There is a lot of evidence to suggest that people want to be sexually active and don't want children until they have an existential crisis and many existing people are unplanned happenstances.

Educated people don't have enough children to replace themselves, and its not because they are too busy.

Give people the choice and most choose to do things preventing children until they have existential crisis. Remove the choice and people are have children at twice the rate, pretty much coinciding with when they become sexually active.

It's not that hard to understand or perceive.