Women are often advised not to combine breast milk from different pumping sessions. For citing reasons such as increased bacterial contamination and temperature differences. This advice can lead to excess worry and more work for those who are trying to breastfeed their babies.
Infants with food allergies create enough challenges for families to deal with – on top of having a new baby! You’re changing your diet, label reading, and making so many sacrifices to continue breastfeeding.
Let us dig deeper into this myth and discuss what it could mean for parents. Especially those who have higher risk babies such as preterm or babies with food allergies.
What Does The Scientific Research Say About Combining Breast Milk?
The biggest concern about combining breast milk from different sessions is whether or not it will contaminate the milk with bacteria. You want the best for your baby and we understand. Let’s look into some science.
Research on Bacterial Contamination of Pooled Breast Milk
Scientists performed a study on pooled breast milk versus non-pooled milk over 4 days from 19 participants.
Day 1: 24-hour pooled breastmilk and Day 2: 24-hour breastmilk separated by pump session (non-pooled). The following week, participants repeated the collection on alternate days (day 1 separate, day 2 pooled).
Bacteria analysis through plating found colony counts ranging from 0 to >100,000 per milliliter (mL). Counts over 100,000/mL were observed in 31 of 211 non-pooled samples (14.7%) and 3 of 35 pooled samples (8.6%). This means that there were more bacteria found in the non-pooled samples than the pooled samples.
These findings support that pooling breastmilk does not increase bacterial contamination. These results can be explained by a separate study that found the largest contributing factor for breast milk bacterial contamination is collection and storage material cleanliness. 
Go on and pool your breast milk, just make sure the parts are clean and you follow proper storage recommendations for breast milk.
The Nutritional Review of Different Pumping Sessions
Breastmilk nutrition content varies throughout the day. Some parents attempt to match when the milk was expressed to when the baby receives that milk (morning pump sessions fed to baby another morning). This is not always feasible. Instead, pooling a days’ worth of pump sessions creates more consistent macronutrients.
In the same study of pooled vs. non pooled milk, scientists found a macronutrient variability range of ±32% for protein, ±46% for fat, ±30% for carbohydrates, and ±29% for total calories.  This is especially vital for preterm and NICU infants who may be receiving expressed and fortified breastmilk as consistent nutrition that can improve health outcomes. 
The advice to not combine freshly pumped milk with chilled refrigerated milk is based on the assumption that it will significantly impact bacterial contamination and nutritional content due to the temperature changes.
Temperature fluctuations in pooled milk do not lead to increased bacteria counts or nutritional breakdown. The study found that the average nutritional content of 24-hours’ worth of individually stored milk was nearly identical to that of the pooled samples and that pooled milk did not have increased bacterial contamination. 
Breast Pumpers Need Ease and Convenience
Families who tried both pooling and non-pooling methods reported pooling to be easier overall, citing items such as less plastic usage and easier storage . It makes sense, right? If you’re pumping and separating every session, that’s a lot of dishes and milk bags!
From Dr. Trill’s personal experience, pooling was an absolute necessity while she was on military duty, traveling or simply forgot to bring extra containers to work. We’re human, remember? But we always want what’s best for our babies.
It’s essential for those who pump to not feel as though they’re somehow giving “subpar” milk to their infants. If anything, it should be encouraged to make your life easier and babies healthier. Pumping is hard work and you should feel empowered and proud in doing this for your little ones.
Have you met Annie yet? She pumped for TWO YEARS with her firstborn!
What about Combined Pumped Breast Milk and Babies with Food Allergies?
The general rule of thumb should be dependent on your baby’s reactions. If ingested foods in your diet cause food allergy symptoms in your baby, you should be aware of how pooling could impact your milk.
We know that allergen protein concentrations typically spike around 2 hours post-ingestion and steadily decrease after that. [4-7] Pooling milk will reduce the overall allergen protein concentration if you don’t continue to consume the food throughout the day. This may be an option if you know what your child is allergic or sensitive to and symptoms are not life-threatening.
Essentially, if you would feel comfortable giving all of the expressed milk to your little one separately, then pool-on!
But if you don’t know what foods are causing reactions, pooling can add another layer of mystery to your already difficult situation. You won’t know which pumping session contained the allergen to investigate your diet thoroughly. The Free to Feed App can help you track foods you and your baby are eating, symptoms, diapers, and more! As with all things food allergy-related, consult your physician for further assistance.
If you know you’re producing allergen-free breastmilk and your baby is at baseline then combine those pumping sessions with freedom! Many families like to label their allergen-free milk (top 12 free, DF/SF, etc) for clarification for other members of the family. This allows everyone to be on the same page and takes the guesswork out of feeding the baby.
Important Takeaways on Pooled Breast Milk For Your Baby
Feel confident pooling your pump sessions! Having a single container to hold expressed milk can help decide how to best portion out what will be used immediately and what needs to be frozen. Reduce dishes and how much plastic wear you need to lug around on your daily commute or when traveling.
Understand that when labeling milk and determining how long it is good, you should use the oldest expression date and time for all of the pooled milk.
Example: You pumped the night of 4-19 and the morning and afternoon of the 20th. You should label your milk: 5oz 19th PM.
Always add more details to help if you feel the need to do so!
If you are in the trenches and need help, consider a consult with one of our infant food allergy experts!
Stellwagen, Lisa M., et al. "Pooling expressed breastmilk to provide a consistent feeding composition for premature infants." Breastfeeding Medicine 8.2 (2013): 205-209.
Haiden, N., et al. "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." Journal of Hospital Infection 92.3 (2016): 226-228.
Heiman, Howard, and Richard J. Schanler. "Enteral nutrition for premature infants: the role of human milk." Seminars in Fetal and Neonatal Medicine. Vol. 12. No. 1. WB Saunders, 2007.
Vadas, Peter, et al. "Detection of peanut allergens in the breast milk of lactating women." Jama 285.13 (2001): 1746-1748.
Palmer, D. J., M. S. Gold, and M. Makrides. "Effect of maternal egg consumption on breast milk ovalbumin concentration." Clinical & Experimental Allergy 38.7 (2008): 1186-1191.
Zhu, Jing, et al. "Discovery and quantification of nonhuman proteins in human milk." Journal of proteome research 18.1 (2018): 225-238.
Picariello, Gianluca, et al. "Excretion of dietary cow’s milk-derived peptides into breast milk." Frontiers in Nutrition 6 (2019): 25.