The more educated a population of women are, the fewer children they have.
More schooling means delaying marriage and waiting for someone that checks off the professional, educational, financial and physical requirements. Back in the day, women either quickly settled for the attractive jock turned coal miner or the nerdy pencil pusher because he made good money.
Modern women don't have to settle...and most won't settle (unless it's for an attractive bum).
We don't have a bunch of family farms anymore. Where our great-grandmothers had 10, 12, 15 kids, a woman in 2024 would be hard pressed to have more than 2.