Parenting expert shares breast milk research that could make life easier for parents

Emily Oster, Harvard-trained economist and New York Times best-selling author of "Expecting Better," "Cribsheet," "The Family Firm," and "The Unexpected," recently weighed in with her thoughts on the “extremely strict” CDC guidelines about tossing breast milk right after a baby takes a bottle.

Oster called the guidelines "way too cautious" in a recent deep dive for her ParentData website (behind a paywall, viewable with a 7-day free trial), in which she reviewed the knowns and unknowns of safe breast milk storage. 

Moms who pump (and their partners), especially those who have trouble producing enough milk, will likely raise a bottle to Oster's analysis.

Key Takeaways:

• The Centers for Disease Control and Prevention (CDC) currently has strict recommendations for how to handle and store breast milk.

• Oster claims that some of these older beliefs are mistaken, and she supports this with cited research.

• Reasonable, research-led guidelines on storing breast milk can make newborn life less stressful for new parents.

The Backstory:

Currently, the CDC says (paraphrasing): Fresh milk out for four hours at room temperature? Toss. Fresh milk in the fridge for four days or longer? Toss. Refreezing a defrosted bottle? Toss. Mixing multiple temperatures of milk, like adding cold milk to a warm bottle, or vice versa? Forget about it. Didn’t finish a thawed, previously frozen bottle within 2 hours? Pitch. 

The CDC also notes that breastfeeding reduces risk of “[a]sthma, severe lower respiratory disease, obesity, Type 1 diabetes, acute otitis media (ear infections), sudden infant death syndrome (SIDS), gastrointestinal infections, which can cause diarrhea and vomiting, [and] necrotizing enterocolitis (NEC) (death of intestinal tissue) for preterm infants.” But with guidelines like the above, is it any wonder that so few moms (especially those who work outside the home) exclusively breastfeed, breastfeed until six months, or even breastfeed at all? 

The current guidelines represent an abundance of caution about the possibility of bacteria multiplying in breastmilk under certain conditions. Heat spurs bacterial growth, which is why you don’t leave supper out on the counter overnight. 

But that concern over breastmilk is a bit overblown, Oster argued. She pointed out that bacteria are already present in breast milk, as they are present on the mother’s breast and on every surface everywhere. The question isn’t whether we can avoid bacteria in breastmilk, but rather, how much is too much?

The Details:

The Research

⒈ Oster cited a small, old, but still helpful, study finding that breastmilk, even when left out for 24 hours at 59 F, had negligible bacterial growth:

• In a 77 F room, which is considered room temperature, bacterial growth became concerning after eight hours.

• In a 100 F room, it took just four hours to reach concerning levels.

⒉ A larger, more recent study, this time from 2013 and in Nigeria, produced similar results. 

⒊ Oster also summarized a small 2026 preprint study of 44 infants, of whom 17 received breastmilk and 27 received formula. While this study has not yet undergone peer review, it is still promising:

• Researchers collected milk samples and then measured them at specific time intervals and temperatures.

• Breastmilk bacterial content remained stable even after 24 hours at room temperature, and beyond. 

⒋ Oster also referenced a 2016 review of research, which found that, even after four days in the refrigerator, bacterial growth in fresh-never-frozen breastmilk was minimal. Of note, the four day timeframe is the same as the current CDC recommendation. Studies of the safety of longer durations in the fridge are still needed. 

⒌ Oster pulled in research from a 2006 paper as well as a 2016 review article to show that breast milk can likely be safely thawed and refrozen. 

Summing up Oster’s research-based adjusted guidelines for breastmilk storage: 

• A freshly pumped (or hand-expressed) bottle should be good to go for up to 8 hours on the counter. 

• Fresh breastmilk should be ok in the fridge for up to four days (no change from CDC guideline). 

• Freshly pumped or expressed milk can safely be mixed with refrigerated milk. 

• After a baby has begun a bottle, it should be safe for up to 8 hours at room temperature and 24 hours in the refrigerator. 

The CDC's Reasoning

The reason the CDC guidelines discourage mixing of freshly-pumped with refrigerated breastmilk is because of the slight temperature difference. In other words, the warm fresh milk will heat up the refrigerated milk a little. Remember, heat multiplies bacterial growth. 

But, Oster insisted, the difference in temperature is not concerning because, per the above, it takes much longer and higher temperatures to create bacterial growth problems. 

CDC guidelines on throwing away milk that has already touched the baby’s mouth come from the fact that our mouths contain bacteria. As the reasoning goes, added bacteria from baby’s mouth could make bacteria in breastmilk grow faster. Oster pulled from multiple studies, and found that even reused formula samples, stored in the refrigerator between feedings, didn’t have marked bacterial growth.

As Oster acknowledged, a few situations do warrant extra caution along the lines of the CDC recommendations. These scenarios include babies born preterm, babies who are in the NICU, and babies who receive breast milk from donors. 

The Bottom Line:

Relaxed guidelines on breast milk storage can matter a great deal to mothers who are committed to nourishing their babies with breast milk, but struggle with low supply or have to pump often or exclusively.

Oster’s recommendations will warm their hearts, if not their bottles.