The negative correlation between the developed economy and childbearing has been a thing as long as there have been things.
In a developing economy your children are your retirement plan, and you have as many as you can so the burden of caring for you when you’re old gets spread out amongst all of them. Plus, you might be a farmer, and they can help you in the fields when they’re kids.
In a developed economy, your children are not your retirement plan, and they cost you a boatload to bear and raise. So, you have as few of them as needed to meet your personal requirements. :-)