would reject.
- Is a no-result the same as a high-risk result?
- No — it is neither. Some laboratories do separately flag a no-result sample as higher chance, so the exact wording on your report matters.