First of all we need to know the cause of down syndrome-
1. Non-dysjunction that occurs in 95% cases
2. Translocation- 4%
3. Mosaicism- 1%
For a non-dysjunction recurrence risk is same as the population for that maternal age.
For Translocations- t(21:21) in parents is associated with 100% recurrence of down syndromein subsequent pregnancy.
t(4:11) is associated with 5-7% recurrence rate
If a mother has Down Syndrome, she can still be fertile and able to reproduce, with chance of having a baby with downsyndrome by 50%