These three tests (‘Harmony Prenatal Test’, ‘MaterniT21 PLUS’, and ‘Non-invasive verifi prenatal test’) are examples of different brands of Non-Invasive Prenatal Testing (NIPT). NIPT uses new technology to detect fetal aneuploidy during pregnancy. Genetic material (DNA) in human cells is packaged into 46 chromosomes. Aneuploidy is defined as an extra or missing chromosome. Changes in the number of chromosomes can influence human health. The most common form of aneuploidy is called Down syndrome or ‘trisomy 21’ (trisomy 21 means that there are three copies of chromosome 21 rather than two copies). NIPT currently provides accurate information for Down syndrome as well as other less common trisomies, Trisomy 18 and Trisomy 13 (and possibly other trisomies depending on the specific test). In the future, NIPT has the potential to test for other kinds of aneuploidies and conditions caused by changes in single genes.
How does NIPT look for aneuploidy?
As a baby develops in the womb, DNA from the baby circulates in the mother’s blood. NIPT involves testing the mother’s blood (drawn from a vein in the arm, for example). The blood is sent to a laboratory to analyze the fetal DNA in the mother’s blood to determine whether or not the baby is aneuploid. It takes about a week for the laboratory to analyze the blood. In rare cases, there is not enough fetal DNA in the mother’s blood to obtain conclusive results, so the test may be re-done.
How is NIPT different from other prenatal screening tests?
NIPT can be distinguished from other kinds of prenatal screening tests (e.g. maternal serum screening, fetal ultrasound) for three reasons. First, NIPT has a higher detection rate, which means that NIPT detects a larger number of affected pregnancies than other tests. Second, NIPT results are usually available earlier in the pregnancy than other kinds of prenatal screening. For example, detailed fetal ultrasounds are performed usually after 18 weeks of gestation while NIPT can be performed after 10 weeks of gestation. Third, NIPT does not provide information about open neural tube defects (e.g. spina bifida) while maternal serum screening and fetal ultrasound can provide information about these kinds of conditions.
What are the next steps after a positive test result?
While NIPT has a very high detection rate (~99%), it is not currently considered a diagnostic test because there is a small chance of a false-positive result. The false positive rate is approximately 0.2% which means that 1 in 500 unaffected pregnancies will test ‘positive’ or ‘consistent’ with aneuploidy. Therefore, it is recommended that further testing is pursued to confirm positive test results. Further testing may involve invasive procedures, called chorionic villus sampling (CVS) or amniocentesis. The risks associated with these invasive procedures include the risk of miscarriage (0.5-1%) and other adverse pregnancy complications. Fetal ultrasound may also provide more information for pregnancies that test positive on NIPT.
Where can I get NIPT?
In Canada, NIPT is not yet widely available. Paying privately for this testing may be an option. In the United States, NIPT is available but insurance coverage is variable.
If you have any questions about NIPT, contact your local medical genetics clinic.
In Canada: http://cagc-accg.ca/component/option,com_sobi2/Itemid,30/
In the United States: http://www.nsgc.org/FindaGC/tabid/64/Default.aspx
For those of you outside of North America, contact your local medical genetics clinic or health care provider for assistance.