Freshly expressed breastmilk can be kept in the back of a refrigerator for three to five days. Some keep their milk up to 8 days in the fridge with no issues (if stored under sterile conditions). Always label your expressed milk with a time and a date.
If you have already left your milk out at room temperature, it is advised to use it as soon as possible.
Keep in mind that the longer your breast milk stands, the higher the loss of vitamin C and antioxidants. Even though some components of breast milk are lost, breast milk that has been frozen for many months is still far superior to formula.
Milk that has been thawed can be kept in the refrigerator for 24 hours at 32-39°F / 0-4°C.
Once the milk has been removed from the fridge, it can be kept at room temperature for up to six hours - four hours if it is a hot day.
Freshly pumped milk should be stored separately or chilled before adding to other already-cold milk in the refrigerator. If batches of milk are mixed, the milk should be used according to the refrigeration time of the first, older batch of milk.
You may also decide to keep your milk in an insulated cooler (with ice packs), in which case it can be kept for one whole day.
Breast milk can be stored in the back of the freezer for 12 months. Milk that is stored closer to the outside of the freezer can be stored for 6 months. Never store your milk in the door compartment where it might defrost due to continuous opening.
Milk storage guidelines may differ for those babies who are preterm or hospitalized.
Resources and References