Freezing and Thawing Breast Milk

If you look up storage times, you’ll quickly find three or four different answers. That doesn’t mean someone was careless. The recommendations come from different expert groups, and each assumes slightly different conditions:

• Germany’s National Breastfeeding Committee is the German expert group on breastfeeding. Its 2024 statement applies specifically to milk you store at home for your own healthy child. It’s the basis for this article.

• An international breastfeeding medicine society, a professional society of physicians, has its own protocol for healthy, full-term babies at home. It often gives two values: an optimal one and one that is still acceptable under very clean conditions.

• The CDC and the UK health service publish their own time limits, sometimes a little shorter, sometimes a little longer.

There are solid reasons for the differences. US recommendations assume a refrigerator of about 39 °F (4 °C), while kitchens in Germany usually run warmer. It also matters how cleanly the milk was expressed and how warm the room is where it’s sitting. And honestly: many studies on this topic are older and hard to compare with one another. The experts say that themselves.

That’s why you’ll find the range for each time limit here, and where sources disagree, we recommend the more cautious side. So you don’t need to memorize charts from three countries. If you follow the numbers in this article, you’re on the safe side.

And one more thought, because it can take the pressure off: milk you’ll need within the next four days keeps best in the refrigerator. Freeze everything else right away—frozen milk is still valuable baby food. Freezing isn’t a last resort you need to justify. It’s a tool that gives you freedom.

Storage starts with the very first drop, because cold is a pause button, not a cleaning method. Bacteria that get into milk while you’re pumping survive freezing: after weeks in the freezer, thawed milk contains about as many live bacteria as fresh milk. Most of them are harmless and can even support your baby’s gut flora. Even so, the rule is simple: the fewer germs in the milk to begin with, the less they can multiply later. All the timelines in this article assume you pumped hygienically. That sounds like a lab procedure, but it’s actually quick and simple:

1. 1

   Wash your hands: thoroughly with soap and water, including under your fingernails. Dry them with a paper towel or with a towel you change every day. At home, you don’t need hand disinfectant—it doesn’t make the milk any cleaner. No sink nearby, like at work or in the car? Then an alcohol-based hand sanitizer is the next-best option.

2. 2

   Don’t wash your breasts specially: clear water during your daily shower is enough. Soap dries out the skin and can make sore nipples more likely. You also don’t need to discard the first drops; they aren’t more contaminated than the rest.

3. 3

   Pump directly into the storage container: every transfer is another chance for germs to get in. So if possible, pump straight into the bottle or bag the milk will stay in.

4. 4

   Clean the pump parts after every use: take apart everything that touched milk, clean it promptly with dish soap and hot water, rinse under running water, and let it air-dry or dry with paper towels. Cover the dry parts with a clean cloth.

Instead of washing by hand, you can also clean the parts in the dishwasher at at least 149 °F (65 °C), ideally putting small parts in a closed basket. Boiling does not replace washing: it kills germs, but it does not remove milk residue. For a healthy, full-term baby, thorough cleaning is enough, according to the German breastfeeding experts. If your baby is younger than two months, premature, or sick, disinfect the cleaned parts once a day as well: boil for at least five minutes, use a steam sterilizer, or use a microwave steam bag. The microwave is fine here because it’s for the parts, never for the milk. Experts advise against chemical disinfectants because residue can get into the milk. Also follow the pump manufacturer’s cleaning instructions.

If you don’t have a pump yet: in the US, your health insurance has to cover a breast pump. Ask your plan which one and when you can get it. For on the go, a good manual pump or a rechargeable pump can be practical.

First, the good news: whether you use glass, plastic, or storage bags, you don’t need a special system. What matters is that the container is meant for baby food or breast milk, seals tightly, and can be cleaned thoroughly. Each option has its strengths:

• Glass bottles last a long time and are easy to keep clean. When freezing, they need headspace: leave about 1 in. (2.5 cm) at the top, because milk expands as it freezes and an overfilled bottle can crack.

