The ability to perform early, non-invasive prenatal tests such as the Harmony Test is due to the discovery of cell free fetal DNA (cffDNA). As the name implies, cell free DNA can be found free floating in the bloodstream. In the blood of a pregnant woman, both maternal cell free DNA and fetal cell free DNA can be found in the mother's blood. The fetal DNA comes from trophoblastic cells, the cells on the outer layer of a blastocyst. Some of these trophoblastic cells go through apoptosis (programmed cell death), and the DNA can end up in the mother’s bloodstream. The fetal DNA is present in the mother’s blood after five to seven weeks of gestation, and the amount of cffDNA present increases as the pregnancy continues. One challenge of studying the cffDNA is that the majority of the free DNA in the bloodstream is maternal, not fetal. In order to accurately assess the karyotype of the fetus, it must be possible to analyze the fetal DNA without any of the maternal DNA. The ratio of maternal cfDNA and fetal cfDNA can vary greatly between each individual, with some reports claiming 3-6% of the DNA being fetal and others claiming 11-13.4% being fetal. The major difference allowing for differentiation between the two sources of DNA is that fetal DNA segments are around 200 base pairs long, which is significantly shorter than the maternal DNA fragments. To collect the DNA, the blood sample from the mother is spun down in a centrifuge to separate out the plasma, from which the cffDNA can be isolated and purified via methods such as PCR or use of a mass spectrometer.
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