What is prenatal testing? And what is NIPT?

Jun 2, 2023 | Resources

 

Prenatal testing helps doctors and obstetric specialists understand how your pregnancy is progressing. It looks for complications or issues that may be affecting you or your unborn baby. 

Prenatal testing refers to a range of tests which can be performed in a pregnancy to either give a prediction or probability of complications, or to offer definitive diagnosis. This includes screening for maternal complications such as pre-eclampsia, but most commonly refers to screening for chromosomal differences. There are a range of available prenatal tests, and they are either conducted as part of a routine process (this is called screening), or as part of a diagnostic process to diagnose a specific condition, or when screening has suggested a high probability of complication (this is called a diagnostic test). Prenatal screening may be as simple as a blood test, or, if diagnosis is needed, may involve an invasive procedure.

At QUFW, we can offer a range of prenatal screening , including non-invasive prenatal testing (“NIPT”), and diagnostic testing in the form of CVS (chorionic villus sampling) or amniocentesis. We’re also passionate about educating our patients and community about prenatal testing. 

This fact sheet answers these questions:

What is prenatal screening?

Prenatal screening is available to all pregnant women. There are different types of prenatal screening available, which differ in both accuracy of their predictions and their cost. They also vary in the detail of information that they can provide. In making the decision of which test, if any, is right for you, it is important to understand not just what the test can provide, but also its limitations. 

It is your choice whether or not to have prenatal screening for chromosomal anomalies. Your healthcare provider will be able to help you in making the decision of whether prenatal screening is right for you, and if so, which type of screening you prefer.

At QUFW, we provide both First Trimester Combined Screening and NIPT (non-invasive prenatal screening).

In certain circumstances, such as high probability result on screening, or known history of inherited genetic conditions, diagnostic testing may be indicated in a pregnancy. In this circumstance, we can guide you as to the next steps. 

Image showing fetal heart rate confirming live pregnancy.

 

What is a genetic abnormality?

A genetic abnormality occurs when a person’s genes or chromosomes don’t function like they normally should. Genetic or chromosomal abnormalities occur in different ways. For example:

  • An extra chromosome may be present (this is called ‘trisomy’)
  • A chromosome may be missing (this is called ‘monosomy’)
  • Copy number variants (CNVs) include missing (deletions) or extra pieces (duplications) of chromosomes
  • Single gene mutations can occur. These impact a very small amount of genetic information, but sometimes in a critical way that impacts its function. Single gene deletions can cause conditions such as cystic fibrosis, thalassemia and sickle cell disease.

Prenatal screening tests differ in their sensitivity (accuracy of prediction), and in the range and depth of chromosomal or genetic changes that they can predict.

 

What conditions does prenatal screening look for?

The prediction of prenatal screening depends upon the type of test that you choose. 

  1. Combined first trimester screening (CFTS) is a combination of a blood test and early ultrasound. It provides a prediction of common aneuploidy (an extra copy of a chromosome) only: 
  • Trisomy 21 (Downs Syndrome)
  • Trisomy 18 (Edwards syndrome)
  • Trisomy 13 (Patau syndrome) 

This test provides a number based prediction of probability of one of the above major chromosomal conditions based on association only. That means that the calculation of risk looks at features, both of the mother and the pregnancy, and how likely they are to be associated with a pregnancy impacted by one of the above chromosomal anomalies. It is important to remember then that there can be many other reasons for a high probability result on this type of testing, not just aneuploidy as listed above. 

  1. Non-invasive prenatal testing (NIPT)

NIPT is a non-invasive way of screening for major chromosomal anomalies. It involves a blood test which is analysed in the lab to detect fragments of circulating placental DNA. Using the assumption that placental and fetal DNA are the same (true in most, but not all cases), the NIPT can then analyse the DNA sample to look for abnormalities in the amount of chromosomal material.

 There are various platforms for this testing, which, depending on their depth, can offer a prediction for: 

  • Major trisomies i.e. an extra copy of a chromosome (Down syndrome (Trisomy 21), Edwards syndrome (Trisomy 18) and Patau syndrome (Trisomy 13)).
  • Rare trisomies (ie. an extra copy of any of the chromosomes other than chromosome 21, 13 or 18. 
  • Sex chromosome abnormalities (eg. Turner syndrome, Klinefelter syndrome, Triple X syndrome, and XYY syndrome).
  • Conditions caused by changes to chromosome structure (missing or extra pieces of chromosomes).
  • Microdeletions (such as DiGeorge syndrome and Prader-Willi syndrome).

A result from a NIPT test will be reported as ‘low probability’ or ‘high probability’ only. 

NIPT is considered to be a highly sensitive screening test. This means that, when it returns a low probability result, it is very rare that the test would ‘miss’ reporting a major chromosomal anomaly. 

There can however be ‘false positive’ results – this is when a NIPT reports a high probability result, where diagnostic testing later proves that the fetus is not affected. 

 

Can prenatal testing detect all birth defects and/or genetic disorders? How accurate is it?

It is important to remember that even highly sensitive screening tests such as NIPT are not diagnostic, and only offer a prediction or probability of a chromosomal condition.

No screening test in pregnancy will be able to totally exclude the possibility of any genetic anomaly in an unborn baby.

 

Can prenatal testing tell me the sex of my baby?

NIPT can predict the sex of your baby by detecting the presence or absence of the Y chromosome.

 

What if I don’t want to know the sex of my baby?

If you have a prenatal test and don’t want to know the sex of your baby, please tell your doctor or obstetric specialist. It is not compulsory to be told the sex of your baby before it is born.

 

Is prenatal screening compulsory?

Prenatal screening is not compulsory. It is a choice for your family. Your medical practitioner can guide you through the process of making this decision. 