• Plastic bottles and cups are shatterproof, which makes them practical for the freezer. Use only ones specifically labeled for infant or toddler food. In the EU, bisphenol A has been banned in baby bottles since 2011. Good options include polypropylene (PP), polyethylene (PE), or PET. But rely on labels such as “for baby food” or “for breast milk,” not on the recycling number alone. Older polycarbonate baby bottles—hand-me-downs or thrifted ones from before 2011, for example—are best avoided.

• Breast milk storage bags save a lot of space in the freezer. They should be sturdy, well sealed, and stored so nothing can damage them, for example in a rigid box. Ordinary household freezer bags or ice cube bags aren’t suitable because they aren’t pretreated to be low-germ for this use.

Skip jam jars and other containers that were really made for something else: they often can’t be cleaned thoroughly enough. Sample cups from a hospital, such as those used for urine, are also off-limits, because there’s no proof they’re chemically safe for babies.

So which is better, glass or plastic? Studies have looked at whether fat or immune cells stick to the container walls, and the results are mixed. Overall, glass and polypropylene perform similarly. Only stainless steel showed noticeably fewer live cells in some studies—it isn’t harmful, and you don’t need to throw away any stainless steel bottles you already have. So use whatever works best for you day to day. You can also mix and match: bottles for the refrigerator and cooler bag, bags for your freezer stash.

Thawed milk can’t go back into the freezer, and whatever your baby leaves in the bottle ends up down the drain. So if you freeze large portions, sooner or later you’ll waste precious milk. Small portions are the easiest way to prevent that.

What counts as “one portion”? Amounts vary a lot from baby to baby, but a few guideposts help with planning:

• In the first days of life, the amounts are tiny: on day 1 often just ½ to 2 teaspoons (2 to 10 ml) per feeding, on day 3 about ½ to 1 fl oz (15 to 30 ml), and on day 4 about 1 to 2 fl oz (30 to 60 ml).

• Between 1 and 6 months, exclusively breastfed babies drink a surprisingly steady total of about 25 to 27 fl oz (750 to 800 ml) a day even as they grow. Spread across about six to eight feeds, that works out to roughly 3.4 fl oz (100 ml) per feed. But the range is wide: some babies drink a little over 1.7 fl oz (50 ml), others more than 6.8 fl oz (200 ml) at a time.

• Once solids begin—in Germany, recommended no earlier than the start of the fifth month and no later than the start of the seventh month, depending on development—the amount of milk slowly decreases. But breast milk remains the most important source of nutrition for quite a while.

In practice, that means: freeze portions that roughly match one of your baby’s feeds — for many babies that’s 2 to 4 fl oz (60 to 120 ml) — and also keep a few small portions of ½ to 1 fl oz (15 to 30 ml). Those are worth their weight in gold when your baby isn’t quite full after the first bottle. Then you can thaw a small top-up instead of sacrificing a whole bottle. For a newborn, plan smaller portions accordingly.

A quick example if you’ll be away for a few hours: a 3-month-old baby feeds about every 2 to 4 hours during the day. If you’ll be gone for 8 hours, your baby will likely need about two to four feeds in that time. At around 3.4 fl oz (100 ml) per feed, that’s roughly 7 to 13.5 fl oz (200 to 400 ml), plus a small backup portion. It’s a guide, not a rule. Your baby will show you how much they actually need.

Label every portion right away with something that won’t rub off: the date you pumped it, and if you pump more than once a day, the time too. If the milk is going to daycare or another caregiver, add your child’s name. And because all time limits start when the milk was pumped, the rule for using it up is: oldest first.

Once you’ve done it a few times, the whole routine takes only a few minutes. Here’s how:

1. 1

   Get ready: wash your hands, use a clean pump set, and pump directly into the storage container.

2. 2

   Seal and label: add the date and, if needed, the time and name.

3. 3

   Chill it right away: Put the milk in the fridge as soon as possible. At room temperature (59–77 °F, or 15–25 °C), it will keep for up to 4 hours, but it’s better to cool it right away. If you want to bring the temperature down faster, hold the sealed container briefly under cold running water first.

4. 4

   Decide: will you need the milk within the next 4 days? Then it stays in the fridge. If not, put it straight into the freezer — not on day four when the time limit is nearly up.

