Pumping allows you to build up a reserve of milk, which can come in handy if you need to be away from your baby unexpectedly. After you express your milk, store it in the refrigerator for up to 72 hours, or in the freezer for about three months. It's best to put breast milk in plastic or glass feeding bottles that have secure caps to seal in freshness. You can also use plastic bags made especially for storing breast milk or even disposable baby bottle liners, though they both may break more easily because the plastic is thin. Remember to write the date on the bottle or bag BEFORE putting it in the refrigerator or freezer so you'll know WHEN you pumped it.