A defined piece of single-stranded nucleic acid which is used to identify specific complementary-sequence nucleic acid molecules in an assay intended to be used to evaluate a clinical specimen to diagnose, monitor or predict one or multiple congenital aneuploidies, including (but not limited to) trisomy 13 (Pätau syndrome), trisomy 18 (Edwards syndrome), trisomy 21 (Down syndrome), and sex chromosome aneuploidies.