NIPS Worldwide - NIPS to Expecting Parents | How NIPS Works

Factor Analysis

The completed sequence is matched in a computer database, and chromosome ratios are determined. Under normal circumstances, the first chromosome takes up 8% of the total and the 21th takes up about 1.5%. However, babies with Down syndrome have an extra copy of chromosome 21; therefore, the ratio of chromosome 21 to other chromosomes will be higher in these cases.

Normally, the ratio of maternal DNA to fetal DNA is 1:1; however, abnormalities in fetal DNA cause this ratio to change.

If the fetus has Down syndrome (which is caused by an extra chromosome 21), the percentage of fetal chromosome 21 in the mother’s blood will be higher than that found in normal pregnancies (i.e. 1.5%). The percentage of chromosome 21 in maternal blood is calculated according to the following formula: % cell-free DNA of the mother (usually around 90%) * expected % of fetal chromosome 21 (i.e. 1.5%) * expected ratio+Cell-free DNA of fetus%*the 21th chromosome%*expected ratio =90%*1.5%*2/2+10%*1.5%*3/2 =0.0135+0.00225 =1.6% Therefore, if the fetus has Down syndrome, the concentration of chromosome 21 in maternal blood is 1.6%, which is 6% higher than for normal pregnancies (1.5%). However, note that different companies apply different algorithms to determine whether fetal DNA contains abnormalities.