Trisomy 21 is a type of Down Syndrome where the unborn child has 3 copies of their 21 chromosome instead of the normal 2. It can lead to a delay in physical and mental abilties once the child is born among other issues. When you are pregnant there are screenings available to test for the possibility of Down Syndrome or Trisomy 21 in you unborn child. Check out my hubs, I have one of Trisomy 21 that explains the various parts of the disorder, and another on screenings and diagnostic testing. My 11 month old son has Trisomy 21.