What are some of the causes for a child to be born with down syndrome?

Genetics: Down syndrome is a genetic condition, meaning it is part of your cells. It is not something inherited like the color or your eyes, but is due to a defect when the egg, sperm or embryo cell is made. Most causes are from trisomy 21 (having three copies of the chromosome 21 instead of two). There are no environmental or behavioral risk factors. It is seen mostly in women of advanced age.

Down syndrome: Down syndrome is a genetic disease. There are risk factors that increase risk of Down syndrome like advanced maternal age, advanced father's age or chromosomal translocation in families.

Several possible: The features of Down syndrome are dependent on the addition of one extra segment of a 21 chromosome which is more likely as women age .You can get this several ways. The traditional DS (~95%) has 3 copies of a 21 chromosome. About 4% have the specific extra 21 piece stuck on a regular chromosome (trans-location).About 1% have a mosaic of some normal and some 21 trisomy in variable amounts.