NIPT (Non-Invasive Prenatal Testing)
Non-invasive prenatal testing, commonly known as NIPT, is a way to test a fetus for the presence of certain chromosomal abnormalities—particularly Down syndrome—without the risks associated with more invasive procedures like amniocenteses. The necessary information can be obtained from a sample of the mother’s blood, which contains some of the fetus’ DNA. In addition to detecting possible birth defects, NIPT can tell you the sex of your baby (if you want to know).
Why NIPT is Done
The purpose of non-invasive prenatal testing is to screen for some of the most common chromosome anomalies:
- Trisomy 21 (Down syndrome)
- Trisomy 18 (Edwards syndrome)
- Trisomy 13 (Patau syndrome)
- Monosomy X (Turner’s syndrome)
NIPT is safe because it is non-invasive, unlike other forms of prenatal testing, such as amniocenteses or CVS (chorionic villus sampling), which carry slight risks of causing miscarriage (albeit only about a 1% risk). The safety and non-invasiveness of this procedure, the accuracy of which is comparable to amniocenteses, are the best reasons to recommend it.
While its accuracy in predicting Down syndrome is better than 99%, NIPT is less accurate for predicting Patau syndrome (Trisomy 13) than for predicting the other conditions for which it tests. A positive finding for Patau syndrome can still be useful, however, because it alerts your doctor to the need for further testing.
Non-invasive prenatal testing can be done for any pregnancy, but it is especially recommended for pregnancies in which there is a significant risk for chromosomal abnormalities. These are pregnancies in which:
- The mother is over the age of 35 by the due date
- There have been abnormal results from a pregnancy ultrasound
- There is a family history of chromosomal abnormalities, such as the birth of a previously affected child
NIPT is strongly recommended for women over the age of 35. A woman is born with all the eggs she will ever have, and as she gets older the risk increases that some of those eggs will develop abnormalities. By the time a woman turns 30, the risk that she will give birth to a baby with Down syndrome is one in 1,000; by age 35 the risk rises to one in 300; and by age 40 the risk is one in 100.
NIPT can be performed if the mother-to-be is carrying twins, but the results may not be clear-cut. For example, if the NIPT shows male DNA present (that is, if it detects the Y chromosome), then one or both of the babies is predicted to be male. If NIPT shows possible Down syndrome, it can be difficult to know which fetus is affected, so in all likelihood, an amniocentesis for each baby would be necessary for a final determination. But if the test comes back normal or negative, this is still extremely useful for ruling out chromosomal abnormalities in both fetuses, without the need for invasive testing. NIPT cannot be performed for triplets or higher order multiples.
When NIPT is Done
It is possible to get a useful result from an NIPT test anytime after the 10th week of gestation, and the test can be performed at any time throughout the remainder of the pregnancy. However, the optimal time is between the 11th and 13th weeks, and the procedure is typically performed between weeks 10–22. Sometimes an abnormality is seen during an 18- or 20-week ultrasound, in which case the next step is to draw blood for the NIPT.
How NIPT Works
Chromosomes come in pairs, but people with Down syndrome have an extra copy of chromosome 21—they have three, rather than two; this is what NIPT testing detects. Edwards syndrome and Patau syndrome work similarly: affected individuals have an extra copy of chromosome 18 or 13, respectively.
Fetal cells can persist in maternal blood for years, but NIPT looks for circulating fetal DNA that is not attached to any cell. This cell-free fetal DNA remains after fetal cell breakdown but disappears within hours, meaning that any such DNA found in mom is from the baby she is currently carrying, not an older sibling. Thus, with the right technology (which took more than ten years to develop), specialized labs can now isolate and measure the amounts of fetal DNA present in the mother’s blood. One day this test will be able to detect the DNA of all 24 chromosomes (22 autosomes plus X and Y).
Your NIPT Test: What to Expect
When you have an NIPT test done, your doctor or a qualified lab technician will take about 20 ml of your blood, which will be sent to a lab for testing. That’s all there is to it—a quick jab with a needle, and you’re done! NIPT test results take approximately one to two weeks, depending on what lab they are sent to.
The cost of NIPT testing varies, but it is generally in the neighborhood of $700–$1,000. If you have a valid indication for the test, most insurance plans cover it. This test is available to pregnant women of any age, although women who are age 34 and under may have to pay the full cost out-of-pocket.
Is the NIPT Test Reliable?
Most sources say that there is no more than a 1% chance of a false positive result (i.e., the baby is normal but the NIPT test indicates an abnormality). According to one company that conducts non-invasive prenatal testing, the chance of a false negative test result (i.e., the test result was normal but the baby is born with Down syndrome or Edward syndrome) is less than one in 1,000, although other sources put the chances of any kind of false result at closer to 1% (which is still comparable to amniocenteses). Also, depending on the lab, in 1–4 percent of cases a DNA sample can’t be obtained the first time, although subsequent attempts may yield better results. For fewer than one percent of pregnant women who have NIPT testing performed, no result can be obtained at all.
There are a number of reasons why false positive results happen; for example, if the test is done early in the pregnancy, it may detect DNA left over from a vanishing twin that has been absorbed by the mother or by the surviving twin. In rare cases, maternal cell-free DNA may be mistaken for the baby’s, and the mother’s genetic problems may be attributed to the baby.
If the mother is overweight or obese, there may be too little baby DNA in her blood, which may cause a false negative test result or an indeterminate result.
There is another possible reason for inaccurate NIPT results: cell-free fetal DNA comes from the placenta, and there are sometimes “cell lines” that grow in the placenta but not in the baby. This may cause a false negative test result if an abnormal cell line is present in the baby but not present in the placenta, or a false positive if present only in the placenta, not in the baby.
What if the Results Are Positive?
If non-invasive prenatal testing indicates that an abnormality is present that suggests Down syndrome or some other birth defect, further testing will be necessary to confirm this. The next step will most likely be CVS (chorionic villus sampling) and/or amniocenteses.