Down Syndrome - Cause
Down syndrome is caused by abnormal cell division in early embryo development. Normally, a child inherits 46 chromosomes, 23 from each parent. Each chromosome carries DNA, called genes, which tell how the brain and body should develop.
But a fetus with Down syndrome has extra or abnormal chromosomes. Having extra genetic material changes the way the brain and body develop. The type of Down syndrome depends on how many cells have the extra or abnormal chromosomes.
Medical experts believe the cell changes most often start in a woman's egg before or at conception. Less often, the error occurs in sperm at conception. It is not known what causes the cells to divide abnormally. One type of Down syndrome, called translocation-type, may be passed down through families (inherited).