As people and places become richer, women tend to have fewer children. The most obvious reason being that poverty, as in the past, means seeing a quarter of your babies die before their fifth year. So, as wealth increases, this happens less, and it is necessary to have fewer births in order to still ensure that only meaningful result of life, the production of grandchildren.
It is also true that a richer place provides more for people to do other than just pump out more babies, and so, people do more of those other things. In formal economic terms, the opportunity cost (what you must give up to have something) of having children rises as careers and long vacations and all of that open up for women. That glorious economic liberation of women of the past decades, plus a generally richer place, thus means fewer children.
