Freezing breast milk in storage bags is straightforward: express your milk, pour it into a bag designed for breast milk storage, leave about an inch of space at the top, seal it tightly, label it with the date, and lay it flat in the freezer. Breast milk stays good in a standard freezer for about 6 months at peak quality, and up to 12 months total. The details below will help you do it efficiently and avoid wasting a single ounce.
Choosing the Right Bags
Use bags specifically made for breast milk storage, not regular sandwich bags or general food storage bags. Breast milk bags are pre-sterilized, made from food-safe plastic that won’t leach chemicals, and designed with double-zipper seals that hold up in the freezer. Most brands are also pre-marked with volume lines so you can measure without a separate container. You can find them at any pharmacy or baby supply store.
How Much to Put in Each Bag
Store milk in smaller portions, typically 2 to 4 ounces per bag. Newborns eat about 2 to 3 ounces per feeding, while older babies may take 4 to 5 ounces. Smaller portions thaw faster and reduce waste, since you can’t refreeze breast milk once it’s been thawed. If your baby regularly takes larger feeds, you can always thaw two small bags and combine them.
Breast milk expands as it freezes, so leave about one inch of empty space at the top of the bag before sealing. Overfilling risks a burst seal, which means a messy freezer and lost milk.
Labeling Each Bag
Write the date you expressed the milk on every bag before it goes in the freezer. If your baby is in daycare, add their name as well. A permanent marker works directly on most storage bags, or you can use a piece of tape. Including the volume is helpful too, especially when you’re pulling bags to prepare bottles later. Always use the oldest milk first.
Freeze Bags Flat for Faster Thawing
Once sealed, lay each bag flat on a baking sheet or directly on the freezer shelf. Flat bags freeze into thin slabs that stack neatly, saving significant space compared to bags frozen upright in lumpy shapes. More importantly, flat bags thaw much faster than thick, rounded ones. Once a bag is fully frozen solid (usually a few hours), you can stand the slabs upright in a bin or line them up like files in a folder for easy organization.
Store milk toward the back of the freezer where the temperature is most consistent. The door shelf is the warmest spot and experiences the most temperature swings every time you open the freezer.
How Long Frozen Breast Milk Lasts
In a standard freezer attached to a refrigerator, breast milk keeps for about 6 months at best quality. It remains safe up to 12 months, though some fat and vitamin content gradually declines over time. A deep chest freezer that stays at a more stable temperature can help preserve quality toward the longer end of that range.
For context, freshly expressed milk is good at room temperature (77°F or cooler) for up to 4 hours, and in the refrigerator for up to 4 days. If you know you’ll use the milk within a few days, refrigerating is simpler than freezing and preserves more of the milk’s immune properties.
Adding Fresh Milk to a Bag Already in the Freezer
You can add freshly expressed breast milk to a bag that’s already frozen or refrigerated, but only after cooling the fresh milk first. Place the freshly pumped milk in the refrigerator for at least an hour before combining. Pouring warm milk directly onto a frozen bag will partially thaw the frozen layer, which compromises safety and quality. The amount you add should be less than the amount already in the bag, so the warm milk doesn’t bring the frozen portion above a safe temperature.
Thawing and Warming Frozen Bags
The safest way to thaw a frozen bag is in the refrigerator overnight. If you need it sooner, hold the sealed bag under lukewarm running water or place it in a bowl of warm water. Gently swirl the bag as it thaws to mix the fat layers that naturally separate during freezing. Don’t shake it vigorously.
Never use a microwave to thaw or warm breast milk. Microwaves heat unevenly, creating hot spots in the liquid that can burn your baby’s mouth. They also break down some of the milk’s nutritional and immune components. When warming thawed milk, keep the temperature at or below 104°F (40°C) to preserve nutrient quality. Test a few drops on the inside of your wrist before feeding.
Once breast milk is fully thawed in the refrigerator, use it within 24 hours. Milk that’s been warmed to feeding temperature should be used within 2 hours. Never refreeze breast milk after it has thawed.
Quick Reference for Storage Times
- Room temperature (77°F or cooler): up to 4 hours
- Insulated cooler with ice packs: up to 24 hours
- Refrigerator: up to 4 days
- Freezer: 6 months ideal, 12 months acceptable
- Thawed in refrigerator: use within 24 hours