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How to read nipt results


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The non-invasive prenatal test (NIPT) is an accurate screening test for common trisomies, sex chromosome aneuploidies and other selected chromosome abnormalities.

NIPT differs from conventional prenatal screening methods in two important ways. First, conventional screening methods which use the concentration of serum proteins in maternal blood and on ultrasound have higher false-positive rates (incorrect “high risk” results in an unaffected pregnancy) and false negatives rates (incorrect “low risk” results in an affected pregnancy) than NIPT. Second, conventional screening methods report an individual risk score for a patient which takes into account ultrasound findings, the results of other screening tests and relevant family history. NIPT relies primarily on the relative concentrations of DNA fragments from different chromosomes in the mother’s blood.

NIPT results are reported as either ‘High Probability’ or ‘Low Probability’ for the chromosome abnormality being tested.  The accuracy of this result can be viewed in two ways:

  • The accuracy of the test in general i.e. the proportion of pregnancies in which the test correctly identifies the fetus as having (or not having) the disorder in question. This refers to the sensitivity and specificity (respectively) of the test for that disorder. These measures of test performance are based on the analysis of thousands of pregnancies. For example, the Table shows the sensitivity and specificity of Sonic’s NIPT for different autosomal trisomies:

  • The accuracy of this test in a particular woman i.e. the chance that her own result correctly identifies whether the fetus has (or does not have) the disorder in question. This refers to the positive and negative predictive value (respectively) of the test for that woman. The answers depend both on the sensitivity and specificity (as above) and the chance that this particular woman could have an affected fetus.

For example, some general scenarios are considered below.

Woman who is at a very low prior risk, e.g. 1 in 5,000 (0.02%) of her fetus having trisomy 21.

This may apply to a young woman with normal fetal ultrasound, or a woman with a low probability result from conventional first trimester screen.

If the NIPT result is LOW PROBABILITY of trisomy 21

The negative predictive value of the test for this patient would be >99.99% i.e. it is almost certain that the result is correct, and that the fetus does not have trisomy 21. However, there is a very small chance (less than 1 in 10,000) that this normal test result is incorrect.

If the NIPT result is HIGH PROBABILITY of trisomy 21

The positive predictive value of the test for this patient would be approximately 20% i.e. it is possible but not definite that the result is correct, and that the fetus has trisomy 21. However, there is an 80% chance that this abnormal test result is incorrect.


A high probability NIPT result is more likely to be correct than a high probability conventional first trimester screen. Nonetheless, a high probability NIPT result must be confirmed by invasive testing by CVS or amniocentesis before making major medical decisions regarding the pregnancy.

Woman who is at low prior risk, e.g. 1 in 1,000 (0.1%) of her fetus having trisomy 21.

This may apply to a young woman with normal fetal ultrasound, or a woman with a low probability result from conventional first trimester screen.

If the NIPT result is LOW PROBABILITY of trisomy 21

The negative predictive value of the test for this patient would be approximately 99.99% i.e. it is almost certain that the result is correct, and that the fetus does not have trisomy 21. However, there is a very small chance (approximately 1 in 10,000) that this normal test result is incorrect.

If the NIPT result is HIGH PROBABILITY of trisomy 21

The positive predictive value of the test for this patient would be approximately 50% i. e. it is likely but not definite that the result is correct, and that the fetus has trisomy 21. However, there is a 50% chance that this abnormal test result is incorrect.

A high probability NIPT result is more likely to be correct than a high probability conventional first trimester screen. Nonetheless, a high probability NIPT result must be confirmed by invasive testing by CVS or amniocentesis before making major medical decisions regarding the pregnancy.

Woman who is at increased prior risk, e.g. 1 in 100 (1%) of her fetus having trisomy 21.

This may apply to an older woman, or a woman with a high probability result from conventional first trimester screen.

