Cell-free (cf) DNA analysis of maternal blood for trisomies 21, 18 and 13 is superior to other methods of screening, but it's expensive. One strategy to maximize cfDNA testing at reduced cost is to offer it contingent on the results of the currently used first-trimester test.
A new study in Ultrasound in Obstetrics & Gynecology found that such contingent screening could potentially lead to detection of a higher proportion of affected pregnancies; however, prenatal detection of trisomies depends not only on performance of screening tests but also on parental choices. Only 60% of women considered high-risk after undergoing standard first trimester screening chose to undergo cfDNA testing. Consequently, adding cfDNA testing may only have a small effect on the rate of live births with trisomy 21.