Here’s everything you need to know when combining milk from different pumping sessions. 

If you’re a breastfeeding mom, chances are you’ve combined pumped milk from separate sessions throughout the day. Known as “milk pooling” or “the pitcher method,” this technique can be both convenient and beneficial to your baby — especially if you’re focused on weight gain.

While pooling your milk is recommended by healthcare providers and organizations like the American Academy of Pediatrics (AAP), there are a few things to keep in mind to ensure that you’re doing it as safely and carefully as possible. So we asked Dr. Laurie Jones, a pediatrician, IBCLC, and the founder of Dr. MILK (Mothers Interested in Lactation Knowledge), to share everything moms need to know. Read on to learn: 

• What breast milk pooling is 
• The benefits of milk pooling
• Best practices for safe milk pooling 
• Risks and when to avoid milk pooling 

Let’s dive in!

What is Breast Milk Pooling?

Milk pooling refers to the practice of combining milk from multiple pumping sessions within a 24 hour period. This can be done in large pitchers (hence the “Pitcher Method” nickname), bottles, storage bags, or even a portable cooler if you’re on the go! 

Some moms choose to store milk from each session in a separate container in the fridge, and combine right before giving their baby a bottle — adding cold milk to cold milk. In other cases, moms pour freshly-pumped milk into a refrigerated pitcher or cooler as they pump throughout the day, collecting the full days’ worth of milk in a single place. In this case, they’re adding warm milk to cold milk. 

There’s no perfect or singular way to pool — although certain things may increase the risk of bacteria growth, which can be consequential if your baby is premature, in the NICU, or otherwise immunocompromised. More on this below! 

Benefits of Milk Pooling 

Combining milk from different pumping sessions can help ensure that your baby is getting more complete nutrition from each bottle — and therefore, it is especially encouraged for babies who are in the NICU or struggling to gain weight in the newborn phase. This is because milk varies in its nutritional and caloric content depending on the duration of your session, how long it has been since you last pumped, the number of times you’ve pumped that day, and the time of day! 

How your milk content varies: 

• Minerals like magnesium, zinc, and potassium may be higher in morning milk 
• Fat content is often higher in nighttime milk 
• Levels of protein and lactose vary in milk throughout the day 
• Fat content increases in milk throughout your pumping session 
• Hormones like cortisol are higher in morning milk, while hormones like melatonin are higher in nighttime milk 

When you mix your milk, you’re more likely to evenly distribute all of these nutrients and hormones, and also ensure that your baby is getting as many calories as possible. This is backed by science: “Twenty-four-hour pooling of human milk reduces nutrient and caloric variability,” confirms the National Institutes of Health (NIH). It does this “without increasing bacterial counts.”

Beyond nutrition, milk pooling can simply make it easier to store milk in your fridge — and reduce the number of storage containers that you need to clean throughout the day. 

Best Practices for Safe Milk Pooling 

If you’ve heard or read about any risks of milk pooling, chances are they had to do with the temperature of milk — more specifically, about adding warm milk to already-chilled milk. 

This isn’t a perfectly straightforward issue. There’s so much nuance, in fact, that the American Academy of Pediatrics changed their guidelines around pooling milk of different temperatures as recently as 2021! 

So here’s what’s important: 

• The colder you keep your milk, the safer it will be for your baby. The potential for bacteria growth increases each time you remove your milk from the fridge and give it the opportunity to warm up.
• The potential for bacteria growth also increases each time that you handle your milk. This is because your hands, kitchen counter, and other surfaces that your milk may come into contact with have likely been exposed to germs. 
• The older your baby gets, the less worried you need to be about the potential spread of bacteria. (If you have a healthy full-term baby, this also should give you peace of mind!)

Given this, you should only pool milk that has been pumped throughout the same 24-hour period. This is one of the best ways to ensure that you’re handling a days’ worth of milk as little as possible.  

You’ll also want to avoid taking your pitcher or storage bag out of the fridge or ice pack and leaving it at room temperature for more than a few minutes. Set it on the counter, pour the fresh milk into the existing milk, and get it back into a cool place as quickly as you can! (Making sure that your hands and countertops are clean is critical as well.)

Finally, if possible you’ll want to store your pooled milk in the fridge, since it will keep it colder than a cooler bag with ice packs. Remember: the colder your milk, the better!

A recap from the AAP: 

“Mothers should aim for cleanliness but realize milk is a biologic substance and full of probiotic and commensal bacteria. Hand washing, using clean or new containers, minimizing milk transfers (which waste fat and calories) and pumping directly into storage containers are recommended. Plastic or glass containers are fine, no evidence supports one over the other.”

When to Avoid Milk Pooling                         

If you have a baby who is premature or immunocompromised, you’ll want to talk to your doctor before adding freshly-pumped milk to chilled milk.  Even though the risk of bacteria growth is low, you can’t be too careful when you’re dealing with babies in this condition.  

And remember, you should never pour freshly pumped milk into milk that has already been frozen.

Now You’re Ready to Get Pooling! 

For more tips from Dr. Jones and other pumping and breastfeeding experts, make sure to check out our blog and subscribe to our YouTube channel. You can also read more about this method and its potential risks in this comprehensive article, which lays out recommendations, guidelines, and more. 

You can also shop the Portable Breast Milk Cooler which stores up to 16 oz of milk at a safe temperature for up to 24 hours — and can be game-changing if you need to pool your milk while you’re on-the-go. (But remember: ideally, you’ll do this in the fridge!) 

You can always book a session with a lactation consultant for guidance on milk pooling or anything else. Explore the IBCLCs with our partners at SimpliFed, or ask your OBGYN or healthcare provider for a local recommendation.