Does Testing Breast Milk Every Day Help Very Small Premature Babies Grow?

A German study by Gebauer and colleagues compared measuring and adjusting each day's milk with a simpler fixed recipe that was already rich in protein

Very small premature babies fed breast milk need extra protein and energy added to it, and some hospitals now test the milk every day to fine-tune what they add. A careful study of 110 babies in Germany found that when the standard recipe already contained plenty of extra protein, daily testing did not make babies grow any faster or develop differently by age two. Neither approach did better than the other.

Why breast milk needs a boost for the smallest babies

Breast milk is the best food for babies born very early, and doctors encourage it for good reasons. But a baby born weighing less than 1500 grams (about 3.3 pounds), called a very low birth weight baby, is trying to grow at the speed they would have grown in the womb. Breast milk on its own does not contain enough protein and energy for that [1]. So hospitals add a powder called a fortifier, which is a concentrated mix of protein, carbohydrate, vitamins and minerals. This is called fortification.

How hospitals have tried to get the recipe right

Adding fortifier has been routine for many years, but the researchers behind the new study point out that its benefits are real yet modest [1]. A large review that combined 18 studies involving 1456 premature babies found that fortified milk helped babies gain weight a little faster in hospital, but there was not enough evidence to say whether it changed growth or development later in childhood [2]. Doctors still care about good growth in hospital because, in a large group of very small babies followed to about one and a half years of age, those who grew faster in hospital tended to have fewer developmental problems, although that kind of study cannot prove that faster growth was the cause [3].

There has long been a hidden problem with the standard recipe. It assumes every mother's milk contains an "average" amount of protein. In reality, breast milk varies a great deal from one mother to the next, and it changes as the weeks go by after birth [4]. When researchers tested fortified milk at the bedside, about half of the samples still had less protein than recommended [5], and babies were often getting noticeably less protein than staff thought [6].

Hospitals tried two ways to fix this. One adjusted the amount of added protein based on a blood test from the baby [7]. The other, called target fortification, measures each batch of milk with a special analyser and adds exactly what is needed to reach a goal. In a study at McMaster University in Hamilton, Canada, babies whose milk was adjusted this way gained weight faster than babies on the standard recipe [8]. A review of several such studies agreed that adjusting the milk probably helps babies gain weight in the short term, but none of those studies looked at serious developmental problems later on [9]. That left an important question: was it the measuring that helped, or simply the fact that babies ended up getting more protein?

What the Leipzig study did

The new study was carried out in one hospital, the University of Leipzig Medical Center in Germany, which has one of Europe's largest breast-milk banks [1]. Between 2013 and 2016, it included babies born between 24 and 31 weeks of pregnancy, weighing 400 to 1499 grams. They were fed only their mother's milk or donated breast milk, never formula.

Every baby first got the same standard fortifier. Once a baby was drinking a set amount of milk, usually at about four weeks of age, a process like flipping a coin decided which of two plans they would follow until they reached 36 weeks (counting from the start of pregnancy).

In the first plan, the "modified-standard" plan, a fixed extra scoop of protein and a small amount of a special fat were added to every feed, whatever the milk contained. The milk was still measured every day, but the results were not used to change the amounts. This made the standard recipe much richer in protein than usual. In the second plan, the "target" plan, a milk-bank team measured each day's milk and added just enough protein and fat to reach set goals. These goals followed European expert recommendations for premature babies [10].

Importantly, neither the parents nor the nurses and doctors knew which plan a baby was on. The milk-bank team prepared all the feeds out of sight. This is called a double-blind design, and it helps make sure that people's expectations do not affect the results.

The main thing the researchers measured was how fast the babies gained weight, using a well-tested way of calculating growth speed for very small babies [11].

What the researchers found

Of the 110 babies who started, 106 completed the study, 54 on the modified-standard plan and 52 on the target plan. No baby died [1].

