Freezing breastmilk is straightforward: express it, store it in a clean container with some room for expansion, label it with the date, and place it in the back of your freezer where temperatures stay most consistent. Frozen breastmilk stays at its best for about 6 months, though it remains safe for up to 12 months. The details that matter most are choosing the right container, filling it to the right level, and knowing the rules for thawing so nothing goes to waste.
Choosing a Storage Container
You have three main options: breastmilk storage bags, plastic bottles, and glass bottles. Each works, but they come with trade-offs worth knowing about.
Storage bags designed for breastmilk are the most space-efficient option in a crowded freezer. They lay flat, stack neatly, and are pre-sterilized. The downside is durability. Bags can tear or develop small punctures when shuffled around among frozen items, and plastic is porous enough to harbor bacteria in tiny scratches. Some parents also prefer to avoid plastic because it can leach microplastics into milk over time.
Glass bottles eliminate chemical leaching concerns entirely. Glass is nonporous, easy to sanitize, and completely freezer safe. The main drawback is space. Round bottles take up more room, and you’ll need lids that seal tightly. Stainless steel lids are a good pairing because they won’t rust or degrade the way aluminum can. If you go with glass, avoid filling the bottle to the very top, since milk expands as it freezes and can crack a full container.
Plastic bottles made from food-grade material (typically marked BPA-free) are a middle ground. They’re lighter than glass and more durable than bags. Whatever container you choose, make sure it’s designed for breastmilk or food storage, not a repurposed household container.
How Much to Store Per Container
Freeze milk in the amounts your baby typically eats in a single feeding, usually 2 to 4 ounces. This avoids waste, since you can’t refreeze milk once it’s thawed. Smaller portions also thaw faster when you need them.
Leave about an inch of space at the top of any container. Milk expands as it freezes, and overfilled bags can burst open, while overfilled bottles can crack or pop their lids. If you’re using storage bags, squeeze out excess air before sealing, then lay them flat on a baking sheet until frozen. Once solid, the flat bags stack like books and save a surprising amount of freezer space.
Labeling and Organizing
Write the date you expressed the milk on every container before it goes into the freezer. If your baby is in daycare, add their name as well. Use a system that lets you grab the oldest milk first. One simple approach: line bags or bottles up from left to right by date, always adding new ones to the right side. Some parents use small bins or freezer baskets to keep batches separated by week or month.
Where to Place Milk in the Freezer
Store containers toward the back of the freezer, away from the door. Every time the door opens, the temperature near the front fluctuates, which can partially thaw and refreeze your milk over time. The back wall maintains the most stable temperature. If you have a standalone deep freezer, that’s even better for long-term storage because it holds a steadier temperature than a freezer attached to a refrigerator.
Storage Time Limits
Freshly expressed breastmilk can sit at room temperature (77°F or cooler) for up to 4 hours before it needs to be refrigerated or frozen. In the refrigerator, it stays good for up to 4 days. In a standard freezer, 6 months is the ideal window. Milk stored up to 12 months is still safe, but the longer it sits, the more some vitamins and fats break down. The nutritional loss is gradual, not sudden, so milk at 7 or 8 months is still valuable.
Adding Fresh Milk to Frozen Milk
You can combine freshly pumped milk with already frozen milk, but there’s one important step: chill the fresh milk in the refrigerator first. Adding warm milk directly to a frozen container causes the frozen portion to partially thaw, which compromises its safety. Once the fresh milk is fully cooled, you can pour it into the container with the frozen milk and return it to the freezer. The same rule applies when combining fresh milk with refrigerated milk from an earlier session. Cool it down, then combine.
How to Thaw Frozen Breastmilk
Three safe methods work well. The gentlest is moving a container from the freezer to the refrigerator the night before you need it. It will be ready by morning. For faster thawing, hold the container under lukewarm running water or set it in a bowl of lukewarm water. Swirl the container occasionally to distribute the warmth evenly.
Never use a microwave. Microwaves heat unevenly, creating hot spots that can scald a baby’s mouth, and they also break down some of the milk’s beneficial nutrients. A bottle warmer set to a gentle temperature is fine if you have one, but plain lukewarm water works just as well.
Time Limits After Thawing
Once milk has fully thawed in the refrigerator, use it within 24 hours. That 24-hour window starts when the milk is completely thawed, not when you first moved it out of the freezer. If you bring thawed milk to room temperature or warm it for a feeding, use it within 2 hours. Any milk left in the bottle after a feeding should be discarded, and thawed milk can never be refrozen.
What to Do During a Power Outage
A full freezer holds its temperature for roughly 48 hours if the door stays closed, and about 24 hours if it’s half full. Once power returns, check your stored milk. If ice crystals are still visible in the container, the milk is considered frozen and can safely stay in the freezer. If the milk has thawed completely but still feels cold, move it to the refrigerator and use it within one day. If it has thawed and feels warm, or if you’re unsure how long the power was out, discard it. The CDC’s guidance is simple: when in doubt, throw it out.
Quick-Reference Storage Chart
- Room temperature (77°F or cooler): up to 4 hours
- Refrigerator: up to 4 days
- Freezer: 6 months ideal, up to 12 months acceptable
- Thawed in refrigerator: use within 24 hours of fully thawing
- Thawed and warmed: use within 2 hours