Why did my baby get downs?

Problem in chromasms: Babies are supposed to get 23 chromosomes from mom & 23 from dad at he point of conception.In ds babies, an extra # 21 chromosome joins the mix for reasons that are not clear.As the egg grows the extra becomes part of every cell & baby is born with ds & all its problems. We know there is risk of ds for any pregnancy & somewhat higher for mom <17 or > 40. Genetic counseling could give u specifics.