Abstract: A method for the diagnosis of aneuploidy of a chromosome in a fetus is provided using the polymerase chain reaction. The method utilises a multiplex PCR assay comprising a plurality of chromosome-specific short tandem repeat markers. The method can be used to diagnose fetal trisomy and monosomy responsible for disease conditions such as Down Syndrome and Turner Syndrome, respectively. The method can also be used to diagnose the presence of other genetic conditions such as cystic fibrosis.