How long breast milk can sit out on the counter versus how long you can store it in a refrigerator or freezer

• Freshly expressed breast milk can sit out at room temperature for four hours.
• Store breast milk in the back of the fridge for up to 4 days and in the freezer for 6 to 12 months.
• To avoid wasting milk, freeze 2 to 4 ounces in a food-grade container with a tight-fitting lid.

Being hooked up to a breast pump is a reality for many new moms.

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But like any other milk in the fridge, your breast milk can, and will, go bad.

To maximize safety and freshness, here's what you need to know about using and storing pumped breast milk.

Breast milk can sit out at room temperature for up to 4 hours

Doctors agree that it's best if you store breast milk within 4 hours of expressing. Especially if the room is hotter than 77 degrees Fahrenheit because bacteria in the milk will cause it to go bad faster at warmer temperatures.

If your baby did not finish a bottle of breast milk, you can leave it on the countertop at room temperature and reuse it within two hours.

The time that you can let used breast milk sit out is shorter than for freshly expressed milk because the baby's mouth introduces new bacteria into the milk, which may cause it to go bad sooner.

Use refrigerated breast milk within a week of storage

The CDC recommends storing freshly-expressed breast milk in the refrigerator for up to four days.

When storing in the refrigerator, Syeda Amna Husain, MD, a pediatrician at Pure Direct Pediatrics, tells Insider you should place breast milk in the back of the fridge, not the door. That's because, compared to the door, which you're always exposing to room temperature, the back of the fridge stays cooler. Double-check and make sure the temperature in your fridge is 39 to 40 degrees Fahrenheit.

When you're ready to use it, place the bag or container in a pot of warm water or run it under warm water for a few minutes.

Note: You can also feed your baby cold breast milk right from the fridge, but if your baby is used to nursing from your breast they may prefer warm milk. Just always remember to use the oldest milk first.

For extended use, store breast milk in the freezer

Whether you're creating a stash for later or you have a surplus of breast milk, you can safely store it in the freezer for six to 12 months. Though the shorter time it spends there, the better.

One small 2012 study, published in the Academy of Breastfeeding Medicine, reported that, after three months of storage, breast milk samples had fewer calories and less fat compared to fresh milk.

As with the fridge, you should store your milk in the back of the freezer at 0 degrees Fahrenheit or colder. And the colder, the better. The same 2012 study reported that fat and caloric content in milk decreased if the milk was stored at temperatures above -4 degrees Fahrenheit.

When you're ready to thaw the milk, take the oldest milk out of the freezer first and place it in the back of the refrigerator. Do not place it on the counter to thaw at room temperature, because this can give bacteria a chance to grow and spoil the milk before it's completely thawed.

Once thawed in the refrigerator, use the milk within 24 hours. After you take thawed milk out of the fridge and leave it at room temperature, use it within 2 hours.

Never refreeze breast milk after it has been thawed.

The rule of fours

If you're worried that you'll forget these dates and time frames, consider using the rule of fours. "Four hours at room temperature, four days in the fridge," says Husain.

Husain also suggests that, if you plan on storing your milk, you should label it with the date and time you pumped it before placing it in the fridge or freezer.

How to tell if breast milk is bad

No matter how careful you are, sometimes your breast milk may still go bad before you use it. You can tell it's bad if it smells or tastes sour - similar to cow's milk when it spoils.

But don't mistake thawed milk for bad milk. Sometimes, thawed breast milk will have a different smell, often a soapy smell, or consistency than fresh milk. As long as you've followed the storage and thawing guidelines, the milk should be safe to use.

The best way to preserve breast milk

After pumping, place breast milk in storage bags made specifically for breast milk or clean food-grade containers with tight-fitting lids.

For the milk you plan on using within 4 days, date it, and place it in the back of the refrigerator. Otherwise, freeze it right away. To avoid wasting milk, freeze 2 to 4 ounces in each bag or container.

Insider's takeaway

How you store breast milk depends on how soon you plan to use it. If you are not going to use freshly pumped breast milk immediately, you can store it in the refrigerator for up to four days. Or, you can freeze it for up to one year.

However, despite your best efforts, breast milk can still go bad sometimes. Spoiled breast milk will have a foul smell or sour taste. In order to minimize spoilage, store breast milk in clean, food-grade containers with tight-fitting lids.

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