Current prenatal diagnostic methods increase the risk of maternal miscarriage, and this fact has led researchers to seek a new method. In this search, scientists first observed Y chromosome in the circulatory system of a pregnant mother in 1969 . Subsequently, two different pathways have been found, the first one is to collect and examine fetal cells in the mother’s blood, and the second one is to isolate the fetal DNA (cell free fatal DNA-cffDNA), which is circulating in the mother’s blood, and not present in the cell.
Cff DNA was first discovered in mother blood in 1997 and contains about 10% of the non-cell DNA in the mother’s circulation. After the 7th week of pregnancy, fetal DNA is detected in the mother’s blood. These DNAs are released as a result of the destruction of the fetus’s cells in the amniotic fluid (plesental trophoblasts) and are introduced into the bloodstream of the mother via placenta.
The sequencing technology developed in recent years allows the screening of millions of DNA fragments that are independent of each other at the same time, thus enabling fetal DNA to be examined. Sex determination by using Cff DNA, Rh blood group determination and the presence of paternal mutant allele in autosomal dominant diseases are determined by this method. Down syndrome can be detected between the 8th and 10th weeks using this method. The specificity and sensitivity of this test for trisomy 21 is around 99%. This test can also be used to detect Y-crosomomas in the mother’s blood and to determine the sex of the fetus, with false-positive and negative rates between 1-2%.
Despite the accuracy of the test is so high, it is still accepted as a screening test.
Although this new technique is expensive, it is expected that it will gradually replace the old techniques in the following years as there is no miscarriage risk compared to other prenatal investigations, also it can be done early and does not require experience for sampling.
If we go into details, this new generation sequencing is how to detect trisomy 21. In each chromosome, specific sequence points were determined, ie, the sequence of the DNA fragments which were released from the cell to which chromosomes belong. From this point, any chromosome is missing or excess can easily be determined by this method. For example, chromosome 21 constitutes 1.5% of the total DNA. That is, when chromosomes are proportioned according to the sequencing result, chromosome 21 should constitute 1.5% of fetal and maternal DNA. However, if there is trisomy 21 in the fetus, there will be a small but significant increase in this rate. With this calculation it can also be used to detect other aneuploidies.