Pooling milk: Breastmilk is not frail!

We get nerdy with friend and inspiration Dr. Trill so that we can share facts that are often hard to come by: You can pool milk safely if you understand the principles! 

This amazing doctor is here on the Ceres Series to explain with a post covering all the science and citations, and we are so grateful. Read on!

Post by Trillitye Paullin, Ph.D.

Women are often advised not to combine milk from different pumping sessions, citing reasons such as increased bacterial contamination and temperature differences. 

Let us dig deeper into this myth and discuss what it could mean for parents, including those of babies with higher-risks such as preterm birth or food allergies.​ 

Trillitye Paullin, Ph.D.

I am a mother to two beautiful daughters who had severe infant food sensitivities who discovered that many parents experience the same troubling situation. I created Free to Feed to be a place for parents to find answers to their questions about infant food sensitivities and empower them to continue their breastfeeding journey.

Bacterial Contamination 

Over the course of 4 different days, scientists collected milk from 19 mothers. Day 1: 24-hour pooled breastmilk and Day 2: 24-hour breastmilk separated by pump session (non-pooled). The following week, mothers 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 (14.7%) non-pooled samples and 3 of 35 (8.6%) pooled samples. This means that there was more bacteria found in the non-pooled samples than the pooled samples, however the difference was not statistically significant [1]. ​These findings demonstrate pooling breastmilk does not increase bacterial contamination. These results can be explained by a separate study which found the largest contributing factor to expressed breast milk bacterial contamination is collection and storage material cleanliness [2]. 

Nutritional Analysis 

Breastmilk nutrition content varies throughout the day. Some parents may attempt to match when the milk was expressed to when 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 as above, scientist found macronutrient variability range of ±32% for protein, ±46% for fat, ±30% for carbohydrates, and ±29% for total calories [1]. This is especially vital for preterm and NICU infants who may be receiving expressed and fortified breastmilk as consistent nutrition can improve health outcomes [3]. 

Temperature Differences 

The advice to not combine freshly pumped milk to chilled refrigerated milk is based on the assumption that it will significantly impact bacterial contamination and/or nutritional content due to the temperature changes. As discussed above, temperature fluctuations in pooled milk does not lead to increased bacteria counts or nutritional breakdown. In fact, 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[1]. 

Ease of Use 

Mothers who tried both pooling and non-pooling methods reported pooling to be easier overall, citing items such as less plastic usage and easier storage [1]. From personal experience, I can say that pooling has been an absolute necessity while I have been on military duty, traveling, or simply forgot to bring extra containers to work. It is essential for mothers in this situation to not feel as though they are somehow giving “subpar” milk to their infants. If anything, it should be encouraged to make mom’s life easier and babies healthier. 

Food Allergies 

General rule of thumb should be dependent on your baby’s reactions. If ingested foods in your diet cause food allergy symptoms in 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 do not continue to consume it 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 momma! However, if you do not yet know what is causing reactions, pooling can add another layer of mystery to your already difficult situation. This is because you will not know which pumping session contained the allergen in order to investigate your diet thoroughly. As with all things food allergy related, consult your physician for further assistance. 


 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 traveling. Understand that when labeling milk and determining how long it is good, you should use the oldest expression date/time for all of the pooled milk. ​


 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 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. 7. Picariello, Gianluca, et al. "Excretion of dietary cow’s milk derived peptides into breast milk." Frontiers in Nutrition 6 (2019): 25.

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thank you so much for this info!

Vanessa Velasquez-Rousseau

This was valuable information. I appreciate how it was organized and presented. Just what I was looking for! Thank you!!

Holly Eager

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