P816: Atypical sex chromosome findings on prenatal cell-free DNA screenings

Since its introduction, prenatal screening by cell-free DNA (cfDNA) has expanded rapidly, and most laboratories now offer sex determination in addition to autosomal aneuploidy testing. About 0.66% of these screenings return a result of a sex chromosome aneuploidy (SCA). Genetic counseling regarding these results is complex – SCAs show wide variability in clinical presentation and positive predictive values are relatively low (29-74%, depending on the specific SCA). In addition to a High Risk SCA result, some screenings return no result for sex chromosomes with additional qualifying statements explaining an atypical finding involving the sex chromosomes attributable to fetal SCA, maternal SCA, or placental/fetal mosaicism.