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Pumping breast milk takes time and commitment, so you don’t want a drop of it to go to waste. Luckily, it can easily be stored in the refrigerator or freezer, and it can stay fresh for up to six months.

Here’s what you need to know to keep your breast milk as fresh and nutritious as possible for your little one.

No one wants to give their baby less-than-optimal breast milk. How long breast milk lasts depends on where you’re storing it. Follow these guidelines from the Centers for Disease Control and Prevention (CDC) to ensure your breast milk doesn’t spoil.

You can leave freshly expressed or pumped breast milk out at room temperature—77 degrees Fahrenheit or cooler—for up to four hours.

When possible, though, freshly expressed breast milk is preferred over refrigerated or frozen breast milk. Fresh milk contains greater nutrients and immunologic benefits.

“You could never leave formula out for that long, but breast milk is a live substance with antimicrobial properties that prevent bacteria growth,” says Julie Morgan-Vaughn, a registered nurse and a certified lactation consultant in Vancouver, Washington.

Meanwhile, thawed breast milk can stay fresh at room temperature for one to two hours.

Many moms rely on an insulated cooler to transport the milk they’ve pumped at work or while traveling. Breast milk can remain safely in a cooler for up to 24 hours, according to the CDC. Just make sure that the cooler remains closed the entire time you’re in transit, and once you get to your destination, put the breast milk in the refrigerator or freezer right away.

You can keep freshly expressed or pumped breast milk in the refrigerator for up to four days—just make sure your refrigerator is 40 F or colder. Make sure to store it in the back where the temperature is coldest and never store breast milk in the refrigerator door, as the temperature fluctuations could cause the milk to spoil. Note that thawed breast milk, however, stays fresh for only 24 hours in the fridge.

While breast milk is safe in the freezer for 12 months, over time the quality of the milk decreases. Ideally, you should use it within three months to six months for best quality and taste. Make sure your freezer is 0 F or colder.

Regardless of where you’re storing your breast milk, leftover breast milk (if your baby didn’t finish their bottle) needs to be consumed within two hours of the baby finishing feeding.

It’s important to pick an appropriate storage container for breast milk. “And if your child goes to daycare, you may have to work with whatever protocols they have for storing breast milk,” says Nicole Drury, a certified La Leche League leader in Northampton, Massachusetts.

Be picky about the containers you use to store breast milk. Never store it in disposable bottle liners or plastic bags that aren’t intended for this purpose. Options include:

Place a label on the container indicating the date it was expressed. If your child goes to daycare, include their name on the label as well.

“If you’re freezing your breast milk, it can take up a lot of room if you’re pumping frequently,” says Drury. “Some women like storage bags because you can stack them flat in the freezer, which saves space.”

When filling the container to freeze your milk, leave about an inch of space at the top for the breast milk to expand as it freezes. No matter the container you use, freeze the breast milk in small amounts, such as 2 to 4 ounces.

“Anything your baby doesn’t finish within two hours has to be thrown out,” adds Drury. “You can always add more if your baby is still hungry.” The CDC recommends never refreezing breast milk you’ve thawed but didn’t use.

When you’re ready to use the breast milk that’s been stored in the freezer, follow the steps below. Drury cautions not to defrost or heat breast milk in the microwave. “This can destroy nutrients and may create hotspots that can hurt your baby’s mouth,” she says.

Always thaw the oldest breast milk first. Here are three ways you can defrost breast milk safely, according to the CDC:

You don’t need to warm breast milk. You can give your baby milk that has been kept at room temperature or even cold milk from the refrigerator, and many babies like it this way just fine. However, if you choose to warm the milk, follow these steps:

It’s not always easy deciding whether breast milk has gone bad. “You may notice it has a different smell than what you’re used to,” says Morgan-Vaughn. “This is normal. Most foods change after being frozen.”

“How your milk smells is also influenced by what you ate on the day you pumped,” she adds. A small number of moms notice their milk smells soapy, metallic or rancid. “This [soapy smell] is due to a high level of lipase, an enzyme found in all milk. The milk is safe and most babies will take it,” says Morgan-Vaughn.

Milk that smells metallic or rancid after storing or heating could be caused by chemical oxidation. High lipase and chemical oxidation have different solutions. Both milks are safe to feed to your baby, but if your baby refuses the flavor change, take time to learn how to prevent the flavor change.

Still not sure? “If it’s got freezer burn, or it smells or tastes sour, don’t give it to your baby. When in doubt, toss it out,” says Morgan-Vaughn.

Proper Storage and Preparation of Breast Milk . Centers for Disease Control and Prevention. Accessed 4/21/2021.

Milk Issues . La Leche League International. Accessed 4/21/2021.

Eglash A, Simon L, et al. ABM Clinical Protocol #8: Human Milk Storage Information for Home Use for Full-Term Infants, Revised 2017 . Breastfeeding Medicine . 2017;12,7.

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Kate Kelly is a Pittsburgh-based writer who specializes in health and parenting. She is passionate about empowering women with good health information. She was an editor at Parents, American Baby and Shape magazines. She likes hanging out with her three sons and her cat, Maisy, and she’s thinking about getting a dog.