To be able to detect foetal chromosome abnormalities, there must be sufficient foetal cfDNA in the mother's bloodstream. The foetal fraction is the percentage of cfDNA in maternal blood that originates from the placenta. The foetal percentage must normally be greater than 4%, which happens during the ninth week of pregnancy. Low foetal fractions may prevent the test from being run or result in a false negative. Low foetal fractions can be caused by foetal abnormalities, sampling mistakes, maternal obesity, and testing too early in the pregnancy.