The two groups grew at very similar rates: about 18 grams per kilogram of body weight per day in both. To make that concrete, a 1.5-kilogram baby gaining at this rate puts on roughly 27 grams a day, about the weight of a slice of bread. The difference between the groups was tiny, far smaller than the difference the study was designed to detect.

It is worth being precise about what "no difference" means here. Researchers calculate a range of results that are reasonably compatible with their data. In this study that range ruled out the size of growth advantage the researchers had considered important, so a meaningful benefit from daily testing is unlikely. It does not prove the two plans are exactly the same; a very small difference in either direction cannot be excluded. Over the roughly five weeks the plans lasted, the researchers note that a difference of 1 gram per kilogram per day, roughly the edge of that range, would add up to only a few tens of grams of body weight.

At 36 weeks, babies in both groups weighed about 2.4 kilograms on average, and their length and head size were similar.

When the children were about two years old (counting from their due date), 79 of them had a standard developmental check of thinking, language and movement skills. Scores were similar in both groups, and no child in either group had cerebral palsy (a lifelong condition affecting movement and posture).

Daily testing did show how much breast milk varies. Some batches contained more than six times as much protein as others. Measuring the milk made each baby's daily protein intake steadier, but it did not change how well they grew. Babies on the target plan ended up getting a bit more fat and energy, while those on the fixed plan got a bit more protein after 34 weeks.

What this means for your baby

If your baby is in a unit that tests breast milk daily, that is a careful and reasonable approach. If your baby's unit uses a fixed, protein-rich recipe instead, this study is reassuring: in these babies, daily testing did not lead to better growth than that simpler approach, and there was no sign that it would have helped more.

There are some important limits. The study compared daily testing with a standard recipe that had already been boosted with extra protein. It did not compare daily testing with the ordinary standard recipe that many hospitals use, and the researchers themselves say their findings do not answer that question [1]. The babies started their plans only once they were stable and feeding well, so the results do not apply to the first fragile weeks of life. The study took place in a single hospital some years ago, and some products now available, such as fortifiers made from human milk, were not used. Only three in four children came back for the two-year check, so those results are less certain. The fortifier company Milupa paid for the milk analyser and supplied the products, but the researchers report that it had no say in the study.

The study also suggests that more protein is not automatically better. Experts still disagree about exactly how much protein very small babies need: one recent review suggested quite high amounts may help growth [12], while another found that high protein intakes might slightly lower the chance of surviving without developmental problems [13]. Extra energy beyond a certain point also tends to be stored as fat rather than building lean tissue [14], so the balance of protein and energy may matter as much as the amount of protein alone.

If you have questions about how your baby's milk is fortified, your neonatal team will be glad to explain the plan they use and how they track your baby's growth.

What researchers are working on next

The researchers suggest that future studies should begin fortification adjustments earlier, look closely at the balance between protein, fat and carbohydrate, and measure what babies' growth is made of, such as muscle and fat, rather than weight alone [1]. Earlier research found that babies who gained more lean tissue, rather than fat, in hospital tended to show better development at one year [15]. You can read more on In[Neo]Sight about a long-term follow-up of a different milk-adjustment study and a review comparing many fortification approaches.

The takeaway for families

For very small premature babies who were already feeding well, testing breast milk every day and adjusting it precisely did not lead to better growth or development than a simpler recipe already rich in protein. Neither approach led to better growth than the other. The bigger question, whether daily testing beats the ordinary standard recipe, is one this study was not designed to answer.

