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The CDC has extremely strict rules for breast milk. It can be left for four hours at room temperature, four days in the fridge, or up to 12 months in the freezer. You should never refreeze it once defrosted. You should never mix multiple temperatures of breast milk together (i.e. do not add warm milk to cooled milk). And if you start a bottle, you must use the rest within one to two hours. If you don’t, you should throw it away.
It’s possible that in an ideal world, with unlimited milk supplies and no logistical problems, these regulations might be easily followed. But in the real world — where milk spills, freezers are left open, and babies don’t finish their bottles — there are a lot more gray areas.
What’s the actual data behind storage times, mixing, and reusing a partially used bottle? What if I (heaven forbid) put the bottle back in the fridge and fed my kid again with it four hours later? What would actually happen?
Here’s the data on breast milk storage: when you can be flexible with it, and when you should err on the side of caution.
The underlying concern: bacteria
Underlying all these CDC rules is concern about bacteria. Breast milk, like all substances, has bacteria in it. These can come from the breast, from the air, from handling, etc. If the bacteria is pathogenic (say, something like staphylococci) and if it multiplies to high enough levels, it could make your baby sick. It is important to note that levels matter here. There is bacteria everywhere, in everything, even some bad bacteria. At low levels, that’s okay. At high levels, however, you have to be careful; and there are guidelines about how high these bacterial levels can be before they cause a problem.
Generally, freshly pumped breast milk has low levels of bacteria — far lower than any level that could cause an issue. But when breast milk is stored, bacteria have a chance to multiply. This growth is even faster at higher temperatures. It is widely understood and accepted that if you left breast milk out on the counter in the summer for four days, it would be spoiled. It would smell bad and taste bad, and possibly be dangerous. It’s also widely accepted that leaving it out for an hour is fine. The question, on all of these exposures, is where the line is. So, what does the data say? Are these CDC guidelines necessary to keep bacterial growth low enough to be safe?
How long can breast milk be left out or stored in the fridge?
Since we can study breast milk directly, this question is answerable. The answer is necessarily going to depend on the temperature of the room. Bacteria grow more quickly when it’s hotter.
Let’s look at this study, which took samples of milk from 16 women and studied their bacterial growth at varying temperatures and time frames, up to 24 hours. In this data, the researchers showed that at a temperature of 59℉, there was minimal bacterial growth by 24 hours. At 77°, though, the minimal growth was maintained for only four to eight hours. At 100°, even four hours saw considerable growth.
This suggests that even in a hot room, a period of storage in the range of four to eight hours would be acceptable. Similar data from Nigeria showed that storage for nine hours in tropical conditions (i.e. left out) did not generate bacterial loads that exceeded acceptable levels.
At refrigerator temperatures, storage is considerably longer. This review article points to no appreciable bacterial growth at four days with storage at typical fridge temperatures. In the freezer, measurements at six weeks and longer do not show bacterial growth. Generally, the concerns with long-term freezing are due to a slow decline of vitamins, as well as the fact that the breast milk’s taste is eventually affected.
What we have from this literature is a sense that over some reasonable period (several hours left out; at least several days in the fridge), breast milk bacterial levels are stable. What we do not have is much of a sense of the “reasonable period”’s limits. In the studies of the refrigerated milk, we see little bacterial growth at four days. Does this mean eight days is fine? Twelve? Longer-term analysis of the question of Where is the limit? is missing.
Can I refreeze breast milk if it thaws?
This question is especially important, in my view, because sometimes breast milk thaws by accident. I’ve heard from many people asking what they should do if someone in the house left the freezer door open and the milk defrosted overnight. And if you lose power after hurricanes and other major weather events, what happens to the milk in the freezer?
This is a question that can be studied using donor milk in carefully controlled conditions, as in this 2006 paper. The authors look at both bacterial load (a measure of pathogens) and vitamin and fatty acid content in milk treated in various ways. In addition to their analysis of storage without freezing, they also consider what happens if you take milk that’s frozen, thaw it, and refreeze it.
The frozen-thawed-frozen milk does not show an appreciable bacterial load — it is an order of magnitude lower than the FDA limits. A 2016 review article draws broadly similar conclusions.
There is some human judgment here — if the freezer is left open while you’re on a two-week-long trip, you do not want to refreeze the milk. But if it’s open overnight and everything thaws a bit, there is nothing in the data that would suggest you need to throw it away. Just close the freezer and try to move on.
Can I mix multiple temperatures of breast milk?
If you are like me, you are not a prolific breast milk producer. I used four-ounce bottles when I pumped, but I was never actually producing that much at once. Sometimes there would be half-filled bottles. Was it okay to combine the new two ounces, still warm, with the older milk?
The guidelines say no. Many people wonder why. The simple reason is that the new milk will warm up the cold milk. When breast milk is warmer, it’s more susceptible to bacteria growth. However: we established above that generally breast milk is quite stable, and storage, even at room temperature, is safe for a fairly long time. Adding breast milk to a bottle — that is, slightly warming it for a brief period, while also leaving it in the refrigerator — is a very small change.
Put differently: If you think of yourself as having six to eight hours of room-temperature storage before you get concerned, the briefly warmer period while the milk comes to the same temperature can be counted against this amount of time. All in all, this risk is theoretical at best. You can mix without worrying most of the time.
Can I reuse breast milk after already having used some?
