The reason is that the number of genes on chromosomes 2 1, 18 and 13 is small, and the fetus is likely to survive after trisomy mutation, that is, it can be born. There are many genes on other chromosomes, and if the number changes, the impact on the fetus will be fatal, and basically there will be abortion in the early stage of development. Therefore, in medicine, there are many abnormalities in these three chromosomes (2 1, 18, 13), and there are also a lot of data accumulated by non-invasive DNA detection, so the detection accuracy is high. On the contrary, the accuracy of other chromosomes is low. As a medical technology, if the accuracy is low, it can't be reflected in the report.