Chromosomal abnormalities happen by chance. You did not cause this by anything you did or did not do during pregnancy. A woman is born with a certain number of eggs and does not produce new ones during her life. The longer an egg lives, like any tissue, the more likely something will go wrong. Why this happens in sperm is more of a mystery. New sperm cells are continuously being made. There is no such thing as an “old sperm.” For some unknown reason the risk of nondisjunction occurring in sperm increases in men over fifty.