Reihaneh Pirjani - MD. Associate Professor, Obstetrics and Gynecology Department, Arash women’s hospital, Tehran University of Medical Sciences, Tehran, Iran

Fetal/placental cell free DNA (cfDNA) can be isolated from maternal blood as early as five weeks ofgestation and almost always by nine weeks of gestation.The relative concentration of fetal cfDNAincreases modestly (0.1 percent per week) with gestational age from 10 to approximately 20 weeks,and then increases rapidly (1 percent per week) until term.An adequate amount of fetal/placentalcfDNA must be present to obtain a reliable test result.Four factors can systematically reduce a woman s fetal fraction, which can lead to an assay failureor can result in a false negative result. These factors include: early gestational age, suboptimalsample collection, obesityand fetal karyotype. The fetal fraction is substantially lower prior to 10weeks, so most laboratories require that patients undergo screening at ≥10 weeks of gestation toensure an adequate fetal fraction. As maternal weight increases, the fetal fraction systematicallydecreases. This inverse relationship has been attributed to the dilution of a relatively constantamount of fetal cfDNA in the larger maternal plasma volume of obese women and also to anincrease in the concentration of maternally-derived cfDNA as maternal weight increases. Theaverage fetal fraction at 10 to 20 weeks of gestation is lower in pregnancies with a trisomy 18 fetus(average fetal fraction 9 percent) than pregnancies with a euploid fetus (average fetal fraction 13percent) or pregnancies with a Down syndrome fetus (average fetal fraction 15 percent). This maypartially explain why detection rates for Down syndrome are higher than for trisomy 18, especiallywhen test failures are considered. There are less data for other abnormalities, but it appears thatthe fetal fraction in both trisomy 13 and Turner syndrome is also lower than in euploid fetuses.Triploid fetuses have extremely low fetal fractions.