Breast milk is naturally bacteria-free, but if you are pumping to store for later use, milk can be contaminated by bacteria on the breast shield or the bottle. Mothers should wash their hands first and always use a clean breast shield. Use bottles with screw caps or bags that are specially designed to fit inside of a baby’s bottle (plastic or formula bottle bags can drip or spill) and never add fresh breast milk to a container of older milk.
Breast milk can be stored safely at room temperatures for up to four hours, refrigerated for 48 hours, or frozen at temperatures below 0 degrees Fahrenheit for three to six months. Once frozen, breast milk has been defrosted, it should be used within 24 hours or discarded. And breast milk that’s been thawed should never be refrozen.