If the NIPT result is LOW PROBABILITY of trisomy 21

The negative predictive value of the test for this patient would be >99. 9% i.e. it is highly likely that the result is correct, and that the fetus does not have trisomy 21. However, there is a very small chance (approximately 1 in 5,000) that this normal test result is incorrect.

If the NIPT result is HIGH PROBABILITY of trisomy 21

The positive predictive value of the test for this patient would be approximately 90% i.e. it is very likely that the result is correct, and that the fetus has trisomy 21. However, there is a 10% chance that this abnormal test result is incorrect.

A high probability NIPT result is more likely to be correct than a high probability conventional first trimester screen. Nonetheless, a high probability NIPT result must be confirmed by invasive testing by CVS or amniocentesis before making major medical decisions regarding the pregnancy

Woman who is at high probability, e.g. 1 in 10 (10%) of her fetus having trisomy 21.

This may apply to an older woman, or a woman with a very high probability result from conventional first trimester screen, or evidence of fetal malformations on ultrasound.

If the NIPT result is LOW PROBABILITY of trisomy 21

The negative predictive value of the test for this patient would be 99% i.e. it is likely that the result is correct, and that the fetus does not have trisomy 21. However, there is an approximately 1% chance that this result is incorrect for trisomy 21. In addition, this result does not preclude a chromosome disorder other than trisomy 21 being the underlying cause of her being identified as “high prior probability”. Invasive testing by CVS or amniocentesis with cytogenetic studies may still be warranted to check for a chromosome abnormality other than trisomy 21.

If the NIPT result is HIGH PROBABILITY of trisomy 21

The positive predictive value of the test for this patient would be approximately 99% i. e. it is very likely that the result is correct, and that the fetus has trisomy 21. Invasive testing by CVS or amniocentesis with cytogenetic studies would be warranted to confirm the diagnosis of trisomy 21.

Given that both a “low probability” result and a “high probability” result yield the same recommendation for invasive genetic testing, NIPT is not recommended as a screening test in women if there is a high prior probability of a chromosome abnormality.

Why is NIPT sometimes incorrect?

It is important to note the DNA fragments that are tested by NIPT are derived from the placenta and not the fetus. The placenta and fetus develop from the same cell i.e. the fertilised egg, and almost always have the same number of chromosomes. However, there are exceptions, and these can account for those uncommon situations in which the NIPT result does not match the fetal chromosomes.

For this reason, NIPT must be regarded as a screening test i.e. there is a small well-defined possibility that the NIPT will not reflect the actual chromosome status of the fetus. A “high probability” result should be confirmed by invasive genetic testing (CVS or amniocentesis) before taking irreversible steps in the management of the pregnancy. Conversely, a “low probability” result may still warrant invasive genetic testing if there are other indicators of a fetal chromosome disorder.

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Non-invasive prenatal testing (NIPT) | Pregnancy Birth and Baby

Non-invasive prenatal testing (NIPT) | Pregnancy Birth and Baby beginning of content

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Key facts

  • The non-invasive prenatal test (NIPT) screens your baby for genetic health conditions.
  • You may want to consider genetic counselling before having an NIPT to help you make an informed decision.
  • If your NIPT result shows that your baby is likely to have a chromosomal difference, a diagnostic test such as chorionic villus sampling or amniocentesis can confirm your result.

What is the non-invasive prenatal test (NIPT)?

The non-invasive prenatal test (NIPT) is a very accurate screening test. Screening tests are used to see if your baby has a high chance of a genetic health condition. These conditions include Down syndrome and other chromosomal differences. The NIPT involves a simple blood test that is done in your first trimester of pregnancy.

There are different kinds of prenatal testing available to check the health of your baby. It’s your choice if you would like to have these tests. Talk about your options with your doctor or genetic counsellor and give yourself some time to make your decision.

Screening tests are different to diagnostic tests. Diagnostic tests are often more invasive. However they confirm for certain if the result is positive.

