Breast Milk Storage Bags: How to Choose, Fill, Freeze and Label Them

If you’re building a freezer stash, storage bags are probably going to be your best friend. They take up less space than bottles, they freeze flat, and you can toss them once they’re empty. But not every plastic bag is safe for milk, and a few small habits make the difference between a tidy stash and a freezer full of leaks.

This guide covers what to look for when you buy breast milk storage bags, and how to fill, seal, freeze, label and thaw them using the CDC’s guidance. These times are for healthy, full-term babies. If your baby is premature or in the hospital, follow your care team’s rules.

Bags vs. bottles: which to use when

The CDC says expressed milk can be stored in breast milk storage bags or in clean glass or plastic containers with tight-fitting lids. Both work, and many parents use both.

	Storage bags	Bottles and hard containers
Best for	Freezing, building a stash, travel	Fridge storage, milk you’ll use within a few days
Space	Freeze flat and stack or file upright	Take up more room
Reusable	No, single use	Yes, after washing
Leak risk	Higher; seams and zippers can fail, and frozen bags can split	Lower
Pouring	Can be awkward; some bags have a spout	Easy

La Leche League International (LLLI) suggests glass or hard-sided plastic containers with well-fitting tops for bottles, and advises avoiding plastic made with BPA or BPS (polypropylene is a safe alternative, and glass avoids the risks of plastic completely). The ABM protocol also says to avoid containers made with BPA and that only food-grade plastic should be used.

What to look for in breast milk storage bags

You don’t need a particular brand. Look for these features:

• Made for breast milk. The CDC is clear: never store milk in disposable bottle liners or plastic bags that aren’t intended for breast milk. LLLI explains that general-purpose bags are less strong and tend to leak, that the risk of contamination is greater, and that some types of plastic may destroy nutrients in milk.
• A secure seal. A double zipper or a zipper plus a fold-over strip helps prevent leaks.
• Thick, freezer-safe material. The ABM protocol says plastic bags used for human milk should be sturdy and sealed well.
• BPA-free plastic. Check the label. LLLI recommends avoiding BPA and BPS.
• A write-on label area. You’ll need room for the date and, for childcare, your baby’s name.
• Volume markings. Handy, but they’re approximate, so measure in a bottle if you need an exact amount.
• A flat base or gusset. Bags that stand up on their own are easier to fill.
• Clean, sealed packaging. Bags that come in sealed packaging help keep them clean until you use them.

Optional extras include pour spouts and adapters that let you pump directly into the bag. They’re conveniences, not safety features.

How to fill a storage bag

1. Wash your hands before handling pump parts, bottles or bags.
2. Label the bag first. Writing on a flat, empty bag is much easier than on a full one. Note the date you expressed the milk.
3. Pour in small portions. The CDC suggests storing 2 to 4 oz (60 to 120 ml), or about one feeding’s worth, so less milk is wasted. LLLI suggests 1 to 4 oz (30 to 120 ml).
4. Leave room at the top. Breast milk expands as it freezes. The CDC recommends about 1 inch (2.5 cm) of space at the top.
5. Push out the air and seal. LLLI suggests squeezing out the air at the top before sealing. Run your fingers along the zipper twice to make sure it’s closed.

If you don’t plan to use fresh milk within 4 days, the CDC says to freeze it as soon as possible. Our guide to how long breast milk lasts in the fridge covers fridge storage in more detail.

Freezing bags flat

Freezing bags flat isn’t an official rule in the CDC, AAP, ABM or LLLI guidance, but it’s a popular trick because it saves space, and thin portions should thaw faster:

• Lay sealed bags flat on a tray or a clean, level freezer shelf until they’re frozen solid.
• Once frozen, stand them upright in a bin like files, or stack them.
• Organize the bin so the oldest milk is at the front.

Keep milk toward the back of the freezer, not in the door, where temperatures change each time you open it. HealthyChildren.org (AAP) gives the same advice. Your freezer should be at 0°F (-18°C) or colder.

The ABM protocol says to store bags in an area of the freezer where damage to the bag would be minimized, so avoid crushing them under heavy items.

Labeling: what to write

The CDC says to label milk clearly with the date it was expressed. Add more if it’s useful:

• Date expressed (always)
• Your baby’s name if the milk goes to daycare; ask your provider about their own labeling rules
• Amount, so you can grab the right bag quickly

Use a permanent marker that doesn’t smudge when the bag gets wet.

CDC storage times for bagged milk

Where	Freshly expressed milk
Room temperature, 77°F (25°C) or colder	Up to 4 hours
Fridge, 40°F (4°C) or colder	Up to 4 days
Freezer, 0°F (-18°C) or colder	About 6 months is best; up to 12 months is acceptable
Insulated cooler with frozen ice packs	Up to 24 hours

The CDC says to count frozen milk’s age from when it was first frozen. Other groups differ a little: the AAP gives up to 9 months in a freezer with a separate door (0°F / -18°C) and up to 12 months in a deep freezer (-4°F / -20°C), and says up to 4 days is best in the fridge; the NHS gives up to 6 months in the freezer and up to 8 days in a fridge at 4°C or lower. For a quick use-by date for any bag, try our breast milk storage calculator, and see the full overview in our breast milk storage guide.

Thawing milk from a storage bag

• Thaw the oldest milk first.
• Thaw it in the fridge overnight, in a bowl of lukewarm water or under lukewarm running water. Set the bag in a bowl or cup while it thaws in case it leaks.
• Never use the microwave. The CDC says never to thaw or heat milk in a microwave, because it can destroy nutrients and create hot spots that burn a baby’s mouth.
• Use it in time. The CDC says milk thawed in the fridge should be used within 24 hours of being completely thawed. Once it’s at room temperature or warmed, use it within 2 hours. The AAP says thawed milk is best used within 24 hours but may be used within 48 to 72 hours.
• Don’t refreeze thawed milk.

You can pour the thawed milk into a bottle and serve it cold, at room temperature or warmed. Our guide to thawing frozen breast milk has step-by-step instructions.

Common problems and fixes

• Leaking bags: Check the seal, don’t overfill, and freeze bags in a single layer before stacking. Thaw them inside a container.
• Milk smells soapy or metallic after freezing: This may be high lipase milk. La Leche League says that if the milk was stored properly it is safe and most babies will drink it, but some babies do refuse it.
• Fat stuck to the bag: Milk separates when stored. Swirl the bag gently before pouring to mix the fat back in.
• A torn bag: If you’re not sure whether milk stayed sealed and frozen the whole time, it’s safest to throw it away.

When to ask for help

Talk to your pediatrician or a lactation consultant if your baby is premature or medically fragile, if you’re storing milk for a hospitalized baby, or if you have questions about donating milk. Hospitals often have their own storage rules and containers.