Children with Down Syndrome are born with an extra copy of chromosome 21 (thus the medical term: "Trisomy 21"). It is a chromosomal anomaly, or mix-up, in the genetic blue-print that occurs at conception.
Approximately 1 in 700 live births is a child with Down Syndrome in Canada. It is important to know that nothing the mother does during her pregnancy could cause this to happen. The chance of having a baby with Down syndrome increases significantly with age of the mother.