Why does the likelihood of trisomy 13 trisomy 18 or trisomy 21 increase with maternal age?

The increased likelihood of trisomy 13, trisomy 18, and trisomy 21 in pregnancies among older women is primarily attributed to meiotic nondisjunction—a random failure of chromosome pairs to separate correctly during the formation of gametes (eggs).123

While the exact biochemical basis for this phenomenon remains unknown, it is strongly linked to the aging process of the female reproductive system. As women age, there is a decline in both ovarian function and the quality of the remaining eggs. This degradation in egg quality is believed to contribute to the increased frequency of errors during meiosis, leading to eggs with an abnormal number of chromosomes.241

The risk of conceiving a child with these chromosomal abnormalities rises in an almost exponential manner, particularly after age 35. Because these events are generally independent, the increased risk is associated with maternal age rather than a recurring genetic predisposition in the parents.2