The expressed breast milk may be stored, for example in a refrigerator overnight, so someone else can feed the baby by bottle. A breast pump is also used when the breasts are producing more milk than can be consumed by the infant and the breasts become "engorged" preventing proper latching by the infant. Pumping relieves pressure in the breasts. For babies that do not latch properly for direct breastfeeding but the mother desires the benefits of breast milk, the baby may be fed breast milk in a bottle. A breast pump may also be used to stimulate lactation in women who have not just given birth. Pumping may be desirable to continue lactation and the resulting hormones to recover from pregnancy even if the pumped milk is not used.
Breast pumps may be manual where the woman can directly control the pressure and frequency of pumps. They are often electrically powered. The portions of the pump that comes into direct contact with the expressed milk that will be fed to infants needs to be sterilized like baby bottles.
Breast pumps can be designed to be part of a "feeding system" so that the milk storage portion of the pump is the baby bottle used to feed the infant.