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Warming stored breast milk before serving it to your baby is a personal choice. Many babies like breast milk warm if they take it from a bottle, since breast milk is warm when babies nurse.

Warming up breast milk also helps the consistency after it’s been stored. When breast milk is frozen or refrigerated, the fat tends to separate in the bottle. Warming breast milk, or at least bringing it to room temperature, can help you more easily mix breast milk back to its original consistency.

Read on to learn how to warm up breast milk and safety precautions you should take.

To warm breast milk from the fridge:

  • Take breast milk from the fridge and set it aside.
  • Heat water using either a teakettle or microwave. Pour very warm (not boiling) water in a mug or bowl.
  • Place sealed bag or bottle of breast milk in the bowl of warm water. The milk should be kept in a sealed container for warming.
  • Leave the milk in the warm water for 1-2 minutes until breast milk reaches desired temperature.
  • With clean hands, pout breast milk into a bottle, or, if it’s already in a bottle, screw on the bottle nipple.
  • Swirl breast milk (never shake it) to mix in the fat, if it’s separated.

Before offering the bottle to your baby, test the breast milk temperature. You can do this by pouring a bit on your wrist. It should be warm, but not hot.

To prevent germs getting into the milk, avoid dipping your finger into the bottle.

You can also warm the milk by holding the sealed bag or bottle under very hot running water from the faucet. This takes longer and uses more water. You could also burn or scald your hand.

To warm frozen breast milk, remove the frozen breast milk from the freezer and put it in the fridge to thaw overnight. Then, follow the same instructions for warming breast milk from the fridge.

If you need milk right away and all you have is frozen milk, you can heat the breast milk straight from the freezer using the same method you’d use to heat from the fridge. The only difference is that you’ll need to keep it in the warm water for 10-15 minutes, or longer.

Never put breast milk in a microwave. Microwaves don’t heat food evenly, so they can make hot spots that can burn your baby.

Microwaves are also believed to damage the nutrients and antibodies in breast milk.

You can use a microwave to heat the water used for warming breast milk, however.

Some parents swear by using a bottle warmer to heat breast milk or formula. A bottle warmer is a simple contraption used to help you heat up a bottle.

Manufacturers of bottle warmers claim these devices heat more evenly than a microwave. However, opinions are mixed if they are actually useful or easier than steeping breast milk in hot water.

The potential disadvantage of a bottle warmer is the increased chance of overheating breast milk and killing beneficial nutrients.

In a 2015 study, researchers tested how warm different portions of breast milk can get in a bottle warmer. They found that the milk can get above 80°F (26.7°C), which could have a negative effect on the nutritional value of the milk.

The study doesn’t state which brand of bottle warmer they used in testing. If you’re interested in the convenience of a bottle warmer, it might be worthwhile to use a thermometer and test breast milk temperatures as you use it.

How to warm breast milk in a bottle warmer

To warm breast milk in a bottle warmer, put the whole bottle in the heating area and follow the manual’s instructions.

Most bottle warmers take a few minutes to reach desired warmth. Keep an eye on the bottle warmer so that it doesn’t overheat, and unplug it when not in use.

Do not reheat or restore breast milk that was previously warmed.

Sometimes babies nibble at their food and don’t quite finish it. But after two hours of sitting out, it’s best to throw out any leftover breast milk. This helps prevent milk from going bad or being introduced to germs in the environment.

If your baby eats on and off, or if you’re traveling, breast milk may end up sitting out for a while. Safety of breast milk left out will vary greatly depending on the overall bacteria levels in the environment.

Breast milk is good at room temperature (up to 77°F or 25°C) for:

  • Four hours for fresh breast milk. After four hours you should use, store, or discard it.
  • Two hours for previously stored and thawed breast milk. Discard unused, thawed breast milk after two hours. Do not refreeze or reheat breast milk that was frozen and thawed.

Always keep breast milk covered with the lid on or bag zipped while it’s sitting out.

At least one study indicates that you can store breast milk in an insulated cooler with ice packs for up to 24 hours. Always use bottles and bags specifically designed for freezing human milk.

Plan to store breast milk in 2 to 6 ounces, depending on how much your baby typically takes in a single feeding. That can help reduce the amount of unused breast milk you later have to discard.

Always label breast milk with the date it was expressed, and use the oldest stored breast milk first in order to keep the rotation fresh.

Breast milk can be stored in the refrigerator for four days and in the freezer for up to 12 months. However, after 90 days, acidity in breast milk can rise and nutrients can decrease. So, for the best quality, plan to use frozen breast milk within six months of when it was expressed.

You can mix and store breast milk that was pumped on different days but always use it based on the first, oldest date. And never add fresh breast milk to already frozen breast milk.

If your baby doesn’t like breast milk that was previously frozen, you can try just refrigerating breast milk and working through your supply faster.

In general, refrigerated breast milk is better than frozen because it’s fresher and the nutrients and antibodies will be most current to the baby’s needs.

However, freezing breast milk is a good technique if you need to have a lot on hand, for example, if you’re returning to work. Frozen breast milk is still considered to have more nutrients than formula.

Warming breast milk is a very common practice, but safety and quality standards cannot be guaranteed due to all of the variables that come with storing and reheating.

More research is needed on optimal use of frozen breast milk since many babies rely on it completely for their nutrition.

In general, though, breast milk stores well in the refrigerator and freezer, and can be warmed to help baby take it easier. Always use storage bags or bottles designed especially for breast milk.

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