Combining Breast Milk from Different Pumping Sessions

breast milk in storage bottles on counter

In an ideal world….

…every pumping session would yield a predictable amount of milk 

…you’d always be able to pump the exact amount of milk that your baby is taking in their bottles

…it would be possible to pump anytime and anywhere, even while you are resting or asleep

…both breasts would always yield the same amount of pumped milk

The reality is that we do not pump in an ideal world, and oftentimes find ourselves pumping under less than ideal circumstances! The amount pumped can vary greatly from breast to breast too, and it’s not unusual for women to have one breast produce quite a bit more than their  “slacker” breast.

Although most moms pump just about the amount of liquid gold their babies take per feed during each pumping session, the amount of milk you pump can vary depending on the time of day, the location or environment you are pumping in, accessibility, and/or if you are feeling sick. Infant growth spurts and having to take certain medications while breastfeeding can also create a “mismatch” between your milk supply and demand of what your baby needs. If you opt to use a Haakaa breast pump, or similar breastmilk collection device, to passively collect milk as you nurse, you may collect small amounts of milk at a time which are not large enough for a full feed. For all of these reasons, it’s not unusual to sometimes have to combine breast milk from different pumping sessions for your babies’ supplemental bottles.

Before we dive into common questions and answers about how to combine breast milk, it’s important to review Q&As from our previous blog post about breast milk storage guidelines including which breast milk storage bags to use. 

How long does breast milk last in the freezer?

You can store breast milk in batches in the freezer for up to 6 months. The storage time can vary especially based on where and how you store it. It will last up to 12 months in the back of a deep freezer, but using frozen milk within 6 months is best. Thawed breast milk should be discarded after use. Do not place it in the fridge or refreeze it.

Breast Milk Storage Q&A

How long does fresh breast milk last?

Freshly pumped breast milk can be stored at room temperature for 4 to 6 hours. It’s best to use it within four hours. It should always be covered in a safe storage container and kept as cool as possible, between 66 to 78F. Do not freeze fresh milk after it’s been left at room temperature for more than 2 hours.

How long does breast milk last in a cooler?

After expressing milk, you can store it in a cooler or insulated cooler bag with ice packs for about 24 hours. How cold your cooler is will determine if it will be safe to freeze your milk. Freeze it within 24 hours of chilling it, but only if it passes the sniff test.

How long does breast milk last in the refrigerator?

You can store your milk in the fridge for up to five days at 32° to 39°F, but no more than 72 hours is ideal. Be sure to place your milk in the back of the refrigerator, away from the door or items that could knock it over. As long as breast milk still smells fine, it can be frozen after spending up to 48 hours in the fridge.

FAQs about Combining Pumped Breast Milk

Can I combine milk that I pumped on different days that has been stored in the fridge?

As a rule of thumb, all milk should be clearly labeled and kept in the fridge no longer than 3-5 days before being consumed or frozen. If you combine breast milk from different days, please be sure to cool your fresh breast milk before adding it to the previously refrigerated milk. Also make sure the combined contained of milk is labeled with the date the older milk was pumped, and store it in the freezer if the oldest milk was pumped more than 3-5 days ago.

Can I combine my breast milk with milk from someone else?

The only time this can be safely done is in a hospital setting, where pasteurized donor milk can be used to supplement your own milk.

Can I combine freshly pumped milk with my breast milk that had previously been frozen and is now thawed?

Yes, this can be done, but it’s recommended to cool your freshly pumped warm milk before combining with previously frozen breast milk to prevent the previously stored cold milk from warming up too much. This is because warming milk up can promote bacterial growth. You will not be able to refrigerate or refreeze this combined milk if it is not all used up during a feeding. 

If there is breast milk left over in my baby’s bottle after they are done feeding, can I combine this leftover milk with milk I previously pumped?

It is not recommended to combine milk that is left after a feed with other milk due to a risk of bacterial contamination of the milk that is left over in a bottle. If you do opt to save leftover milk (which should never be done if your baby is a preemie or has a fragile immune system) it should never be mixed with milk from other pumping sessions, and should preferentially be fed and used up during the next feeding session. 

Do you have any additional questions about how to mix breast milk that have not been addressed?

If so, please reach out to a lactation consultant, pediatrics, or our trained breastfeeding professionals at Lactation for assistance.

About the Author

Jessica Madden, MD, is the Medical Director at Aeroflow BreastpumpsDr. Madden has been a board-certified pediatrician and neonatologist for over 15 years. She's currently on staff in the neonatal intensive care unit (NICU) at Rainbow Babies and Children’s Hospital in Cleveland, OH. She previously worked in the Boston and Cleveland Clinic Children’s Hospitals. In 2018 she started Primrose Newborn Care to provide in-home newborn medicine and lactation support. She also enjoys traveling, yoga, reading, and spending time with her children.

Information provided in blogs should not be used as a substitute for medical care or consultation.