Non-invasive prenatal testing (Harmony® test)
The Harmony test is a cell-free DNA-based prenatal blood screen. It is being used in more than 100 countries around the world, and has been used to guide clinical care in over 1.4 million pregnancies. The test can be used in singleton, twin, and egg-donor pregnancies and has been validated for use in pregnant women aged 18 to 48. It can be administered as early as 10 weeks gestation.
The test can screen for:
- Trisomies 21, 18, and 13
- Sex chromosome aneuploidy
- Monosomy X
- Fetal sex
- 22q11.2 deletion
Non-invasive prenatal testing (NIPT) analyses cell-free DNA circulating in a pregnant mother’s blood. It is used screen for Down syndrome (trisomy 21) and other common chromosomal conditions (trisomies 18 and 13). Options are also available to screen for X and Y chromosome conditions or for a deletion in chromosome 22q11.2.
About the test
DNA from the fetus circulates in the mother’s blood. Cell-free DNA (cfDNA) results from the natural breakdown of fetal cells (presumed to be mostly placental) and clears from the maternal system within hours of giving birth.
During a pregnancy, cfDNA can be tested to give the most accurate screening approach in estimating the risk of a fetus having a common chromosome condition sometimes called a trisomy. This occurs when there are three copies of a particular chromosome instead of the expected two. The test looks to detect the following conditions:
- Trisomy 21 is the most common trisomy at the time of birth. Also called Down syndrome, it is associated with moderate to severe intellectual disabilities and may also lead to digestive disease, congenital heart defects and other malformations.
- Trisomy 18 (Edwards syndrome) and Trisomy 13 (Patau syndrome) are associated with a high rate of miscarriage. These babies are born with severe brain abnormalities and often have congenital heart defects as well as other birth defects. Most affected individuals die before or soon after birth, and very few survive beyond the first year of life.
- Sex chromosome conditions occur when there is a missing, extra, or incomplete copy of the X or Y chromosomes. The Harmony test with sex chromosome aneuploidy panel option can assess risk for XXX, XYY, XXYY, XXY (Klinefelter syndrome), and a missing X chromosome in a girl (Turner syndrome).
Options are also available to look for Turner syndrome only (and not the other sex chromosome conditions), and/or to look for a deletion in chromosome 22q11.2. If the mother is interested in having this optional testing, she should talk with her healthcare provider to determine if it is right for her. This option is not available for twin pregnancies.
The testing is non-invasive: it involves taking a blood sample from the mother. The pregnancy is not put at risk of miscarriage, or from other adverse outcomes that are associated with invasive testing procedures such as amniocentesis.
A ‘high probability’ result is indicative of a high probability for a trisomy. In singleton pregnancies, the test identifies more than 99% of fetuses with trisomy 21, 97% of fetuses with trisomy 18, 94% of fetuses with trisomy 13, and 96% of fetuses with Turner syndrome. X and Y analysis provides >99% accuracy for fetal sex. Accuracy for detecting other sex chromosome anomalies varies by condition.
After the test, less than 1% of women need to have a CVS or an amniocentesis procedure.
The Harmony test is considered a prenatal screening test, not a diagnostic test. So if the test results show there is a high risk of the fetus having trisomy 21, 18, 13 or a sex chromosome condition, it does not mean that the fetus definitely has one of these conditions – although it is highly likely. For this reason, in the event of a ‘high risk’ (or positive) result, follow-up testing by an invasive procedure is recommended.
In the same way, if the test results show a ‘low probability’ of the fetus having trisomy 21, 18, 13 or a sex chromosome condition, it is unlikely that the fetus has one of these conditions. However, there is a very small risk that not all trisomic fetuses will be detected.
Who can have this test?
The Harmony test can be ordered by healthcare professionals for women with pregnancies of at least 10 weeks’ gestational age. This test can be requested for any singleton or twin pregnancy, including those conceived naturally or by IVF using the patient’s own egg or a donor egg. Note that, in twin pregnancies, sex chromosome (X and Y) analysis can determine fetal sex but not sex chromosome conditions. The Harmony test also does not assess risk for mosaicism, partial trisomies or translocations.
Results will be ready in approximately 3-5 days. Women still can have their 12-week scan for a detailed examination of the fetal anatomy, including measurement of nuchal translucency, nasal bone and other important factors. In this visit, patients can discuss the DNA and ultrasound results with their obstetricians.
On the basis of the NIPT result and the ultrasound findings, a patient can decide whether or not she wants to have an invasive procedure (for example, CVS or amniocentesis).
There needs to be enough fetal DNA in the maternal blood to be able to provide a result. If there is insufficient fetal DNA in the sample (which occurs in 3% of cases), another blood sample from the mother may be required. This will be processed in the laboratory at no extra charge.
What is the process?
Once the mother has taken an independent personal decision that she wants to have the NIPT performed, she will be asked to sign a consent form and her blood sample can be taken from a vein in her arm.
Who carries out the analysis of the test?
Her sample and completed request form need to be sent to TDL Genetics, where the Harmony test is performed on the DNA extracted from her blood sample.
Will the mother need to have any other tests?
The Harmony test does not provide information on mosaicism, partial trisomies or translocations, or other rare chromosomal abnormalities. If the ultrasound scan shows a high nuchal translucency or other major physical defects such as brain abnormalities, heart abnormalities, the risk for some rare chromosomal defects may be high. In such cases, the mother may choose to have a CVS or an amniocentesis.
The non-invasive prenatal test does not provide information on other physical defects such as spina bifida, or information on fetal growth. It is therefore advisable that the mother has all the usual ultrasound scans during her pregnancy.
Samples must be taken in special tubes provided by the laboratory. These samples must not be refrigerated, but stored at ambient temperature protected by the gel packs provided. The lab must receive the samples within 7 days to allow testing to proceed.
|Non-Invasive Prenatal Testing – common aneuploidy screening from maternal blood|
|Non-Invasive Prenatal Testing – common aneuploidy screening from maternal blood plus 22q11.2 del|