> Instead, the mother delivers and the neonate’s growth rate slows relative to its fetal growth rate, keeping both the offspring’s and the mother’s energy requirements in check.

Growth rate slows after birth, so that would account for some of that. Mother’s also would not need to carry around the weight of the baby, and would be more able to provide for the child.

Women in Africa (in traditional societies), carried their babies on their backs while they worked the fields, carried water, etc.

Baby-wearing is pretty popular in modern times and other parts of the world too.

Though it's not 24/7.