Aneuploidy

It is an anomaly in which the presence of one chromosome more or less than the normal set-up generates syndromes with a very serious clinical picture. The best known is Trisomy 21 (Down's Syndrome); together with Trisomy 18 (Edwards syndrome) and Trisomy 13 (Patau syndrome) are among the chromosomal syndromes that are found most frequently in the fetus.