How is advancing maternal age associated with trisomy 13, trisomy 18, and Down syndrome?

Advancing maternal age is a significant factor in the occurrence of numerical chromosomal abnormalities, including Down syndrome, trisomy 13, and trisomy 18. These conditions generally result from meiotic nondisjunction, an error during cell division where chromosomes fail to separate equally, leading to eggs with an extra chromosome.12

While the exact biochemical mechanism remains unknown, the risk of conceiving a child with these chromosomal abnormalities increases with maternal age in an almost exponential manner, particularly after age 35. For example, the risk of having a child with Down syndrome rises from less than 1 in 1,000 for women under 30 to approximately 1 in 400 at age 35, and between 1 in 20 and 1 in 50 for women aged 45 or older.1

Although these trisomies are associated with developmental malformations, the vast majority of such conceptions are lost to miscarriage. There is no evidence that paternal age has a similar effect on these numerical chromosomal abnormalities.12