Prosperity doesn't make fertility drop, it makes it less necessary to have children because:

• Parents don't need to have 5 children just for 1 or 2 to survive.. people can have 1 or 2 children and expect them to survive now.
• Adults don't need children to survive. Before people expected their children to help them survive when they are elderly so the kids can work the farm and the grandparents can babysit grandkids in their multigenerational home. Now nursing homes can take care of your needs after saving enough money.
• Now that women are gaining equal rights, they can choose to be other things than just a mother or a wife. Life can be full and satisfying without kids now.

But I do agree people can feel that working and being a mother is too much, and decide not to have kids as well.

I don't really know, but as far as actual fertility(the ability to become pregnant, rather than just birth rates (the rate at which people have children). It is more likely to be down to pollution like microplastics and lead in the water and the like.

Possibly even rising temperatures, don't spermatozoa die above 78°F/25.5°C? So if men can't cool off their junk, they can't get anything worthwhile delivered to the egg? I'm no biologist so I'm iffy on that one.

If you make a society where everyone HAS to work in order to survive but also provide no public childcare, schooling and healthcare then obviously people are not going to be able to afford to have babies.

It's not the expectation that women have to work that makes having children difficult, it's that they are expected to work and still do the majority of the childcare that is making having children difficult. Historical remember so many children died, in Ancient Roman times, for example it would take 6 to 9 children per household to keep the population rate stable, with wealthier houses having fewer children, not just because of child survival rates but also because childbirth was so damn risky for the mother. With a 1 in 50 chance of dying, that increased with each subsequent child. Half of all children born would be dead by age 10 so they needed to have so many children.

Ain't urbanisation made fertility drop tho? Like raising a child in the city is bloody expensive while in the country extra child is spare hands for work