You can thaw breast milk in a bag three ways: in the refrigerator overnight, under lukewarm running water, or in a bowl of warm water. The method you choose depends on how quickly you need it ready. Once fully thawed, breast milk is safe in the refrigerator for up to 24 hours, and once it reaches room temperature, you have a two-hour window to use it.
Overnight in the Refrigerator
The simplest approach is moving a frozen bag from the freezer to the refrigerator the night before you need it. Most bags thaw completely in 8 to 12 hours. Place the bag inside a clean bowl or container to catch any milk that leaks, since storage bag seals can loosen as milk expands and contracts during temperature changes.
The 24-hour clock starts when the milk is completely thawed, not when you move it from the freezer. So if you transfer a bag at 10 p.m. and it’s fully liquid by 6 a.m., you have until 6 a.m. the next day to use it. If the milk still has ice crystals, it’s not fully thawed yet and the countdown hasn’t begun.
Warm Water Bowl Method
When you need milk sooner, place the sealed bag in a bowl of warm (not hot) water. Swap the water every few minutes as it cools. A small bag will typically thaw in 15 to 20 minutes this way. You can also hold the bag under lukewarm running water, rotating it so it thaws evenly. Either way, the water should feel comfortable on your wrist, never hot enough to sting.
This method doubles as warming if you keep going after the milk is liquid. Test the temperature by dropping a few drops on the inside of your wrist before feeding. It should feel warm, not hot.
Why You Should Never Use a Microwave
Microwaves heat liquids unevenly, creating hot spots that can scald a baby’s mouth even when the rest of the milk feels fine. The temperature is nearly impossible to control, and bags can burst if heated too long. Beyond the burn risk, high microwave temperatures break down the proteins and immune-protective components in breast milk, reducing its nutritional value. The American Academy of Pediatrics recommends against microwaving any liquid or food intended for an infant.
Handling Separation and Mixing
Thawed breast milk almost always separates into a watery layer on the bottom and a fat layer on top. This is completely normal. Unlike store-bought cow’s milk, breast milk isn’t homogenized, so the fat naturally rises.
Gently swirl the bag or bottle to recombine the layers. Avoid vigorous shaking. While the exact threshold isn’t well defined in research, rough handling can damage some of the bioactive proteins in the milk. A few slow, circular swirls are enough to blend it back together.
Preventing Leaks During Thawing
Leaking bags are one of the most common frustrations with frozen breast milk. The fix is simple: always thaw the bag inside a secondary container. A liquid measuring cup, tall glass, mixing pitcher, or bowl all work well. If a bag tends to leak from the bottom seal, flip it upside down in the container so the zipper end faces down. A measuring cup is especially convenient because you can pour directly into a bottle if the bag does split.
If you’re thawing in the fridge, a lidded container keeps the milk clean in case of a leak. Some parents also give the outside of each bag a quick wipe before thawing, since freezer surfaces aren’t always spotless and any leaked milk could pick up residue from the bag’s exterior.
The Refreezing Rule
Once breast milk has completely thawed, you cannot refreeze it. The CDC is clear on this: never refreeze human milk after it has thawed. However, there is one exception. If the milk has started to thaw but still contains ice crystals, it can go back in the freezer. This sometimes happens during a freezer door being left open or a power flicker. As long as those ice crystals are visible, the milk is considered partially frozen and safe to refreeze.
When Thawed Milk Smells Off
Some parents notice a soapy, metallic, or slightly sour smell when thawing milk that was perfectly fine going into the freezer. This is widely attributed to lipase, a naturally occurring enzyme in breast milk that continues breaking down fats even while frozen. The longer milk is stored, the more noticeable the smell can become.
Interestingly, recent research has challenged the assumption that high lipase levels are always to blame. A 2019 study tested frozen milk that babies had refused from 16 mothers and found none of the samples had unusually high lipase. Oxidation from air exposure during pumping and storage may also play a role. Regardless of the cause, milk with these changes is safe. Most babies drink it without complaint, though some are more sensitive to the taste. If your baby consistently refuses thawed milk, you can try scalding fresh milk briefly before freezing future batches, which deactivates the enzyme.
Quick Reference for Time Limits
- Frozen in the freezer: best used within 6 months, acceptable up to 12 months
- Thawing in the fridge: use within 24 hours of being fully thawed
- At room temperature: use within 2 hours once warmed or brought to room temperature
- Refreezing: only if ice crystals are still present