Breast milk storage: how long it keeps and how to freeze and thaw it safely

Breastfeeding · Updated July 2026 · All articles

Whether you are expressing for a baby in NICU, building a freezer stash before returning to work, or just want to know how long that bottle in the fridge is still safe to use, getting breast milk storage right matters. The good news is the guidelines are straightforward once you know them, and a few practical habits make safe storage easy to maintain every day.

How long expressed breast milk keeps

The storage times below are aligned with NHS and UNICEF Baby Friendly guidance. They assume milk is stored in a clean container and that your hands, pump parts, and storage vessels were all clean when the milk was expressed.

At room temperature (up to 25°C): up to four hours. In a warmer room, use the milk sooner or refrigerate it promptly. If in doubt, refrigerate rather than leave it out.

In the fridge (at 4°C or below): up to five days. Store milk at the back of the fridge where the temperature is most stable, not in the door where it fluctuates every time you open and close it. A fridge thermometer is worth having if you plan to store milk regularly.

In a fridge-freezer compartment (combined unit, separate door): up to six months. In a dedicated chest or upright freezer: up to twelve months, though most guidelines recommend using it within six months for the best quality of fats and immune factors. Freeze in small portions (60 to 120 ml) so you defrost only what you need at each feed and waste as little as possible.

Previously frozen milk that has been thawed: use within 24 hours if thawed in the fridge, or within two hours if thawed at room temperature. Do not refreeze thawed milk.

Choosing the right containers

Food-grade BPA-free plastic bottles or bags designed specifically for breast milk, and clean glass containers with tight lids, are all suitable. Avoid using standard zip-seal sandwich bags, which can split in the freezer and are not sterile enough. Breast milk storage bags are convenient and space-efficient in the freezer, but they are single-use and can be punctured if not handled carefully.

Glass containers are an excellent choice if you plan to warm milk directly in the container, as they do not leach chemicals. Hard-sided plastic containers stack better in the freezer and are less prone to damage. Both work well; the choice usually comes down to personal preference and what fits best in your freezer.

Sterilise containers before first use by boiling, steaming, or using a cold water sterilising solution. After sterilising, they do not need to be re-sterilised between uses as long as they are washed in hot soapy water and dried thoroughly.

Labelling and organising your freezer stash

Label every container with the date and time of expression before you put it in the fridge or freezer. A permanent marker on a piece of masking tape works well on bags; a label on the side of a hard container keeps things organised. When you use milk from the freezer, always reach for the oldest date first to make sure nothing is wasted.

Organising your freezer stash by date in a simple first-in, first-out order makes this automatic. Some parents use a small basket or tray in the freezer with newer milk loaded at the back and older milk at the front. It takes a little setup but saves a lot of time rummaging.

Layering: adding fresh milk to stored milk

You can add freshly expressed milk to a container that is already in the fridge or freezer, but there are two rules. First, the fresh milk must be fully chilled in the fridge for at least an hour before you add it to a frozen container (adding warm milk to frozen milk can partially thaw the outer layer of the frozen portion, creating a temperature risk). Second, the amount of fresh milk you add should be smaller than the amount already frozen, to avoid significant temperature change to the container.

If you are adding fresh milk to a bottle of refrigerated milk, the date on the container should be the date of the oldest milk. Once you add fresh milk, use the whole container before you would need to discard the older milk.

Thawing frozen breast milk safely

The safest way to thaw frozen breast milk is to move it from the freezer to the fridge the night before you need it, letting it thaw slowly overnight. This preserves the most immune factors and produces the most even thaw. Milk thawed this way should be used within 24 hours.

If you need milk more quickly, place the sealed container in a bowl of warm water or hold it under warm running water for a few minutes. Never use boiling water directly, as extreme heat can damage some of the milk's components. Frozen milk can go from frozen to feeding temperature in around 15 to 20 minutes using this method.

Never heat breast milk in a microwave. Microwaves heat unevenly, creating hot spots that can burn your baby's mouth. They also destroy some of the antibodies and living cells that make breast milk valuable. A bottle warmer or a bowl of warm water are the right tools for the job.

Once you warm milk, any unused portion should be discarded after two hours. Do not put warmed milk back in the fridge or freezer for later use.

The lipase issue: soapy-smelling milk

Some people find that their stored breast milk develops a soapy, metallic, or even rancid smell after being refrigerated or frozen. This is almost always caused by an enzyme called lipase, which starts to break down fats in the milk shortly after expression. The milk is safe and nutritious, but some babies refuse it because the taste has changed.

If you notice this, try scalding your expressed milk before storing it: heat it in a pan on the hob until small bubbles appear at the edges (around 82°C), then cool it quickly and store as usual. Scalding deactivates the lipase and prevents the smell from developing. It does reduce a small amount of immune factors in the milk compared to unscalded milk, but the milk remains highly nutritious. You only need to scald if your baby is rejecting the stored milk because of the smell; not every parent experiences this.

Fresh versus frozen: which to use first

Fresh expressed milk contains higher levels of live cells, antibodies, and some vitamins than frozen milk. When you have a choice, fresh or refrigerated milk is preferable to frozen for everyday use. Save your frozen stash for occasions when fresh milk is not available, such as when you are away, when you are ill, or when your baby is at nursery and you are building up a stock.

If you are returning to work and have a freezer stash, using refrigerated milk from today's expressing sessions for that day's nursery feeds, while keeping the freezer stash as a backup, maximises the quality of what your baby receives.

Frequently asked questions

How long can expressed breast milk sit at room temperature?

According to NHS guidance, freshly expressed breast milk can be kept at room temperature (up to 25°C) for up to four hours. If your room is warmer than 25°C, use it or refrigerate it sooner. In practice, if you are not going to use it within two hours, it is safer to put it in the fridge straight away.

How long does breast milk keep in the fridge?

Freshly expressed breast milk keeps in the fridge for up to five days at 4°C or lower. Store it at the back of the fridge, not in the door, where the temperature is most consistent. Previously frozen milk that has been thawed in the fridge should be used within 24 hours and must not be refrozen.

How long can I freeze breast milk?

Breast milk can be stored in a freezer for up to six months in a standard fridge-freezer (with a separate door), or up to twelve months in a dedicated deep freezer. For best quality, aim to use frozen milk within six months. Always label containers with the date expressed so you can use older milk first.

Why does my thawed breast milk smell soapy?

A soapy or metallic smell in thawed milk is usually caused by lipase, an enzyme in breast milk that starts to break down fats after expression. This is safe and nutritious but some babies refuse it due to the changed taste. To prevent it, scald the milk immediately after expressing (heat it to about 82°C, just until you see small bubbles at the edge) before cooling and storing. Scalded milk loses a small amount of immune factors but remains nourishing.

Can I add fresh milk to frozen milk?

You can add freshly expressed, cooled milk to already-frozen milk, but only if the fresh milk has been chilled in the fridge first and the amount you are adding is smaller than the frozen amount. Adding warm milk to frozen milk could partially thaw the outer layer, creating a temperature risk. Always chill fresh milk thoroughly before adding it to a frozen container.

Can I reheat breast milk in the microwave?

No. Microwaving breast milk creates hot spots that can burn your baby's mouth, and it also destroys some of the antibodies and immune factors in the milk. Thaw and warm breast milk by placing the container in warm water or using a bottle warmer. Shake or swirl gently before offering it and always test the temperature on your wrist before feeding.

This article is for general information only. Always consult your midwife, health visitor, GP, or a registered lactation consultant for advice about your specific situation.