This is because some of the trisomies (one extra chromosome, three instead of two) are fatal, births cannot survive.
Theoretically, trisomies can occur with any chromosome, but often result in miscarriage.
The types of autosomal trisomy that survive to birth in humans are:
Trisomy 8 or Warkany syndrome 2
Trisomy 9
Trisomy 13 or Patau syndrome
Trisomy 18 or Edwards syndrom
Trisomy 21 or Down syndrome.
