The two safest ways to defrost frozen breast milk are slowly overnight in the fridge, or more quickly by standing the sealed bag or bottle in a bowl of warm (not hot) water. Once it's thawed, warm it gently to body temperature by running it under warm water or using a bottle warmer. Never use boiling water or a microwave, and never refreeze breast milk once it has thawed.

Defrosting that carefully-built freezer stash can feel high-stakes — every bag represents time and effort at the pump. This guide walks you through thawing and warming frozen breast milk the right way, so none of it goes to waste and every feed is safe for your baby.

How to defrost frozen breast milk

There are two safe methods. Which you choose depends on how much time you have.

Method 1: Thaw in the fridge (the gold standard)

Move the frozen bag or bottle from the freezer to the fridge the night before you need it. It can take up to 12 hours to defrost fully, so a good habit is to move tomorrow's milk across each evening. Thawing slowly in the fridge keeps the milk at a safe, steady temperature the whole time — which is why it's the method most health professionals recommend.

Method 2: Thaw in warm water (the quick way)

Need milk sooner? Stand the sealed container in a bowl of warm water and gently swirl it. As the water cools, top it up with a little more warm water and keep moving the container until the milk is fully liquid. This usually takes around 20 minutes. You can also hold the bag or bottle under warm running water (no hotter than about 37°C) for five to ten minutes.

How to warm thawed breast milk

Once your milk is liquid, many babies happily take it at room temperature or cool — there's no need to warm it at all. If your baby prefers it warm, do it gently:

• Warm water bath: stand the bottle in a jug of warm water for a few minutes.
• Bottle warmer: a warmer brings milk to a consistent body temperature without overheating it. This is especially handy for night feeds and on the go — explore our portable bottle warmer range.
• Swirl, don't shake: gently swirl the bottle to mix the layers of milk and even out the temperature. Vigorous shaking can damage some of the milk's components.

Always test the temperature on the inside of your wrist before feeding — it should feel lukewarm, never hot.

A portable bottle warmer designed for breast milk warms gently and evenly, protecting those precious nutrients. You'll find warmers and milk storage together in our feeding gift packs.

Why you should never microwave frozen breast milk

Microwaves heat unevenly, creating hot spots that can scald your baby's mouth even when the bottle feels cool. High heat can also break down some of the antibodies and nutrients that make breast milk so valuable. Boiling water does the same. Stick to gentle warming every time.

How long does thawed breast milk last?

• At room temperature: use within 2 hours of thawing.
• In the fridge: once fully thawed, store for up to 24 hours.
• Never refreeze: breast milk that has been thawed should not be put back in the freezer.
• Once baby has fed: use within a couple of hours, then discard any leftovers.

The Australian Breastfeeding Association has detailed guidance on preparing and using expressed breast milk. For the full picture on freezing and fridge times, see our breast milk storage guidelines and how long breast milk lasts after pumping.

A note on the smell of thawed milk

Some thawed breast milk smells or tastes soapy or metallic. This is usually caused by an enzyme called lipase breaking down the fats, and the milk is still perfectly safe for most babies. If your baby refuses it, you can scald fresh milk briefly before freezing future batches to reduce lipase activity. Storing milk in good-quality, well-sealed bags also helps keep it fresh — see how in our guide to using a breast milk storage bag.

Frequently asked questions

How long does it take to defrost frozen breast milk?

In the fridge it can take up to 12 hours, so take it out the night before. In a bowl of warm water it takes around 20 minutes, and under warm running water about five to ten minutes.

Can you refreeze thawed breast milk?

No. Once breast milk has fully thawed it should never be refrozen. Use it within 24 hours if it's been kept in the fridge, or within 2 hours at room temperature.

Can you microwave frozen breast milk?

No. Microwaving creates hot spots that can burn your baby and can destroy some of the milk's antibodies and nutrients. Thaw and warm gently with warm water or a bottle warmer instead.

How do you warm breast milk without a bottle warmer?

Stand the bottle or bag in a jug of warm water for a few minutes, or hold it under warm running water, swirling gently until it reaches body temperature. Test a few drops on your wrist before feeding.

Is it safe to defrost breast milk at room temperature?

It's best avoided. Thawing on the bench lets the milk sit in warmer temperatures where bacteria grow. Thaw in the fridge or in warm water instead, which keeps the process safe and controlled.

Why does my thawed breast milk smell soapy?

This is usually due to the enzyme lipase breaking down fats during storage. The milk is generally still safe. If your baby won't take it, briefly scalding fresh milk before freezing can help reduce the smell in future batches.

This article is general information for Australian mums and isn't a substitute for advice from your midwife, lactation consultant or child health nurse. Sources: Australian Breastfeeding Association; Raising Children Network.