You can safely defrost frozen breast milk three ways: in the refrigerator overnight, under lukewarm running water, or in a bowl of warm water. Each method has tradeoffs between speed and convenience, but all three preserve the nutrients your baby needs. The one method you should never use is a microwave.
Three Safe Thawing Methods
Refrigerator (slowest, most hands-off): Place the frozen container in the fridge the night before you plan to use it. A standard bag takes roughly 8 to 12 hours to fully thaw, so putting it in before bed means it’s ready for a morning feeding. This is the easiest option if you can plan ahead.
Warm water bath (moderate speed): Set the sealed bag or bottle in a bowl of warm or lukewarm water. As the water cools, swap it out with fresh warm water until the milk is thawed. This typically takes 20 to 30 minutes depending on the volume and how frozen it is.
Lukewarm running water (fastest): Hold the sealed container under lukewarm running water, rotating it so all sides get even exposure. This is the quickest option when you need milk right away. The water should feel comfortable on your wrist, not hot.
With any method, always thaw the oldest milk first. Use a first-in, first-out approach so nothing sits in the freezer longer than necessary.
Why Microwaves Are Off Limits
Microwaves heat liquids unevenly, creating hot spots in the milk that can scald your baby’s mouth even when the bottle feels lukewarm on the outside. The American Academy of Pediatrics and the CDC both advise against microwaving any liquid or food intended for an infant. Beyond burn risk, microwave heating can destroy proteins and other nutrients in breast milk. Bottles heated too long can also burst.
Temperature matters more broadly, too. Exposing breast milk to temperatures above about 176°F (80°C) alters its fatty acid profile and damages amino acids that are essential for infant growth. Lukewarm water, not hot, is the safest choice for thawing and warming.
How Long Thawed Milk Stays Safe
Once breast milk is completely thawed, you have 24 hours to use it if it’s stored in the refrigerator at 40°F or colder. The countdown starts when the milk is fully liquid, not when you first move it out of the freezer. Any milk left after 24 hours should be discarded.
If thawed milk has been sitting at room temperature (for example, in a bottle your baby didn’t finish), use it within about two hours. Bacteria multiply quickly in milk that’s been warmed, so don’t save partially consumed bottles for later.
Can You Refreeze Thawed Milk?
Once breast milk has fully thawed, you cannot put it back in the freezer. Refreezing breaks down the milk’s protective components and increases bacterial risk.
There is one exception: if the milk still contains visible ice crystals, it’s considered frozen. Breast milk that has started to thaw but still has ice crystals in it can safely go back into the freezer. This is helpful during power outages or if you accidentally pulled out more bags than you needed.
Warming Thawed Milk to Feeding Temperature
Thawed milk from the refrigerator is cold, and while some babies will drink it that way, most prefer it closer to body temperature. To warm it, place the bottle or bag in a bowl of warm water for a few minutes or hold it under warm running water. Swirl the container gently to mix the fat layer that naturally separates during storage. Don’t shake it vigorously.
Test the temperature by dripping a few drops onto the inside of your wrist. It should feel neutral or slightly warm. If it feels hot, let it cool before feeding.
What to Do About a Soapy or Off Smell
Some parents thaw a bag of milk and notice it smells soapy, metallic, or slightly fishy. This is almost always caused by lipase, a naturally occurring enzyme in breast milk that breaks down fats to aid your baby’s digestion. Lipase levels tend to peak around five to six months into lactation, and in some parents the enzyme is active enough to change the smell of stored milk.
High-lipase milk is safe to feed your baby. Most babies drink it without complaint, though some refuse it because of the taste. If your baby consistently rejects thawed milk, you can prevent the issue in future batches by scalding fresh milk before freezing it. Heat the milk in a small pot until it reaches 165°F (just until tiny bubbles form at the edges, not a full boil), then cool it quickly and freeze. Scalding deactivates the lipase so the flavor stays neutral in storage. This step only works on milk that hasn’t been frozen yet; it won’t fix milk that already smells off.