5. 5

   If you want to combine milk: you can collect chilled milk from several pumping sessions over 24 hours and then freeze it together. Freshly pumped milk that is still body-warm should never be added straight to cold or frozen milk; cool it in the refrigerator first. The simplest option is to freeze each portion separately.

6. 6

   Leave space: in bottles, leave about 1 in. (2.5 cm) below the rim. With bags, gently press out the air, seal well, and lay the bag flat to freeze. Bags frozen flat stack neatly and thaw faster.

7. 7

   Put it in the right place: toward the back of the freezer, not in the door. In self-defrosting appliances, don’t place it right against the walls, because it can get warmer there from time to time.

8. 8

   Keep order: put new portions in the back and older ones in front. That way you automatically reach for the oldest milk first.

Not every spot in the fridge is equally cold. The coldest place is usually near the back wall in the lower to middle area, close to the cooling element. That’s exactly where the milk should go. The door and top shelves aren’t suitable: they’re warmer, and every opening makes the temperature fluctuate. The German committee recommends 39–43 °F (4 to 6 °C) and considers the milk usable there for up to 4 days.

Some sources say up to eight days. But that applies only to a refrigerator that reliably stays at 39 °F (4 °C) or colder, or to milk expressed under especially clean conditions. If you don’t know how cold your refrigerator is, or the thermometer reads more than 43 °F (6 °C), it’s safer to plan on three days. That’s also what the UK health service advises for refrigerators between 41 and 50 °F (5 and 10 °C). If the thermometer reads more than 50 °F (10 °C), the milk isn’t really refrigerated there, so the room-temperature time limit applies.

For freezing, it depends on what kind of appliance you have:

• The small freezer compartment inside the refrigerator often stays only around 5 °F (−15 °C). Milk keeps there for no more than two weeks. A freezer thermometer shows how cold yours gets: if it only reaches about 21 °F (−6 °C) or 10 °F (−12 °C), it’s better not to store milk there at all. You want 0 °F (−18 °C).

• A separate freezer, a chest freezer, or a freezer compartment at 0 °F (−18 °C) or colder is the right place for your stash. The milk is best used within six months; up to 12 months is considered acceptable. For a stash longer than two weeks, the German experts recommend only a compartment that cold or a separate freezer.

So why not just always aim for 12 months? Because milk can lose quality in the freezer. Studies are mixed: in some, milk after about three months contained a little less fat, protein, and energy than fresh milk; vitamin C dropped in the first months; and some immune factors decreased. In another study, the main nutrients were unchanged even after nine months. For you, the simple rule is this: the shorter the storage time, the more is preserved. Six months is a good goal, and 12 months is the absolute outer limit.

If you pump at work, store the milk upright in the staff fridge in your own closed box, separate from other foods, labeled with your name and the date. Breast milk doesn’t need special precautions like other body fluids, so a shared fridge is completely fine.

The gentlest way to thaw breast milk is in the fridge. It takes about 12 hours, so a small evening routine helps: take the next day’s portion out of the freezer and put it in the fridge. Slow thawing also causes less fat loss than thawing in warm water.

If you need it faster, you have two options:

• Under running water: hold the sealed container first under cold water, then under lukewarm water — no warmer than body temperature, about 99 °F (37 °C).

• In a water bath: place the sealed container in a bowl of lukewarm water and top it up with more lukewarm water as it cools.

Milk thawed this quickly should be used within 2 hours. For bags, it’s best to place them in a bowl while thawing in case they leak, and dry the outside before opening. The German committee also considers thawing at room temperature possible; that takes 1 to 4 hours. If you do it this way, check on it now and then and move the milk to the fridge as soon as it’s liquid. The CDC doesn’t include that option. If you want to keep it simple, stick with the fridge.

Never use the microwave. It heats unevenly, creating hot spots that can burn your baby’s mouth, even if the outside of the bottle feels only lukewarm. It also significantly lowers important immune factors such as immunoglobulin A and lysozyme. The stovetop and very hot water are just as off-limits: even in a water bath at 176 °F (80 °C), overheated spots form in the milk.

When does the clock start? From the moment the milk is fully thawed, not from when you take it out of the freezer. Use it soon, and no later than 24 hours — some sources allow longer, but we’re sticking with the cautious side. If thawed milk has been at room temperature or you’ve warmed it, it should be drunk within 1–2 hours and should not go back in the fridge.

