Trisomy 16. Karyotype of human chromosomes showing the disorder trisomy 16, in which the 16th set of chromosomes is a trio rather than the normal pair. It is the most common human trisomy, occurring in as many as 1% of pregnancies. The condition causes a miscarriage of the foetus as it is invariably fatal. In some cases not all the body's cells may carry three copies of the chromosome, a condition called mosaicism. In these cases it can lead to poor foetal development, heart and lung defects and unusual facial features. Affected foetuses are also likely to be born prematurely.