It is important to remember that antenatal diagnosis of major chromosomal anomalies in a fetus can help medical providers to tailor your care during pregnancy. This means that even if antenatal diagnosis of a major chromosomal difference would not alter your decision making in a pregnancy, the knowledge that your baby has a chromosomal or genetic difference can help improve care, surveillance and tailor delivery planning to ensure the best outcomes for you and your baby. 

The choice of whether to have screening, and what to do with the result of that screening, is yours every step of the way. 

 

What happens if prenatal screening returns a high probability result? 

If you receive a high probability result on a prenatal screening test (greater than 1:300 risk from CFTS or a high probability result on NIPT), your doctor or obstetric specialist will explain what it means and will counsel you on your options. They will answer your questions and help you make the best possible decision for you and your pregnancy. 

Remembering that, as screening tests, CFTS and NIPT are not diagnostic, further testing may be offered. Diagnostic tests available in a pregnancy include chorionic villus sampling (CVS) and amniocentesis. These are both needle tests which carry a small procedure-related risk which will be explained by the specialist if you choose to have a test. 

  • Chorionic Villus Sampling (CVS): This test takes a sample of cells from the placenta through a thin needle inserted, most commonly, through the mother’s abdomen. It can be performed from a gestational age of eleven weeks. 
  • Amniocentesis: This test involves a thin needle placed through the maternal abdomen to take a sample of amniotic fluid from around the fetus. Amniocentesis can be performed from a gestational age of fifteen weeks. 

If diagnostic testing is required you will be referred for a consultation with a specialist who will explain the types of testing available, risks associated, and the procedure itself. There may be some circumstances in which an amniocentesis may be beneficial instead of a CVS, and this will be explained. 

Additionally, the doctor will explain the testing which will be done in the lab on the sample that has been obtained. This includes a short term result (FISH) and long term result (chromosomal microarray). In certain circumstances there may be indication to also perform more detailed testing such as whole exome sequencing (WES). In these circumstances, consultation with a Clinical Geneticist will be arranged to ensure you are adequately informed of the testing. 

 

Can I opt to have diagnostic testing i.e. CVS or amniocentesis without having a screening test?

Yes. If you want certainty about your baby’s chromosomes and some genetic conditions, you can opt to have a CVS or amniocentesis without having NIPT or CFTS prior.  

It is important to note that with current technology even a diagnostic test will not exclude all genetic conditions but it will be able to exclude chromosomal abnormalities and many genetic conditions. 

 

If I have a high-risk pregnancy, will I need more testing than others?

This depends on the nature of your pregnancy. Remember that there are different types of risks, and risk of growth restriction or maternal complications does not necessarily mean an increased risk of chromosomal anomalies. Please speak to your doctor or obstetric specialist for more information about your specific situation.

 

Can prenatal testing be harmful to my baby?

Screening tests, such as CFTS (combined first trimester screening) and NIPT, are non invasive.  In the case of NIPT, only a blood test from the mother is needed. In the case of CFTS, only a blood test and ultrasound is needed. These do not carry any risk to your baby. 

Diagnostic tests such as CVS and amniocentesis do carry some risks to your pregnancy.  These include a small (1/800-1/100) risk of miscarriage if performed <24 weeks or preterm birth if performed >24 weeks gestation.  There is also a very small risk of premature rupture of membranes (PPROM) or infection.  There is also a very small risk of damage to maternal bowel which can result in a very serious infection in the mother. 

If you are concerned about a certain prenatal test and the risk it carries, please discuss it with your doctor or obstetric specialist. 

 

Can prenatal screening be performed if I’m carrying multiple babies (twins, triplets, etc.)?

In short, yes.

If opting for CFTS, the indices used in the calculation may differ slightly, but a risk can be provided. 

If opting for NIPT, the multiple pregnancy must be noted on the request form. In the case of triplet pregnancy, only some providers of NIPT are able to provide a result, and so this testing should be ordered in consultation with your Obstetrician.

 

If I opt for the NIPT why do I still need an early ultrasound? 

NIPT is a highly sensitive screening test for major chromosomal anomalies. 

However, not all major fetal structural anomalies e.g. anencephaly, spina bifida, are associated with major chromosomal anomaly. Whilst a low probability NIPT test may provide excellent reassurance of the absence of aneuploidy (an abnormal number of chromosomes), it cannot comment at all on the structural development of your baby. 

For this reason, a detailed structural ultrasound at around 13 weeks is recommended in all pregnancies. If you opt instead for the CFTS, this will be performed at the time of the nuchal translucency ultrasound. 

 

How much does a NIPT test cost?

At the moment, there is no government funding for NIPT. This means you are required to pay for your NIPT test. If you would like further information about the NIPT test, including costing and timeframes, please speak to us. The cost will be affected by the brand/company used.  

 

What is prenatal carrier screening? 

Prenatal carrier screening should be offered to all couples planning a pregnancy. This is a different test to NIPT, and screens the parents genes rather than predicting whether a baby is affected. It tests for whether parents carry a range of serious genetic conditions which could be inherited by their baby. Ideally, prenatal carrier screening should be done prior to a pregnancy to allow for appropriate counselling if both parents are found to be carriers, however it can also be performed during pregnancy. 

There are different platforms for carrier screening. The basic ‘trio’ test looks for carriage of: 

  • Cystic fibrosis
  • Fragile X syndrome
  • Spinal Muscular Atrophy

Extended carrier screening tests are also available that can test for carriage of >500 conditions. 

It is important to remember that many genetic conditions such as Cystic Fibrosis are recessive – meaning that an individual would need to inherit two copies of an affected gene, one from each parent. This means that the absence of a family history of genetic conditions does not mean there is no risk of having a baby with a genetic condition. In fact, most cases of these recessive conditions occur for the first time in families with no known history. 

 

Further reading

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