Maternal age effect
Editor-In-Chief: C. Michael Gibson, M.S., M.D.  Phone:617-632-7753
The maternal age effect describes the exponentially increasing risks for numerical chromosomal abnormalities among her gametes as a prospective mother ages. This increase reflects the overall increase in the rate of nondisjunction with maternal age.
However, 50% of Down Syndrome cases are actually due to paternal age effect, as are most non-familial genetic disorders. This is because a woman's ova only divide a total of 24 times throughout life, the first 23 of which happen in utero, whereas men's sperm divide continuously throughout life, leading to gradually increasing copy errors. Trisomy 21 derived from maternal age effect is due to an error when the egg splits.