I hope that child never realizes his parents...
apparently would've preferred him dead than being born (don't know if the child is a boy or girl so I'm writing generically). Down's Syndrome kids are typically pretty innocent so perhaps he'll never realize this.
Depending on the mother's age, the chances of giving birth to a child with Down's Syndrome ranges from quite low (1 in 1,250 at age 25) to pretty substantial (1 in 106 at age 40).
If the mother was much over 30, she should consult with her doctor and determine if amniocentesis is necessary. Amniocentesis can easily detect Down's Syndrome because it is a trisomy condition of chromosome 21.