Women who pump or hand-express milk for their babies know that breast milk is like liquid gold. A lot of time and effort goes into getting that milk for your little one. No one wants to see a drop go to waste.

So, what happens if a bottle of breast milk is forgotten on the counter? How long can breast milk sit out before it’s no longer safe for your baby?

Here’s what you need to know about properly storing, refrigerating, and freezing breast milk, and when it needs to be tossed.

Whether you hand-express breast milk or use a pump, you’ll need to store it afterward. Remember to begin with clean hands and use a clean, capped container made of glass or hard plastic free of BPA.

Some manufacturers make special plastic bags for breast milk collection and storage. You should avoid using household plastic bags or disposable bottle liners because of the risk of contamination.

Your storage method will determine how long expressed breast milk will safely keep. Proper storage is critical so you can preserve both the nutritional content and the anti-infection properties.

The ideal scenario is to refrigerate or otherwise chill breast milk immediately after it’s expressed.

The Centers for Disease Control and Prevention (CDC) shares these guidelines for breast milk storage:

  • Freshly expressed breast milk can sit at room temperature 77°F (25°C) for up to four hours. Ideally, the milk should be in a covered container. Fresh milk can last up to four days in the refrigerator at 40°F (4°C). It can last 6 to 12 months in the freezer at 0°F (-18°C).
  • If the milk has been previously frozen, once thawed, it can sit out at room temperature for 1 to 2 hours. If thawed milk is put in the refrigerator, use within 24 hours. Do not re-freeze previously frozen breast milk.
  • If the baby didn’t finish the bottle, discard the milk after 2 hours.

These guidelines are intended for healthy, full-term babies. You should speak to your doctor if you’re pumping milk and your baby has health complications, is hospitalized, or was born prematurely.

Milk that’s stored for longer periods of time than mentioned above in either the fridge or the freezer will lose greater amounts of vitamin C. Also be aware that a woman’s breast milk is tailored to her baby’s needs. In other words, your breast milk changes as your baby grows.

If breast milk is left out after being used for a feeding, you may wonder whether it can be used for a subsequent feeding. Milk storage guidelines recommend discarding leftover breast milk after two hours because of the potential for bacterial contamination from your baby’s mouth.

And remember, freshly pumped milk that has been left unrefrigerated for longer than four hours should be thrown away, regardless of whether it’s been used in a feeding or not. Previously frozen milk should be used within 24 hours once thawed and refrigerated. If left on the counter, throw out after 2 hours.

Follow these best practices for storing expressed milk:

  • Keep track of stored breast milk with clear labels showing the date that the milk was collected. Use labels and ink that are both waterproof and include your baby’s full name if you’ll be storing expressed milk at your baby’s day care.
  • Store expressed milk at the back of the refrigerator or freezer. That’s where the temperature is most consistently at its coldest. An insulated cooler can be used temporarily if you can’t get expressed milk into the fridge or freezer right away.
  • Store expressed milk in containers or packets in smaller sizes. Not only does breast milk expand during the freezer process, but you’ll also help reduce the amount of breast milk that’s thrown away after a feeding.
  • While you can add freshly expressed milk to breast milk that’s been refrigerated or frozen, make sure it’s from the same day. Cool the fresh milk completely (you can put it in the fridge or in a cooler with ice packs) before you combine it with the milk that’s already chilled or frozen.

Adding warm breast milk can cause frozen milk to thaw. Most experts don’t recommend re-freezing thawed milk. This can further break down milk components and lead to an increased loss of antimicrobial properties.

It’s best to chill, refrigerate, or freeze breast milk immediately after it’s expressed.

If expressed milk is left out unrefrigerated, but it’s in a clean, covered container, it can sit at room temperature for between four and six hours. Milk that has been left out for longer should be thrown away.

If you’re in doubt about how long expressed breast milk has been left out, err on the side of caution and toss it. It can be difficult to throw away expressed breast milk (all that hard work!) but remember: Your baby’s health is the most important thing.