Expressing breastmilk takes time and energy. There is the pumping, the washing up, the planning and, sometimes, the effort of collecting enough for just one feed. Pouring that milk away can feel particularly frustrating when your baby has only taken a few sips.
So it is reasonable to ask: do all of our storage rules reflect what the research actually shows?
Emily Oster's February 2026 ParentData guide brings this question into focus. Her interpretation of the evidence suggests more flexibility in some situations, especially with unfinished bottles. It is a useful conversation to have, provided we are clear about what the studies measured and what remains uncertain.
What does Emily Oster's guide say?
For healthy, full-term babies, ParentData summarises storage as follows:
| Storage | Australian guidance | Oster's interpretation |
|---|---|---|
| Room temperature | 6–8 hours | Up to 8 hours; potentially longer |
| Refrigerated | Up to 72 hours | Up to 4 days |
| Frozen | 3 months in separate-door freezer; 6–12 months in deep freezer | 6–12 months |
| Unfinished bottle | Discard after feed | Up to 8 hours at room temperature or 24 hours refrigerated* |
*Oster's longer timeframes are interpretations of emerging evidence, not updated Australian clinical guidelines. Applies primarily to healthy, full-term babies.
The guide also describes adding freshly expressed milk to refrigerated, unused milk as safe. It mentions that fresh milk may last longer than eight hours at room temperature, but I would not turn that possibility into a routine 24-hour countertop recommendation.
These are Oster's evidence interpretations, rather than updated Australian clinical guidelines. Her guide excludes automatic application to babies born preterm, babies with a NICU history and donor-milk situations, for which individual advice is needed. [1]
Australian advice already allows more than four hours
If you have seen the American “four hours on the bench” rule, you may be surprised to learn that Australian advice already differs.
The Australian Breastfeeding Association (ABA), drawing on NHMRC guidance, lists 6–8 hours for freshly expressed milk at 26°C or below, with refrigeration preferred when available. It lists 72 hours in a fridge at 5°C or below. [2]
That means eight hours for fresh milk is not a completely new idea in Australia. The more substantial difference is what happens after your baby has started drinking: ABA advice is to discard the remainder after the feed, whereas Oster's guide allows longer storage. [1,2]
What does the new leftover-milk study actually show?
A February 2026 preprint examined leftover infant milk. Its laboratory component included breastmilk from 17 infants, with samples studied at 4°C and 20°C over 4, 8 and 24 hours. [3]
Median bacterial counts rose immediately after feeding. They remained stable at four hours; increases in post-feed samples at eight hours were not statistically significant. At 24 hours at 20°C, the researchers observed substantial growth, although this was also not statistically significant. [3]
This is encouraging evidence that deserves follow-up. It also illustrates why “not statistically significant” needs careful interpretation: a small study can miss meaningful differences. It does not mean that bacteria did not grow.
The study assessed bacterial counts, rather than testing whether babies could safely drink stored leftovers. Parents were contacted about illness, but the stored laboratory samples were not a feeding safety trial. The source cited here is a preprint, so its findings should be treated as preliminary. [3]
Does that mean the old rules are wrong?
The evidence gives us reason to question whether every recommended discard time is necessary. It does not establish that all longer storage periods are safe.
A cautious storage limit and a proven point of danger are different things. Parents deserve to understand that distinction, especially when following a rule means discarding milk they worked hard to express.
I would also be careful with the claim that guidelines “overestimate bacterial growth”. That implies a specific prediction has been disproved. A more accurate description is that some recommendations use a precautionary margin where direct evidence is limited.
Oster is interpreting existing and emerging research; she did not conduct the leftover-milk study herself.
A practical approach to wasting less milk
You do not need to stretch storage times to make feeding expressed milk less wasteful.
Start with a smaller serving. Keep the rest separately chilled and offer more if your baby remains hungry.
Label and rotate your milk. Use the oldest suitable milk first and freeze portions you will not use within your chosen fridge guidance.
Keep a reliable cold chain. Store milk at the back of the fridge and use an insulated cooler with ice packs when transporting it.
Warm only what you need. Milk does not have to be warm if your baby accepts it cool. Avoid microwaving. [2,4]
If pooling milk, keep track of the oldest portion. Oster supports mixing warm and chilled unused milk; ABA advice remains to chill the new milk first. Cooling before combining is a straightforward approach consistent with local guidance. [1,2]
Fresh, thawed and unfinished milk need different rules
The fresh-milk times above should not be applied to milk that has been frozen and thawed. For thawed milk, follow separate handling advice. CDC guidance is to use refrigerated milk within 24 hours of complete thawing and never refreeze fully thawed milk. [5]
Likewise, the small leftover-milk study does not justify repeated cycles of warming, feeding and refrigerating. It tested specified conditions, rather than every combination that might happen at home.
For premature, unwell or immunocompromised babies, or babies with a NICU history, ask your care team for a storage plan. Donor milk and hospital feeding may have separate requirements. [1,2]
My perspective
Breastmilk storage advice should recognise both infant safety and the maternal load involved in expressing.
Oster's review makes a valuable case for revisiting rules that may contribute to avoidable waste. I welcome that discussion. I also want families to have an honest account of the evidence: promising bacterial-count findings are a reason to investigate further, rather than proof of a universal safe limit.
For now, use a consistent storage plan suited to your baby, your equipment and your circumstances. You deserve practical support with that decision, without guilt about milk you have already discarded.
Want a simple reference to keep on your phone? Download my free Breastmilk Storage: Less Waste, Clearer Choices guide for a comparison of Australian guidance and Oster's interpretation, plus practical ways to reduce waste.
Need help making expressing work for your family? Explore clinic and online support here.
Sources
ParentData by Emily Oster. Storing Your Breast Milk, February 2026. https://parentdata.org/wp-content/uploads/2026/02/Storing-Your-Breast-Milk.pdf
Australian Breastfeeding Association. Storing expressed breastmilk. https://www.breastfeeding.asn.au/resources/storing-ebm
Zychlinsky Scharff A et al. Leftover Infant Milk After Bottle Feeding: Parental Practices and Microbiological Findings. February 2026 preprint. https://www.medrxiv.org/content/10.64898/2026.02.13.26346179v1.full
Australian Breastfeeding Association. Preparing expressed breastmilk. https://www.breastfeeding.asn.au/resources/preparing-ebm
CDC. Breast Milk Storage Questions and Answers. https://www.cdc.gov/breastfeeding/php/guidelines-recommendations/faqs.html
General education for home use, primarily for healthy, full-term babies. Individual medical advice and hospital or childcare policies may differ. Reviewed November 2026.