References

  1. Gebauer CM, Ascherl RG, Stamm N, et al. Target Versus Modified-Standard Fortification of Human Milk and Growth of Very Low Birth Weight Infants: A Double-Blind Randomized Trial. Nutrients. 2026;18(19):3241. doi:10.3390/nu18193241 ↩
  2. Brown JV, Lin L, Embleton ND, Harding JE, McGuire W. Multi-nutrient fortification of human milk for preterm infants. Cochrane Database Syst Rev. 2020;6(6):CD000343. doi:10.1002/14651858.CD000343.pub4 ↩
  3. Ehrenkranz RA, Dusick AM, Vohr BR, Wright LL, Wrage LA, Poole WK. Growth in the neonatal intensive care unit influences neurodevelopmental and growth outcomes of extremely low birth weight infants. Pediatrics. 2006;117(4):1253-1261. doi:10.1542/peds.2005-1368 ↩
  4. Gidrewicz DA, Fenton TR. A systematic review and meta-analysis of the nutrient content of preterm and term breast milk. BMC Pediatr. 2014;14:216. doi:10.1186/1471-2431-14-216 ↩
  5. Corvaglia L, Aceti A, Paoletti V, et al. Standard fortification of preterm human milk fails to meet recommended protein intake: Bedside evaluation by Near-Infrared-Reflectance-Analysis. Early Hum Dev. 2010;86(4):237-240. doi:10.1016/j.earlhumdev.2010.04.001 ↩
  6. Arslanoglu S, Moro GE, Ziegler EE. Preterm infants fed fortified human milk receive less protein than they need. J Perinatol. 2009;29(7):489-492. doi:10.1038/jp.2009.50 ↩
  7. Arslanoglu S, Moro GE, Ziegler EE. Adjustable fortification of human milk fed to preterm infants: does it make a difference? J Perinatol. 2006;26(10):614-621. doi:10.1038/sj.jp.7211571 ↩
  8. Rochow N, Fusch G, Ali A, et al. Individualized target fortification of breast milk with protein, carbohydrates, and fat for preterm infants: A double-blind randomized controlled trial. Clin Nutr. 2021;40(1):54-63. doi:10.1016/j.clnu.2020.04.031 ↩
  9. Fabrizio V, Trzaski JM, Brownell EA, et al. Individualized versus standard diet fortification for growth and development in preterm infants receiving human milk. Cochrane Database Syst Rev. 2020;11(11):CD013465. doi:10.1002/14651858.CD013465.pub2 ↩
  10. Agostoni C, Buonocore G, Carnielli VP, et al. Enteral nutrient supply for preterm infants: commentary from the European Society of Paediatric Gastroenterology, Hepatology and Nutrition Committee on Nutrition. J Pediatr Gastroenterol Nutr. 2010;50(1):85-91. doi:10.1097/MPG.0b013e3181adaee0 ↩
  11. Patel AL, Engstrom JL, Meier PP, Jegier BJ, Kimura RE. Calculating postnatal growth velocity in very low birth weight (VLBW) premature infants. J Perinatol. 2009;29(9):618-622. doi:10.1038/jp.2009.55 ↩
  12. Sanchez-Holgado M, Johnson MJ, Witte Castro A, et al. Systematic review and meta-analysis of enteral protein intake effects on growth in preterm infants. Pediatr Res. 2025;98(5):1696-1710. doi:10.1038/s41390-025-04115-9 ↩
  13. Das S, McClintock T, Cormack BE, Bloomfield FH, Harding JE, Lin L. High protein intake on later outcomes in preterm children: a systematic review and meta-analysis. Pediatr Res. 2025;97(1):67-80. doi:10.1038/s41390-024-03296-z ↩
  14. Hay WW. Amino acids and protein for preterm infants: How much and for what? Semin Fetal Neonatal Med. 2025;30(2):101633. doi:10.1016/j.siny.2025.101633 ↩
  15. Ramel SE, Gray HL, Christiansen E, Boys C, Georgieff MK, Demerath EW. Greater Early Gains in Fat-Free Mass, but Not Fat Mass, Are Associated with Improved Neurodevelopment at 1 Year Corrected Age for Prematurity in Very Low Birth Weight Preterm Infants. J Pediatr. 2016;173:108-115. doi:10.1016/j.jpeds.2016.03.003 ↩