Pumping is a good way to assess milk supply. Many mothers become worried when they are only able to pump a few ounces per pumping session, believing that the amount of breast milk they have expressed indicates how much milk is in their breasts. This is NOT true, and a cause of much worry and concern. Milk supply and measurable milk output are not the same thing. Milk supply refers in this case to how much milk is available in the breast, whereas milk output is the amount of milk expressed.