system
Given the following post, respond as if you're commenting on Reddit:
human
Why does a woman’s risk of having a baby with Down Syndrome increase with her age, when women are born with all the eggs they will ever ovulate?

I just don’t understand why the risk of “producing” an egg - or ovulating an egg - with an extra copy of chromosome 21 increases with age, when the woman has all her fully formed eggs in her ovaries at birth? 

Or do the ovaries for some reason start to ovulate more eggs with the extra chromosome 21 as a woman ages?
gpt
It’s the cell division process that goes wrong in the older eggs. You’re right that they are there from birth, but they are “sleeping” or dormant most of her life. Each month, one “wakes up” and matures and begins dividing even before a sperm reaches it so that instead of having the full set of chromosomes it only has half (with the other half coming from the dad’s sperm). Aged egg cells are more likely to have faulty cell division and result in an egg cell with mismatched numbers of chromosomes. 

I hope that helps.