As found on http://en.m.wikipedia.org/wiki/Cell-free_fetal_DNA
Cell-free fetal DNA
cffDNA originates from the trophoblasts making up the placenta. It is estimated that 2-6% of the DNA in the maternal blood is fetal in origin. The fetal DNA is fragmented and makes its way into the maternal bloodstream via shedding of the placental microparticles into the maternal bloodstream (figure 1). Studies have shown that cffDNA can first be observed as early as 7 weeks gestation, and the amount of cffDNA increases as the pregnancy progresses. cffDNA diminishes quickly after the birth of the baby, so that it is no longer detectable in the maternal blood approximately 2 hours after birth. cffDNA is significantly smaller than the maternal DNA in the bloodstream, with fragments approximately 200bp in size. Many protocols to extract the fetal DNA from the maternal plasma use its size to distinguish it from the maternal DNA.