Q&A: How Does My Body Know?
Research shows that once a mother reaches full milk production — at about five weeks after birth — milk production won't further increase and doesn't need to. As a baby grows, his growth rate slows down (meaning he'll never grow as quickly as he did in the very beginning again), so he'll be able to achieve healthy growth with the same amount of milk per day.
Babies who are fed formula, however, consume much more milk than breastfed babies, and sometimes breastfeeding mothers think their breastfed babies should be taking as much milk as their formula-feeding neighbor's baby. Not so.