Thawed milk should not be refrozen. German breastfeeding experts, the CDC, and the NHS all agree on this, because germs can multiply as the milk starts to thaw, and refreezing does not undo that. If the milk is partly thawed, keep it in the refrigerator and use it within 24 hours. Small portions make this rule much easier.

Breast milk doesn’t have to be warm. Many babies will drink it at room temperature or even cool from the fridge, though some clearly have a preference. It’s worth trying, especially if you need to move quickly or someone else is feeding. Every warming step takes time and, if it gets too hot, also costs valuable milk components.

If you warm it, do it gently — and never in the microwave or on the stove:

1. 1

   Place the sealed bottle in a container of lukewarm water, no warmer than about 99 °F (37 °C), or use a bottle warmer. Expect it to take 15 to 20 minutes.

2. 2

   Gently swirl the bottle back and forth. It’s normal for a creamy layer of fat to settle on top. Swirling redistributes it. The UK health service also says light shaking is fine.

3. 3

   Drip a little milk onto the inside of your wrist. It should feel pleasantly warm, never hot.

4. 4

   Clean the bottle warmer every day, because germs can multiply in warm water.

Why not warmer? Above about 104 °F (40 °C), immune factors and other sensitive components of breast milk are lost. And baby skin burns faster than adult skin: liquids at about 122 °F (50 °C) or higher can cause scalds very quickly. Especially at night, when you’re tired, “a little cooler is better” is the safer rule. And only warm milk when your baby is truly awake and hungry: warmed milk doesn’t go back in the refrigerator and should not be warmed a second time.

As soon as your baby drinks from the bottle, germs from their mouth get into the milk through the nipple. For bacteria, that’s an ideal breeding ground. That’s why leftovers have their own short time limit: ideally, throw out what’s left one hour after the first sip. The absolute limit is two hours, counted from warming: by then, warmed milk is done, even if your baby started drinking late.

That can hurt when you know how long you spent pumping that milk. A few tricks help keep leftovers smaller:

• Offer a slightly smaller amount first, and thaw one of the small backup portions if needed.

• Don’t put a started bottle back in the fridge to offer again at the next feed, and never pour leftovers back into untouched milk.

• If your baby regularly leaves a large amount, freeze the next portions a little smaller.

• Write down how much your baby actually drinks for a few days. Then you’ll see which portion size fits.

It’s easier if you log every bottle in Moonsleep: the stats show you how many bottles your baby drinks in a day and how much milk that adds up to, and whoever is feeding can see the same information you do.

You thaw a portion, take a sniff, and the milk smells soapy, metallic, or even slightly rancid. Fresh milk smelled completely different, right? No need to panic. Changes like this can happen with stored breast milk and are usually harmless.

An enzyme called lipase is behind it. It’s a natural part of breast milk and breaks down milk fat into fatty acids, even in the fridge and freezer. The odor probably develops when those fatty acids react with oxygen. After longer freezer storage, from around 3 months on, the milk can also taste slightly soapy. The surprising part: this breakdown works against germs and slows their growth in thawed milk.

Milk stored properly with this smell is safe, and most babies drink it without hesitation. There’s no good evidence that babies commonly refuse it for that reason. And if your baby does turn up their nose? Try this:

• Use it earlier: the soapy taste shows up mainly after longer freezer storage. Freeze only as much as you’re likely to need, and use the milk within weeks rather than months if you can.

• Do a test portion: freeze a small amount, thaw it after a week, and smell it. That way you’ll know how quickly your milk changes before you build a large stash.

• Vary the temperature: some babies prefer milk cool rather than warm, or the other way around.

• Don’t boil it: online you may see the tip to heat fresh milk strongly before freezing to stop lipase. The international breastfeeding medicine society advises against this, because heat above 104 °F (40 °C) can destroy many immune factors.

It’s different if the milk has truly spoiled. If it smells sour or rotten, or looks stringy or pus-like, don’t give it to your baby. The same goes if the time limit has passed, no matter how the milk smells. And if you’re unsure, it’s better to throw it out once than worry about it for hours.

