You’ve just pumped 90 mL at 2 p.m. The bottle is sitting on the counter, the baby is asleep, and you’re trying to remember whether it’s fine there for an hour or whether it needs to go in the fridge right now.

Good news: the rules for storing breast milk are short, and the main ones fit on a sticky note. This guide covers how long milk keeps, where to put it, how to thaw and warm it, and what to do with the half bottle your baby didn’t finish.

The numbers below come from the CDC’s breast milk storage guidelines, with notes where the AAP or the NHS say something different. They’re written for healthy babies at home. If your baby was born early or is in the hospital, ask the care team what they want you to do.

How long breast milk keeps

Here’s the CDC’s chart for milk you’ve just expressed:

Where	Temperature	How long
Counter or table	77°F (25°C) or colder	Up to 4 hours
Refrigerator	40°F (4°C)	Up to 4 days
Freezer	0°F (-18°C) or colder	About 6 months is best, up to 12 months is acceptable

A few other times worth knowing, also from the CDC:

An insulated cooler with frozen ice packs keeps breast milk for up to 24 hours when you’re traveling.
Thawed milk in the fridge should be used within 24 hours, counted from when it was completely thawed, not from when you took it out of the freezer.
Milk that’s thawed or warmed and sitting at room temperature should be used within 2 hours.
Never refreeze milk that has thawed.

Where the AAP and NHS differ

Health organizations don’t all give the same numbers. Here’s how the other two compare.

The AAP’s page on freezing and refrigerating breast milk agrees on 4 hours at room temperature and 4 days in the fridge. For the freezer it says up to 9 months in a freezer at 0°F (-18°C) and up to 12 months in a deep freezer at -4°F (-20°C).
The NHS page on expressing and storing breast milk allows up to 8 days at the back of a fridge at 4°C or lower, or 3 days if you’re not sure how cold your fridge is. It gives up to 6 months in a freezer at -18°C or lower, and 2 weeks in the ice compartment of a fridge.

If you want one rule that sits inside all three, the CDC’s 4 hours, 4 days and 6 months is a cautious place to start. When in doubt, ask your pediatrician, midwife or lactation consultant.

Where to put it

The feeding schedule by age has typical bottle amounts by month, which helps you choose a portion size.

Keep it at the back, not in the door. The CDC says not to store breast milk in the door of the fridge or freezer, and that storing it further inside helps protect it. The NHS and AAP also say the back of the fridge.
Use the right container. The CDC recommends breast milk storage bags or clean, food grade containers. It says never to use disposable bottle liners or plastic bags that aren’t made for breast milk. The NHS also mentions sterilized containers.
Store small amounts. Both the CDC and the AAP suggest 2 to 4 ounces (about 60 to 120 mL) per container, or the amount your baby takes at one feed. Smaller portions mean less milk poured away when a feed ends early.
Leave room at the top. Milk expands as it freezes, so the CDC says to leave about an inch of space.

Label every container

Write the date the milk was expressed on each bag or bottle. The CDC, AAP and NHS all say to label it.

Then use the oldest milk first. The CDC calls this “first in, first out.” It’s easy to forget when the freezer drawer fills up, so some parents keep the newest bags at the back and pull from the front.

If you share feeds with a partner or a caregiver, a clear label also saves a question at 3 a.m. Our guide to sharing night feeds with your partner has more on handoffs that don’t need a text.

How to thaw frozen milk

The CDC lists three ways to thaw breast milk:

In the fridge overnight
In a bowl of warm or lukewarm water
Under lukewarm running water

The NHS says slow thawing in the fridge is best, with warm water if you need it sooner.

Never thaw or heat breast milk in a microwave. The CDC, AAP and NHS all say this. The NHS explains that a microwave can cause hot spots, and the AAP warns of the risk of scalding your baby. The NHS says to skip the stove too.

Once milk is thawed, the clock is shorter: 24 hours in the fridge, 2 hours at room temperature, and no going back in the freezer.

Do you need to warm it?

No. The CDC says breast milk doesn’t need to be warmed and can be served at room temperature or cold. Some babies don’t mind cold milk at all. Others prefer it warm, and you’ll find out quickly which kind you have.

If you do warm it, the CDC suggests setting the sealed container in a bowl of warm water, or holding it under warm (not hot) running water for a few minutes. Test a few drops on your wrist before feeding.

Stored milk often separates, with a creamy layer on top. That’s the fat, and it’s normal. The CDC says to swirl the milk to mix it back in.

What to do with leftovers

Babies rarely finish a bottle on cue. The CDC and the AAP both say to use the leftover milk within 2 hours after the feed, and to throw it away after that.

The NHS is stricter here: once your baby has drunk from a bottle of breast milk, it says to use it within 1 hour.

This is the main reason small portions help. A 60 mL bottle that ends half full wastes a lot less than a 150 mL one.

A quick checklist

Here’s the sticky note version, using the CDC’s numbers:

Wash your hands before pumping or handling milk.
Fresh milk: 4 hours on the counter, 4 days in the fridge, about 6 months in the freezer (12 at most).
Back of the fridge, never the door.
2 to 4 ounces per container, with space at the top for freezing.
Date on every label, oldest milk first.
Thaw in the fridge or warm water, never the microwave.
Thawed milk: 24 hours in the fridge, 2 hours out, never refreeze.
Leftovers after a feed: within 2 hours (1 hour per the NHS), then pour them out.

Keeping track of what you’ve pumped

Labels tell you how old each bag is. A pumping log tells you the rest: when you last pumped, how much came out, and whether your supply has changed over the last week. If you’re also tracking feeds, our list of what’s worth logging in the newborn weeks helps you keep it short. Once your baby moves on to food, our guide on when to start solids covers the signs of readiness and what to note.

Pen and paper work fine. If you’d rather use your phone, Vivi Baby logs pumping sessions with a timer, the amount from each side or a total, and a note, and pumping is free. You can start a pump timer from your wrist, too. The step-by-step Apple Watch feeding guide shows how, and the Apple Watch feeding tracker page lists what the watch can and can’t log. Get Vivi Baby.

Whatever you use, keep dating the bags. That’s the part that matters most.

Sources

Breast Milk Storage and Preparation - Centers for Disease Control and Prevention
Tips for Freezing & Refrigerating Breast Milk - HealthyChildren.org (American Academy of Pediatrics)
Expressing and storing breast milk - NHS

How to Store Breast Milk: Times and Thawing