Background Analysis of cell-free DNA (cfDNA) isolated from maternal plasma for fetal trisomy 21, 18, and 13 risk assessment has been introduced into prenatal care across the globe. This technology also screens for sex chromosome aneuploidy (SCA). Regardless of the cfDNA test methodology used, small validation studies demonstrate lower test performance than for autosomal trisomy. The focus in scientific literature has been on predictive capabilities of cfDNA testing in successful analyses. Less attention has been given to analyses for which results cannot be provided. For SCA assessment, there are instances where sex chromosome copy number cannot be determined and a statistical likelihood of a disomic (XX, XY) or non-disomic (X, XXX, XXY, XYY, and XXYY) genotype cannot be established. Although infrequent, results may be due to technical or biological factors and represent a counselling challenge.

Objective We present three cases using directed cfDNA analysis that resulted in trisomy risk assessment but no results for SCA. Further evaluation identified a potential explanation.

Results Case #1: the patient opted to pursue diagnostic testing by amniocentesis, which revealed a 45, X[4]/46, XY[14] fetal karyotype. Case #2: peripheral blood chromosome analysis identified a 45, X[8]/46, XX[54] maternal karyotype. Case #3: a previously unreported twin gestation was revealed. Since extensive further evaluation is rarely indicated or pursued, the frequencies of these biological explanations are unknown.

Conclusion Qualitatively, these cases underscore the importance of inclusion of the possibility of no results into pre-test counselling and add to the base of knowledge for use in post-test counselling in these situations.