If a Late Preterm Baby Needs Extra Milk, Donor Milk and Formula Work About the Same

What a carefully designed study of 201 babies tells families about topping up when mom's milk is still coming in

When a baby is born a few weeks early and the mother's own milk has not fully come in yet, the baby often needs a temporary top-up. A large, carefully blinded study from Australia tested whether that top-up should be donor human milk or infant formula, and found that the choice made no difference to how quickly babies reached full feeds or how they grew by the time they went home. The main practical takeaway for families is reassuring: when a top-up is needed for a moderately or late preterm baby, both options lead to about the same feeding progress, and the decision can rest on what is available and what the family prefers.

Why This Question Matters

Most babies born early are not extremely premature. Roughly five out of six preterm births happen between 32 and 36 weeks of pregnancy — what doctors call moderate-to-late preterm [1]. These babies usually do well, but they are not simply small full-term babies. They more often need help with breathing, keeping warm, and feeding in their first days, and as a group they carry slightly higher risks of learning and developmental difficulties later in childhood than babies born at full term [2], [3]. Feeding is one of the very first decisions parents and care teams face, often within a day or two of birth, while a mother's milk supply is still building up.

How Families and Clinicians Used to Face This

For many years, there was a clear order of preference for feeding a preterm baby, and it still stands: the mother's own milk is best; when there is not enough of it, pasteurized donor human milk — breast milk that other mothers have generously donated and that has been screened and heat-treated for safety — is the next choice; and infant formula comes after that [4], [5]. But that ranking was worked out mostly by studying the tiniest, most fragile babies, born much earlier than 32 weeks. In those very premature infants, formula carries a higher risk of a dangerous bowel illness called necrotizing enterocolitis than donor milk does, while formula tends to produce faster weight gain [6]. Another major study found that donor milk did not improve brain development at 18 months in very-low-birth-weight babies compared with formula [7].

The problem was that nobody had run a high-quality test in the much larger group of babies born just a few weeks early. As donor milk banks grew, many hospitals began offering donor milk to these more mature babies too — hoping that, because human milk is often gentler on the stomach, it might help them reach full feeds and go home sooner. But that was a hope, not a proven fact, and hospitals varied widely in what they did [8]. A small pilot study hinted that donor milk might help with breastfeeding, but it was too small to give a firm answer [9]. This new study was built to fill that gap with real evidence.

What the Researchers Did

The researchers enrolled 201 babies born between 32 and almost 37 weeks at two Australian hospitals, all of whom needed a top-up because their mothers did not yet have enough milk [1]. Each baby was randomly assigned — like a coin flip — to receive either donor human milk or term infant formula as the top-up for up to eight days. Random assignment matters because it makes the two groups alike in every other way, so any difference at the end can be credited to the milk itself.

This study did something especially careful: it kept everyone "blinded." The milk was prepared out of sight and the bottles and syringes were covered with tape, so neither the parents nor the nurses nor the researchers measuring the results knew which baby was getting which milk [1]. That prevents people's expectations from coloring the results — a common weakness in feeding studies. The main thing the team measured was how many days it took each baby to reach "full feeds," meaning the baby was getting all of its nutrition as milk.

What They Found

The two groups were essentially identical on the main measure. Babies reached full feeds in an average of about 5.7 days on donor milk and 5.8 days on formula — a difference small enough to be meaningless [1]. Most babies got there within a week either way. Other measures pointed the same direction: there was no difference in feeding problems, in how long babies needed intravenous fluids, or in how long they stayed in the hospital, which was about three weeks on average in both groups [1]. Because the study was blinded, these results are especially trustworthy — no one's expectations about donor milk or formula could have nudged the outcome one way or the other.

There was one real difference worth knowing. Babies fed donor milk took about two extra days, on average, to get back up to their birth weight — roughly 11 days versus 8 days [1]. This is because donor milk tends to have a bit less protein and fewer calories than formula, so the early catch-up is a little slower. The encouraging part is that this gap did not last: by the time babies went home, and again at two and four months of age, the two groups were the same in weight, length, and head size [1]. Care teams can keep an eye on early weight gain and add a fortifier to donor milk if a baby needs extra calories, using standard growth charts to track progress [1], [10].

The study also offered a hint — not a proof — that families given donor milk were somewhat more likely to be feeding their baby only breast milk at discharge and in the months after, though the numbers were not strong enough to be certain [1]. For example, at the time of going home, about 73 out of every 100 donor-milk babies were getting only breast milk, compared with about 64 out of every 100 formula-supplemented babies — a gap that looks encouraging but could still be down to chance [1]. The researchers were careful not to overstate this, because the study was designed to measure feeding speed, not breastfeeding rates, and it would take a larger study built specifically around that question to know for sure. And importantly, no baby in either group developed necrotizing enterocolitis or a blood infection, which confirms that the bowel-illness danger that makes donor milk so important for the tiniest babies is uncommon in these more mature ones [1].

