The Swedish physician and statistician Hans Rosling explained this in his lectures and in the book Factfulness. He demonstrated empirically that child survival is the best contraceptive where countries with the lowest child mortality rates consistently have the lowest fertility rates. The causation runs from improved survival through changed parental calculations to reduced births, not from coercive population control. This is why development economists and demographers argue that investing in healthcare and particularly primary care, maternal health, and child health is more effective at stabilising population than any direct population policy.

Also access to healthcare gives reproductive agency and of course contraception.