Statistics of the United States, (including mortality, property, etc.) in 1860 (census.gov)
The Southern areas couldn't get out of the 18th century infant mortality pattern. The bulk of the US population lived in Districts I and III and infant mortality was less there and following the long term trend downward. The eastern states in the south couldn't match that pattern.
The explanation is beyond our reach. But the factors that have to be considered are women having children a few years later in the northeast. Smallpox was under control. Production of milk, cheese and eggs was probably increasing. In the north, the enslaved women made up a large percentage of the child bearing women and they probably lived in very harsh conditions. The disease environment in what was then the southwest was very bad. And that kept any advantage those states could have developed to a minimum.
It would take considerable arithmetic computation, but I think many white families in the south were aware of this trend. And those that could, were leaving for the west and the north especially Texas and Missouri.
As the clouds of war gathered in the US, the Atlantic States in the north had more people of the industrial age and more potential taxpayers. The potential states of the Confederacy still had a population that was still very young.
Eventually the consequences became unmistakable. The paid labor states had an enormous advantage in military and farming age men. And the paid labor states had a large class of working age women not ready to start their families.
I suspect the problem was going to persist until maternal health, insect control and better vaccination programs developed in the south.
A war of any kind was only going to make matters worse. That would be especially true if families with means, or newly freed slaves who had the ability, left the southern regions.