During pregnancy, some of the baby’s DNA passes into your bloodstream. The non-invasive prenatal test analyses the genetic information contained in this DNA. It’s used to screen for a number of genetic conditions. The test is particularly sensitive to Down syndrome. It was first offered in Australia in 2012.

In Australia, NIPTs are offered in private centres and involve an out-of-pocket cost. They are sometimes referred to by different names, depending on the company that makes them. They might be called: Harmony, Generation or Percept.

An NIPT is done from 10 weeks into the pregnancy. Before the test you will be asked to give consent.

What does it test for?

The NIPT is a safe and very effective way of screening for certain conditions. These include:

  • Down syndrome (also called trisomy 21)
  • Edwards syndrome (trisomy 18)
  • Patau syndrome (trisomy 13)
  • Turner syndrome

Some laboratories also test the sex of your baby and look for differences with the sex chromosomes.

The test identifies many chromosomal anomalies. It doesn’t screen for genetic disorders such as:

  • cystic fibrosis
  • thalassaemia
  • sickle cell anaemia

Screening will tell you how likely it is that your baby has a chromosomal difference. The only way of knowing for sure is to have a diagnostic test such as:

  • chorionic villus sampling (CVS)
  • amniocentesis

Should I have an NIPT?

The NIPT is very sensitive. It picks up more than 99% of cases of Down syndrome. But it is a screening test rather than a diagnostic test.

It can tell you whether there is an increased chance of having a baby with a genetic condition. It doesn’t give you a definitive answer. For some parents, information from screening tests can help them decide about whether to have diagnostic testing.

You might choose to have an NIPT test if:

  • Your first trimester combined screening test shows you have an increased chance of having a baby with Down syndrome (this test combines results from a blood test at 10 to 12 weeks and an ultrasound at 11 to 13 weeks).
  • You did not have the first trimester combined screening test because it was too late or the test wasn't available in your area.
  • You want to understand your chance of having a baby with Down syndrome before considering diagnostic tests such as amniocentesis or CVS.
  • You have an increased chance of having a baby with Down syndrome because you are older or you or your partner already have a baby with Down syndrome or another genetic condition.

It’s a good idea to consider genetic counselling before you have an NIPT to help you make an informed decision. It’s important to understand the risks and benefits of having the test.

Genetic counsellors can:

  • review your family and medical history
  • give you information about genetic tests
  • help you learn about how the condition is inherited
  • offer advice on support services

If your pregnancy is affected by a genetic condition, your genetic counsellor can:

  • help support you
  • help you understand what the results mean for you and your family

What can I expect from my NIPT results?

It can take up to 2 weeks to get the result of your NIPT.

If your NIPT result shows that your baby is likely to have a chromosomal difference, a diagnostic test such as: CVS or amniocentesis can confirm the result.

You should discuss your options with your doctor, midwife or genetic counsellor.

How much does the NIPT cost?

The NIPT is not covered by Medicare or private health insurance in Australia. You can expect to pay about $400 to $500 for an NIPT.

You may also need to pay for: an appointment with your doctor to get a referral, as well as an ultrasound.

You should speak with your doctor or a genetic counsellor before getting an NIPT. You can find genetic counselling services near you using the healthdirect Service Finder here.

You can read more about genetic counselling here.

If you have any questions about the NIPT, you can call the Pregnancy Birth and Baby helpline.

Speak to a maternal child health nurse

Call Pregnancy, Birth and Baby to speak to a maternal child health nurse on 1800 882 436 or video call. Available 7am to midnight (AET), 7 days a week.

Sources:
The Royal Women's Hospital Victoria (Genetic testing in pregnancy), NSW Health (Genetic and genomic testing for intellectual disability and childhood syndromes), O&G Magazine (Developments in non-invasive prenatal testing: is bigger better?), Allied Health Professions Australia (Genetic Counselling)

Learn more here about the development and quality assurance of healthdirect content.

Last reviewed: September 2022


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