Which trisomy is the most common among viable human births?

1. Most autosomal trisomies fail to develop to birth. 2. Duplications of some smaller chromosomes, numbers 13, 15, 18, 21 and 22, can give offspring who survive. 3. Of these, the most common trisomy among viable births is chromosome 21. 4. That trisomy corresponds to Down Syndrome, which is identified by a third copy of chromosome 21. _Source: OpenStax Biology (1st ed., CC BY 4.0), Ch 13 "Modern Understandings of Inheritance", section 13.2 Chromosomal Basis of Inherited Disorders_