Lower than replacement rates are the demographic model of moribund socialist economies.
And reasonable birth rates are the demographic model of healthy Republics like our own.
Unless you can show any causation, what you keep on repeating is irrelevant.
The total fertility rates for white women in America from 1800-1860 were this:
1800--7.04
1810--6.94
1820--6.73
1830--6.55
1840--6.14
1850--5.42
1860--5.21
I'd like to think that America from 1800 to 1860 was a "healthy Republic," not an "economic basketcase."
You're noting a pattern too?
We also had, in 1800, a continent to conquer and a native population to displace. Excess population found a ready release in “Go West Young Man”.
There are reasons why basket case nations have a high birth rate - many of the same reasons why American women in 1800 had a high birth rate - because the future was uncertain and your children your only retirement plan and you needed to have seven or so to count on having a couple survive until adulthood.