Why is maternal age considered a risk factor for trisomies?

Maternal age is considered a risk factor for trisomies because the likelihood of chromosomal abnormalities, such as Down syndrome (trisomy 21), increases with age. As women age, the quality of their eggs declines, and the risk of improper chromosome division during meiOSis rises. This can lead to aneuploidy, where an embryo has an abnormal number of chromosomes. Specifically, women over 35 have a significantly higher risk of having a child with a trisomy compared to younger mothers.