Can You Mix Fresh and Previously Pumped Breast Milk?

Can you add fresh breast milk to the milk that you pumped earlier? Learn about the potential safety concerns.

If you pump or hand express your breast milk at different times, you wonder if you can mix breast milk from different days into one storage container. Most of the time, combining breast milk is acceptable unless you have a preterm baby or you're in an unclean environment.

That said, you need to follow some specific guidelines to ensure the safety of your milk collection and avoid contamination. Here's what you need to know about mixing breast milk.

Safely combining breast milk from different days

JR Bee

Can You Mix Breast Milk From Different Days?

When you're collecting breast milk, your baby's safety is the top priority. But you can rest assured that combining breast milk from different days is acceptable in most cases—as long as you take precautions to ensure your environment, hands, breast pump, and collection containers are clean.

In fact, one study found that combining breast milk doesn't increase bacterial contamination like people originally thought. Researchers noted, however, that the best way to ensure that your breast milk is free of contamination is to be diligent about your hygiene and ensure your pump and collection supplies are sterile.

Another study supports these findings. Interestingly, researchers noted less variation in the nutrient and caloric content of mixed milk. There also was less bacterial content in breast milk that had been pooled than individual samples.

When You Shouldn't Combine Breast Milk From Different Days

Even though the practice is generally safe, there are certain circumstances in which you shouldn't combine new breast milk with your existing supply. In the situations outlined below, you should store the milk separately or dump it altogether.

  • Possible contamination: If you're not in a clean environment, or you're unable to wash your hands before collecting your breast milk, you shouldn't mix it with breast milk that you previously pumped. If you believe your breast milk can be contaminated, it's best to pump and dump that session rather than compromise your baby.
  • Premature baby: If your baby is premature or has a compromised immune system, you'll likely be given instructions on how to safely collect and store your breast milk. When you collect breast milk for a sick or premature infant, you should seal and store it immediately. Once sealed, don't reopen the container until it's ready to be used. Keeping the bottle sealed prevents contamination.
  • Sharing breast milk: When you're bringing your breast milk to the hospital for a sick or premature baby, or you're collecting breast milk to donate to a human milk bank, follow their guidelines for milk collection and storage.

Keep in mind, every breastfeeding parent's situation is different. Talk to your health care provider or a lactation consultant to determine if combining breast milk from different days is right for you and your baby.

How to Mix Breast Milk From Different Days

Many people opt to mix or pool their breast milk because it cuts down on plastic use, reduces the storage space needed, and provides more ounces that can be used at a time. It can even create breast milk with more consistent macronutrients.

That said, you need to be careful about how you combine freshly pumped milk with milk that was previously pumped. You also need to be sure your health care provider believes it's the right option for you and your baby. Here's how best to combine breast milk.

Adding fresh breast milk to room temperature breast milk

You can add freshly pumped breast milk directly into a bottle of room-temperature breast milk as long as the previously collected milk has not expired. Fresh breast milk can stay at room temperature for up to four hours.

When you add breast milk from a different pumping session into a bottle of room-temperature breast milk, you should base the timing on the oldest pumped milk. So, if you add new breast milk to milk that you pumped three hours ago and left out at room temperature, the entire container, including the fresh breast milk, is now three hours old. It will need to be put in the refrigerator or frozen before the four-hour mark.

Adding fresh breast milk to refrigerated breast milk

When adding freshly pumped breast milk to refrigerated breast milk, you need to cool the freshly pumped breast milk first. Do not combine your body temperature milk with cold milk. It will cause the cool milk to warm up.

Instead, place the fresh breast milk in the refrigerator for 30 minutes to one hour. Then, once it is cool, you can add it to the other container of refrigerated milk. Just keep in mind, if you collected that milk on a different day, all the milk should be labeled with the oldest date of collection.

Adding freshly expressed breast milk to defrosted breast milk

If you have defrosted breast milk and fresh breast milk at about the same temperature, you can put them together in the same bottle to feed to your baby. However, freshly expressed breast milk is more beneficial than previously frozen breast milk, so it's better for your baby to get every drop of your fresh breast milk.

It might not be as convenient, but it's better to give the fresh breast milk on its own first. Then, finish the feeding with the defrosted milk. This way, fresh breast milk will not be wasted. If there's any leftover at the end of the feeding, it will be the defrosted breast milk that gets thrown away.

Safety Tip

You should not add your freshly expressed breast milk to a bottle of defrosted breast milk if you plan to store it long-term. Once you thaw the frozen breast milk, you cannot freeze it again. You must use it within 24 hours or throw it away. If you'd like to store your fresh breast milk, you should keep it separate from any milk that you've already thawed.

Adding fresh breast milk to frozen breast milk

You should not add warm, fresh breast milk to already frozen breast milk. Warm milk can cause frozen milk to thaw out. However, if you cool the fresh milk first, you can add it to the frozen milk.

Place your fresh breast milk in the refrigerator until it's cold. Then, add the cold, refrigerated milk to the bottle of already frozen milk. This process is called layering.

Key Takeaways


Combining breast milk from different expressions can be convenient, especially if you're only getting a small amount at each session. Just make sure you're not adding too much to your storage containers. If your baby doesn't finish the expressed milk, it can't be refrozen or stored, and you'll need to throw it out after two hours. Most experts recommend offering your baby smaller amounts at a time so there's less waste.

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Sources
Parents uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Comparison of Bacterial Counts in Expressed Breast Milk Following Standard or Strict Infection Control Regimens in Neonatal Intensive Care Units: Compliance of Mothers Does Matter. J Hosp Infect. 2016.

  2. Pooling Expressed Breastmilk to Provide a Consistent Feeding Composition for Premature Infants. Breastfeed Med. 2013.

  3.  Human Milk for the Premature InfantPediatr Clin North Am. 2013.

  4. Breastfeeding: Proper Storage and Preparation of Breast Milk. Centers for Disease Control and Prevention. 2022.

  5. Breastfeeding: Frequently Asked Questions. Centers for Disease Control and Prevention. 2021.

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