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(HealthNewsDigest.com) – Prenatal screening has long been associated with increased anxiety due to the complexities of the process and results. For pregnant women with abnormal screens, the thought of undergoing an amniocentesis or other procedures that pose a risk to the pregnancy can be overwhelming and scary.
Now, expectant mothers can take a simple blood test – as early as 10 weeks into the pregnancy – to find out if their fetus has certain chromosomal abnormalities such as Down syndrome. And it gives parents-to-be an opportunity to prepare for unexpected news. For example, one young couple declined traditional prenatal screening for Down syndrome because they were not willing to put the pregnancy at risk through invasive testing. The couple also heard stories from friends about the high chance for false positive results. But an ultrasound during the fifth month of pregnancy raised concerns that the baby had Down syndrome. The couple elected to undergo non-invasive prenatal testing (NIPT), which has an accuracy rate of 99 percent. The test came back positive for Down syndrome and, although the results were difficult to hear, the couple was better able to prepare for the birth of their son and discuss the diagnosis with their doctor, family and friends.
NIPT, also known as non-invasive prenatal screening (NIPS), can empower women with information about their baby, but, as with any medical test, there are limitations for physicians and patients to consider.
Who should use non-invasive prenatal testing?
Research findings on the accuracy and performance of NIPT are largely based on data from a specific population of high risk, single fetus pregnancies. For this reason, some laboratories and practitioners only offer NIPT for patients with high risk indicators, including:
- Women 35 years of age or older at delivery
- Women with an abnormal first or second trimester screen for Down syndrome or trisomy 18
- Women with a personal or family history of certain chromosome conditions such as Down syndrome
- Women with abnormal findings on ultrasound associated with a trisomy
In addition, five leading organizations have published guidelines for the clinical use of NIPT: The National Society of Genetic Counselors (NSGC), American College of Obstetricians and Gynecologists (ACOG)/Society of Maternal Fetal Medicine (SMFM), International Society of Prenatal Diagnosis (ISPD), and American College of Medical Genetics and Genomics (ACMG). Each of these statements recognizes that data in the low risk and multifetal pregnancy populations is limited compared to the high risk population. Some go on to recommend that NIPT be offered only to women with high risk, single fetus pregnancies until more data in other populations is available. Others focus more on the need for pre-test counseling for all patients.
How does NIPT work?
Fetal DNA (tiny pieces of baby’s genetic information) circulates in the maternal blood throughout pregnancy. Fetal DNA enters maternal blood circulation through the breakdown of fetal cells, primarily derived from the placenta. NIPT measures the fetal DNA from specific chromosomes to determine quantity and identify fetuses affected with certain chromosome abnormalities.
How accurate is NIPT?
Currently, NIPT can be used to screen for Down syndrome, trisomy 13, trisomy 18, and some sex chromosome abnormalities such as Turner syndrome and Klinefelter syndrome. Generally, NIPT detects approximately 99 percent of fetuses with Down syndrome or trisomy 18. There is more limited information regarding detection rates for trisomy 13 and sex chromosome abnormalities.
False positive results are also possible with NIPT. The rate of false positives for Down syndrome and trisomy 18 is approximately 0.2 percent. As with detection rates, false positive rates for trisomy 13 and sex chromosome abnormalities are less clear and may be higher than 0.2 percent. See chart A below for a comparison.
Each laboratory varies with regard to which conditions are included in their test. Laboratories will also have their own set of detection rates and false positive rates.
Chart A: Average Accuracy Rates of NIPT
|
Type of Chromosome Abnormality |
Detection Rate |
False Positive Rate |
|
Down syndrome (Trisomy 21) |
99% |
~ 0.2% |
|
Trisomy 18 |
99% |
~ 0.2% |
|
Trisomy 13 |
79-92% |
≤ 1% |
|
Sex chromosome abnormalities |
N/A (limited data) |
N/A (limited data) |
What are the benefits and limitations of NIPT?
The most obvious benefit of NIPT is that it requires only a maternal blood sample; therefore it does not pose a risk to the pregnancy or require a specific skill set from the physician to be performed. Also of benefit, NIPT can be ordered as early as 10 weeks into the pregnancy and anytime thereafter. Finally, due to the high detection rate, the need for invasive prenatal testing may be decreased.
As with any medical test, however, there are limitations to be considered. NIPT is a screening test and does not replace diagnostic tests such as chorionic villus sampling (CVS) or amniocentesis. Although the detection rates for NIPT are quite high, it will not detect all chromosome abnormalities. Women with an abnormal NIPT result should be offered CVS or amniocentesis to confirm the presence of the chromosome abnormality due to the potential for false positive results.
Who can help women decide if NIPT is right for them?
All pregnant women should be counseled about prenatal screening and testing for chromosome abnormalities. The prenatal screening and testing process is complex, and NIPT is only one of several options. Each of these options has its own set of benefits and limitations that should be considered. Because NIPT accurately indicates whether a fetus has certain abnormalities, women should receive counseling from a qualified health care provider such as a certified genetic counselor prior to the taking the test and when they receive the results. Genetic counselors have specialized education and expertise in genetics and counseling, and provide information and counsel to individuals considering or undergoing genetic testing, including NIPT.
For many couples, obtaining the most reliable information about the health of their baby without putting the pregnancy at risk is an ideal situation. NIPT is an option that moves us closer to that goal. As this technology continues to develop, it will be important for physicians and patients alike learn about the options available so they can make informed choices. The National Society of Genetic Counselors is working closely with obstetricians and other doctors who care for pregnant women to ensure that patients understand NIPT and other prenatal tests, and are able to receive genetic counseling.
Jennifer Hoskovec, MS, CGC, is president-elect of the National Society of Genetic Counselors, and the Director of Prenatal Genetic Counseling Services at the University of Texas Medical School at Houston.
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