For trips to work, daycare, or the grandparents, you’ll need a well-insulated cooler bag with ice packs. For longer journeys, an insulated foam cooler with cooling packs keeps things cold even better. Pack it like this:

1. 1

   Cool the milk in the fridge first. Freshly pumped, warm milk shouldn’t go straight into the bag.

2. 2

   Stand the bottles upright inside and place the ice packs between them. Wrap glass bottles individually, for example in a kitchen towel.

3. 3

   Open the bag as rarely as possible while you’re out.

4. 4

   As soon as you arrive, put the milk straight into the fridge or freezer.

If the cold chain stays unbroken, milk will keep in a cooler bag for up to 24 hours. That time counts toward the total: if the milk spent a full day in the cooler bag, it has at most three days left in the refrigerator. German breastfeeding experts, the CDC, and the NHS all agree on that. The condition is that the cooler stays at 59 °F (15 °C) or colder; ideally, 39–43 °F (4–6 °C).

Frozen milk should not thaw while you’re traveling. Pack it tightly with ice packs. If the milk arrives partly thawed anyway, treat it like thawed milk: put it in the refrigerator and use it within 24 hours.

If your child gets expressed milk at daycare or with another caregiver, bring it chilled in a cooler bag and label each portion with your child’s name and the date. Ask ahead how they store and warm the milk. Breastfeeding experts have published separate guidance on handling breast milk in childcare settings. You can point the staff to that if they’re unsure.

When you go back to work: in the US, the PUMP Act gives you the right to breaks and a private space that isn’t a bathroom to pump, for up to 1 year after birth. It’s worth talking with your employer before you return about what this could look like day to day, where you can pump in private, and how you can keep your milk cold.

If you’re flying with pumped milk, check with the airline and the airport ahead of time about how much is allowed in carry-on baggage and how screening works.

These are the stumbling blocks almost everyone hits when storing milk for the first time. None of them is a disaster, but each can cost you milk or peace of mind:

• Freezing it filled to the top: milk expands, glass can crack, and bags can burst. Leave about 1 in. (2.5 cm) of space.

• Storing it in the fridge door: that’s the warmest spot, and every opening causes temperature swings. The right place is low at the back.

• Pouring warm milk onto frozen milk: the top layer starts to thaw. Cool freshly pumped milk in the fridge first, then add it.

• Freezing without a date: after 3 weeks, no one knows which portion should be used first. Label it right away with something that won’t rub off.

• Making portions too large: whatever isn’t drunk has to go. Feed-sized portions plus a few small top-up portions save milk.

• Building your stash in the fridge’s freezer box: milk keeps there for only 2 weeks. For anything longer, you need 0 °F (−18 °C).

• Thawing it overnight on the counter: then the milk sits warm for far too long. Overnight thawing belongs in the fridge.

• “Just briefly” using the microwave: even short heating is uneven and can create hot spots. Use a lukewarm water bath instead.

• Refreezing thawed milk: please never do this. Thaw smaller portions instead.

• Saving the started bottle for later: leftovers get thrown out after 1 hour, and no later than 2 hours.

• Mistaking a soapy smell for spoilage: most often lipase is behind it, and the milk is fine as long as the time limits still fit.

• Using household containers: jam jars, standard freezer bags, and ice cube bags aren’t meant for breast milk.

• Only rinsing pump parts quickly: milk residue is a feast for germs. Take everything apart, wash it with hot water and dish soap, and let it dry—in a separate bowl, not in the sink.

Everything in this article applies to healthy babies at home and to your own milk. If your baby was born early, is in the hospital, or is ill, the unit’s rules apply, even if they differ from these recommendations. There are separate professional guidelines for preterm and sick babies.

It’s best to ask right away: Which containers should you use? How should they be labeled? How long can the milk stay in your refrigerator at home before you bring it in, and how should you transport it? Hospital rules are not about mistrusting you—they protect the tiniest babies. Another woman’s milk should only go to a baby through a donor milk bank. Don’t buy or sell breast milk online or share it privately with other families: untested milk can contain infectious germs, medications, or other substances, and breastfeeding experts explicitly warn against it.