This is in many ways the most interesting question. When your baby drinks from a bottle, bacteria from their mouth get into the milk. For this reason, the baseline bacterial load will be higher right after they’ve drunk the milk compared with right before. The larger bacterial load will mean, of course, more possibilities for bacterial multiplication. This is why the CDC recommends throwing the milk away.
On the other hand, this is also the most interesting question from the standpoint of your life. It’s really, really frustrating to throw away pumped milk.
This question is, in principle, answerable. What you’d need to do is have some babies drink breast milk and measure the bacterial content before drinking and then after drinking and storing for various lengths of time.
When I went looking for evidence like this, the first thing I found was evidence on formula. There are at least two academic papers (here and here) that had adults drink baby formula and then measured the bacterial content of the formula after drinking and then after storage. In these cases, even after up to 24 hours of storage in the fridge, the samples did not produce high bacterial levels.
This is encouraging, but it’s about formula and not breast milk. However, we have other evidence that breast milk is much less prone to bacterial growth than formula. Putting these facts together, we would rationally conclude that the safe storage time of partially drunk breast milk might be higher than partially drunk formula.
None of the published work, though, actually does this direct analysis we want on breast milk. I did, however, find one absolutely amazing article — an undergraduate senior thesis from 1998 — in which the author did precisely what I wanted. She recruited six women with babies and had them feed their babies from bottles of pumped milk. She tested the milk before feeding and after feeding, and then after storing it in the fridge for up to 48 hours. She looked for bacterial colony counts in various ways.
This study is really excellent, I think, because it attempts to model regular breastfeeding practices. The author basically tries to get the women to express and store milk in a normal way. The milk is stored in their regular refrigerators. She uses bottles cleaned in the dishwasher “because this is the method many women use to clean their bottles.”
Here is what she finds.
The bacterial colony counts are largely unaffected by the feeding, and they barely move even in 48 hours of storage in the fridge. This is evidence that milk could be stored in this way for a fairly long time without bacterial growth.
This is a single unpublished study of six women performed by an undergraduate. I’m not suggesting the CDC change its policy. But on top of the logic, I would personally be comfortable storing partially used breast milk in the fridge for a reasonable period (and I did).
Closing thoughts
It’s important to note that there are some situations where we must be extremely careful about pathogens in milk. This includes when a baby is preterm, when the NICU is involved, and when we are discussing donor milk.
But for healthy babies, the guidelines issued by the CDC seem to be overly cautious in terms of the actual evidence on bacterial growth in breast milk. Breast milk appears, based on what we know, to be quite robust.
That “what we know,” though, is too limited, especially if we take the last question about re-feeding milk. The study that the undergraduate did decades ago was basically sound. It would not be difficult to repeat it with, say, 100 women — or even 30! — and in a professional lab. These questions are, in many ways, easy to answer. You do not need a randomized trial; you literally just need basic lab equipment and some breastfeeding women.
You might ask: Why does it matter? Why not just package your milk in tiny one-ounce bags and dole it out as needed? The answer: it’s annoying. And infeasible in many care settings. A lot of people send their children to child care with packaged breast milk, only to find that much of it was thrown away after being partially used. This can be very wasteful. Each individual instance of it is a small cost. But all together, with millions of women breastfeeding for thousands of hours every year, it simply adds up. We can do better on evidence, and we should.
The bottom line
- CDC rules regarding breast milk storage are concerned with bacteria growth. So “safe storage” really means keeping bacterial growth low enough to be “safe.”
- Freshly pumped milk has low levels of bacteria. The bacteria multiplies when stored — and faster when it’s hotter. Even so, over some reasonable period (several hours left out; at least several days in the fridge), breast milk bacterial levels are stable.
- Refreezing milk does not show an appreciable bacterial load. If you accidentally leave the freezer door open overnight, the data doesn’t mandate that you throw it away.
- The actual practice of mixing milk — say, adding new breast milk to a half-filled bottle and then putting it back into the fridge — does not threaten dangerous bacterial growth.
- With already used milk, the data shows that bacterial colony counts are largely unaffected by feeding. It would be reasonable to refrigerate partially used breast milk and reuse it after a reasonable amount of time.
Log in
One way to freeze just an ounce at a time without it being annoying is to buy a silicon freezer tray where each slot is an ounce or half ounce. Haakka & Souper Trays are great. You can then pop out the frozen cubes and still store them in small breastmilk bags or a larger ziplock one. The cubes are also are easier to put in bottles instead of waiting for the bag to thaw and hope you don’t spill pouring it into the bottles.
My first baby I did the one ounce square cubes but they were a bit harder to put into the small milk bags or narrow bottles. Second baby I’m doing half ounce round cubes and they’re def more versatile.
Great information! Does surrogate-provided milk fall into the same category as donor? Is the differentiation actually between breastfed breastmilk and pumped? And how is donor different? Thank you!
Thank you so much for this. Hopefully one day we can have the data to finally put people’s minds at ease but this has been my general approach for all my kids, whether it was formula or breast milk ( with both i would put unfinished milk in the fridge and use at the next feeding). My sense is that CDC and FDA guidelines are usually an order of magnetic lower/higher than thresholds where there’s actual evidence of adverse effects but we need more studies here! I remember years ago hearing my aunt express frustration because my cousin would toss formula that had been out for 61 minutes!