Noninvasive prenatal testing detects the most common foetal chromosomal aberrations: trisomy of chromosomes 13, 18 and 21 and numerical aberrations of sex chromosomes. An additional option includes a test for specific microdeletions (not performed in case of multiple pregnancies).
The test uses free extracellular foetal DNA (cfDNA) present in the mother’s blood plasma so it is possible to perform a prenatal diagnostic test without any risks to the foetus. Thanks to NGS, a method based on massive parallel sequencing, it is possible to test cfDNA and to determine the risk of numerical chromosomal aberrations. Massive parallel sequencing of cfDNA obtained from the mother’s blood plasma used in this study is a proven method to detect aneuploidies in a foetus, and in the case of trisomy 21 its sensitivity is above 99.99% and the specificity is >99.8%.
The test is performed using a 10-mL blood sample from a pregnant woman, therefore it is a safe test which is not invasive for the foetal environment. Free extracellular DNA belonging to a mother and a foetus is isolated from the blood plasma of a pregnant woman. The next stage is to use a method of molecular massive parallel sequencing allowing to identify and test extracellular DNA. It is a quantitative method where sequenced cfDNA fragments are matched with a human genome using databases. The number of individual DNA fragments of analysed chromosomes compared to DNA fragments of reference chromosomes is determined with advanced software for a statistical analysis.
The test report obtained may present one of three results for trisomies of chromosomes analysed: “aneuploidy detected” – if an additional chromosome was found in the test; “aneuploidy not detected” – if the result indicates two copies of a chromosome; and “aneuploidy suspected” – if borderline results for trisomy have been obtained.
With regard to testing of sex chromosomes results are classified as one of six categories: XX, XY, MX (X monosomy), XXX, XXY and XYY.
A prenatal test is a screening test and each result should be consulted by a physician who has ordered the test. Results indicating chromosomal aberrations should be confirmed by invasive prenatal testing on a sample from the amniotic fluid (amniocentesis) or chorion (biopsy).