Can you tell me about causes of high risk of down syndrome during pregnancy?

Chromosome defect: Ds is one of the more common chromosome(c) defects that can survive pregnancy (many others miscarry). It usually happens when an egg with an extra 21c combines with a normal sperm ; creates a baby with the extra 21c in all tissues.Risks vary with age and are lowest in the early 20's(1/2000), rising to 1/100 at about 40 ; 1/12 at 49.Various prenatal tests can pick up signs early on.