Abstract: A method for detecting aneuploidy of a chromosome comprises: a) obtaining a biological sample from a patient; b) preparing DNA from said sample for subsequent analysis; c) subjecting the DNA of said sample to a multiplex PCR reaction, using a multiplicity of markers, at least two per said chromosome of which have a heterozygosity frequency sufficiently high to minimise the possibility that, if two alleles of that chromosome would normally be present in a normal diploid individual, those two alleles would both comprise the same variant of said STR (i.e. would be homozygous); d) comparing the results of said multiplex reaction with the results which might be expected from a normal diploid individual; and e) thereby assessing the probability that said patient might be suffering from chromosomal aneuploidy.