What This Means for Your Baby

If your moderately or late preterm baby needs a temporary top-up while your milk supply builds, this study is genuinely reassuring. It tells you that donor milk and formula will get your baby to full feeds at the same pace and that your baby will be growing the same way by the time you go home, whichever is used [1]. Donor milk is a limited and valuable resource, and hospitals are encouraged to save it first for the smallest, highest-risk babies [4], [5]. So if your hospital offers formula as the top-up for a late preterm baby, that is a sound, evidence-based choice — not a second-best one. If donor milk is available and you prefer it, that is also a safe choice; just know your baby may take a couple of extra days to regain birth weight at first. These findings match another large study from the same period, which also found that the type of top-up did not change how quickly these babies fed [11]. Above all, none of this changes the first principle: your own milk, whenever you can provide it, remains the best food for your baby, and a top-up is simply a temporary, short-term bridge while your own supply catches up.

What Researchers Are Working On Next

The most useful questions now are not about feeding speed — this study has answered that — but about the things families care about over the longer term. Researchers want to learn whether offering donor milk rather than formula helps mothers keep breastfeeding for longer, since that was the one hopeful hint in this study [1], [9]. They also want to test whether adding extra nutrients to donor milk can erase the small early weight-gain difference while keeping the benefits of human milk [8], [11]. Until those answers come in, the message for families is steady and calm: when a late preterm baby needs a little extra milk, both good options are truly good, and you can choose based on what is available and what feels right for your family [1].

References

  1. Rumbold AR, Lai MM, August D, et al. Supplemental Donor Milk vs Infant Formula in Moderate to Late Preterm Infants: A Randomized Clinical Trial. JAMA Pediatr. 2025;179(10):1065–1073. doi:10.1001/jamapediatrics.2025.2365 ↩
  2. Boyle EM, Poulsen G, Field DJ, et al. Effects of gestational age at birth on health outcomes at 3 and 5 years of age: population based cohort study. BMJ. 2012;344:e896. doi:10.1136/bmj.e896 ↩
  3. Cheong JLY, Doyle LW, Burnett AC, et al. Association between moderate and late preterm birth and neurodevelopment and social-emotional development at age 2 years. JAMA Pediatr. 2017;171(4):e164805. doi:10.1001/jamapediatrics.2016.4805 ↩
  4. Committee on Nutrition; Section on Breastfeeding; Committee on Fetus and Newborn. Donor Human Milk for the High-Risk Infant: Preparation, Safety, and Usage Options in the United States. Pediatrics. 2017;139(1):e20163440. doi:10.1542/peds.2016-3440 ↩
  5. World Health Organization. WHO Recommendations for Care of the Preterm or Low-Birth-Weight Infant. Geneva: World Health Organization; 2022. who.int/publications/i/item/9789240058262 ↩
  6. Quigley M, Embleton ND, McGuire W. Formula versus donor breast milk for feeding preterm or low birth weight infants. Cochrane Database Syst Rev. 2019;(7):CD002971. doi:10.1002/14651858.CD002971.pub5 ↩
  7. O'Connor DL, Gibbins S, Kiss A, et al. Effect of Supplemental Donor Human Milk Compared With Preterm Formula on Neurodevelopment of Very Low-Birth-Weight Infants at 18 Months: A Randomized Clinical Trial. JAMA. 2016;316(18):1897–1905. doi:10.1001/jama.2016.16144 ↩
  8. Embleton ND, Moltu SJ, Lapillonne A, et al. Enteral Nutrition in Preterm Infants (2022): A Position Paper From the ESPGHAN Committee on Nutrition and Invited Experts. J Pediatr Gastroenterol Nutr. 2023;76(2):248–268. doi:10.1097/MPG.0000000000003642 ↩
  9. Pithia N, Grogan T, Garg M, Kesavan K, Calkins KL. A Pilot Single-Site Randomized Control Trial: Investigating the Use of Donor Milk in the Late Preterm and Term Infant in the Neonatal Intensive Care Unit. Am J Perinatol. 2024;41(S01):e2427–e2435. doi:10.1055/s-0043-1771261 ↩
  10. Fenton TR, Kim JH. A systematic review and meta-analysis to revise the Fenton growth chart for preterm infants. BMC Pediatr. 2013;13:59. doi:10.1186/1471-2431-13-59 ↩
  11. Alexander T, Asadi S, Meyer M, et al. Nutritional Support for Moderate-to-Late–Preterm Infants — A Randomized Trial. N Engl J Med. 2024;390(16):1493–1504. doi:10.1056/NEJMoa2313942 ↩