How to Use a Bottle Warmer

To use a bottle warmer, add the amount of water specified in your unit's instructions, set it for your milk volume and your target temperature, place the bottle in, warm until the cycle ends, then swirl (do not shake) and test the milk on your inner wrist before feeding. Target lukewarm -- around body temperature (98.6 F / 37 C) -- not hot. This page covers the general method, how to warm refrigerated vs. frozen breast milk, and the safety rules the CDC and AAP use for warmed milk. It is informational only and not medical advice.

For the full storage windows that determine which method to use, see how long does breast milk last . For formula warming, see how long is formula good for in a bottle .

How to use a bottle warmer: step by step

Add water to the reservoir. Use the amount your unit specifies. Too little water means uneven warming; too much can overflow. Always follow the unit's own markings -- amounts vary by model.
Select the setting for your bottle size and milk type. Most warmers have settings for small, medium, and large volumes, and some distinguish between refrigerated and frozen milk. Start with the lowest setting for the given volume and increase if needed -- overheating is the main risk.
Place the bottle in the warmer. Keep the bottle upright and make sure the base is seated correctly so heat reaches the milk evenly.
Warm until the cycle ends. Do not leave the bottle warming unattended for long stretches beyond the recommended cycle. Extended warming raises the risk of hot spots and overheating.
Swirl, do not shake. After warming, gently swirl the bottle to even out the temperature. Shaking breast milk can damage some of its components and introduce air bubbles.
Test the temperature on your inner wrist. A few drops should feel comfortably warm, not hot. If it feels too warm, let it cool for a minute before feeding.
Feed within 2 hours. Once warmed, use the milk within 2 hours. Discard any leftover milk from a started feeding within 2 hours after the feeding ends.

Warming refrigerated breast milk

Refrigerated breast milk (stored at 40 F / 4 C) can go straight into a bottle warmer. Remove it from the fridge just before warming -- keeping it refrigerated until then limits bacterial growth.

If you do not have a bottle warmer, you can warm refrigerated milk by holding the sealed container under warm running water for a few minutes, or by placing it in a bowl of warm (not hot) water. Either way, target lukewarm, not hot.

A useful serving strategy: start with a smaller amount (2 to 3 oz) and offer more only if your baby is still hungry after the first portion. That reduces waste inside the 2-hour use window.

Warming frozen breast milk

Frozen breast milk should be thawed before warming, not warmed from frozen in a single step. The CDC recommends thawing frozen milk in the refrigerator overnight or holding the sealed container under cool then gradually warmer running water. Do not thaw at room temperature.

Once thawed in the fridge, use the milk within 24 hours. Once it has been warmed, use it within 2 hours. See how long does breast milk last for the full storage and thaw windows.

Never refreeze breast milk after it has been thawed. If you thaw more than your baby needs, offer the extra at the next feeding (within 24 hours if kept refrigerated after thawing) rather than refreezing.

Safety rules for warmed breast milk (CDC and AAP)

Never microwave breast milk. Microwaves heat unevenly and create hot spots that can burn a baby's mouth even when the outside of the bottle feels fine. Microwaving can also damage some immune-protective components in breast milk. Both the CDC and the AAP advise against microwaving.
Do not refreeze thawed milk. Once breast milk has been thawed (in the fridge or under running water), do not put it back in the freezer.
Use warmed milk within 2 hours. Discard any portion that has been warmed but not fed, or any leftover in the bottle after a feeding, within 2 hours of warming or of the feeding ending.
Target lukewarm, not hot. Breast milk does not need to be warm -- some babies accept it at room temperature or even cold. Lukewarm (around body temperature, 98.6 F) is a safe and common target. Overheated milk is a burn risk.

Frequently Asked Questions

How do you use a bottle warmer?

Add the amount of water your unit specifies, select the setting for your milk volume, place the bottle in the warmer, run the cycle, then swirl (do not shake) and test the temperature on your inner wrist before feeding. Target lukewarm -- around body temperature (98.6 F) -- not hot.

How do you warm breast milk from the fridge?

Place the bottle in a warmer set for your milk volume, or hold the sealed container under warm running water for a few minutes, or set it in a bowl of warm water. Do not microwave. Warm to lukewarm, test on your wrist, and use the milk within 2 hours.

Can you microwave breast milk?

No. The CDC and AAP both advise against microwaving breast milk. Microwaves heat unevenly and create hot spots that can burn a baby's mouth. Microwaving can also damage some of the immune-protective components in breast milk.

How long can warmed breast milk sit out?

Once breast milk has been warmed, use it within 2 hours. Discard any leftover milk from a started feeding within 2 hours after the feeding ends.

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How to Use a Bottle Warmer