New Growth Charts for Newborns: Getting "Small" and "Large" Right

How two 2026 studies rebuilt the charts that tell doctors whether a baby is small, average, or large at birth — one for intensive-care babies, one for healthy nursery babies

When a baby is born, the first thing the care team does is measure its weight, length, and head size and compare those numbers to other babies born at the same point in pregnancy. Two 2026 studies from the same research group rebuilt these comparison charts using modern methods and very large, recent United States datasets — one chart for babies admitted to newborn intensive care, and a companion chart for healthy babies cared for in the regular newborn nursery. Both new charts do a noticeably better job of correctly spotting the babies who are genuinely small or large, which is the first step toward the right monitoring and care.

Why This Matters, and How Families Used to Face It

Labeling a baby as "small for gestational age" or "large for gestational age" is not just paperwork. It decides which babies get their blood sugar checked, which are examined more closely for problems with growth in the womb, and how the team follows a baby's growth in the days and weeks after birth. To make that call, doctors place a baby's measurements on a chart and read off a percentile — the 10th percentile and the 90th percentile are the usual dividing lines for "small" and "large."

The trouble is that these charts have never fully agreed with one another. For decades, the reference points came from work by Lubchenco in 1963 [1] and the classification introduced by Battaglia and Lubchenco in 1967 [2]. In the United States, hospitals came to rely on charts published by Olsen and colleagues in 2010 [3], by Fenton in 2013 [4], and by the World Health Organization in 2006 [5]. Because these charts were built from different groups of babies using different rules, the very same infant could be called "small" on one chart and "just right" on another. For families, that inconsistency could mean an unnecessary test and worry for one baby, or a missed concern for another — depending only on which chart the hospital happened to use. Two older design choices were especially problematic: the earlier charts left certain pregnancies out of the calculations (which quietly tilted the curves), and they grouped babies by whole weeks of pregnancy instead of counting the exact day, which pushed some babies onto the wrong side of a line.

What the Researchers Did

The same team that made the widely used 2010 charts set out to fix these issues, and they did it for the two very different groups of newborns that hospitals actually care for.

The first study focused on babies admitted to the neonatal intensive care unit, or NICU [6]. It was enormous: 1,152,720 babies born in United States NICUs between 2010 and 2022, ranging from 22 weeks of pregnancy all the way to 41 weeks. Unlike the older charts, this one deliberately kept almost every baby in the calculations — including twins and triplets, babies with birth defects, and babies who did not survive — removing only a tiny number of extreme measurements that were likely recording errors. The researchers were careful to call the result a "reference" (an honest description of what this group of babies actually looks like) rather than a "standard" (a picture of how a baby ideally should grow) [7].

The second study built a matching chart for the regular newborn nursery [8]. It used 129,150 healthy babies born between 35 and nearly 42 weeks across 22 states from 2021 to 2024. Here the team focused on "uncomplicated" babies — those who went home on a normal schedule and did not need extra help such as treatment for low blood sugar or a transfer to intensive care. Both new charts were checked carefully to make sure they were reliable, and then compared against the older 2010 and 2013 charts.

Both studies used the same two modern improvements. First, they counted a baby's age in the womb by the exact day rather than by whole weeks. That sounds like a small detail, but a baby born at 34 weeks and 6 days is meaningfully different from one born at exactly 34 weeks, and lumping them together used to nudge some babies onto the wrong side of the "small" or "large" line. Second, they used a flexible curve-drawing technique that follows the real data more faithfully instead of forcing it into a smooth but slightly inaccurate line. Together these changes let the charts reflect what newborns actually look like, week by week and even day by day.

It also helps to know what the researchers were and were not trying to build. They were careful to describe their charts as a "reference" — an honest picture of how a real group of babies is sized — rather than a "standard," which tries to show how a baby would grow under ideal conditions. Both kinds of chart are useful, but they answer different questions, and mixing them up can lead to confusion about whether a baby is simply on the smaller side or genuinely not growing well.

What They Found

The key finding is simple to state. By definition, about 10% of babies should land below the 10th percentile (small) and about 10% above the 90th percentile (large). The new charts hit that mark at nearly every stage of pregnancy. The older charts often did not: depending on the week, they flagged too many or too few babies as small or large. In the healthy nursery group especially, the older charts identified far fewer than the expected 10% — meaning some genuinely small or large babies would have slipped through [8].

The intensive-care study [6] added a helpful, reassuring message about pregnancy complications: they do not neatly determine a baby's size. Babies of mothers with diabetes were more likely to be large — about 23% were above the 90th percentile — but many were perfectly average. Babies from pregnancies affected by pre-eclampsia leaned small, with about 17% below the 10th percentile, but again they came in every size. In other words, a diagnosis during pregnancy does not tell you a baby's size in advance, and an average size does not rule out a complicated pregnancy. The new intensive-care chart also now stretches down to 22 weeks, reflecting that more of the tiniest babies are surviving than in earlier decades [9].

What This Means for Families and Their Baby's Care

Practically, these two charts give hospitals better-matched tools. If your baby is healthy and in the regular nursery, the nursery chart is the more accurate ruler for the kinds of checks nurseries do — most importantly, deciding which babies need a blood-sugar test. If your baby needs intensive care, the NICU chart is built for that population, including the most premature babies. Better calibration means fewer babies wrongly labeled and fewer missed — which translates into more appropriate testing rather than more testing for its own sake.

Being labeled "small" or "large" is not a diagnosis in itself; it is a flag that tells the care team to look a little closer. A small baby may simply be constitutionally small, following healthy family patterns, or may need a check for causes such as problems with the placenta. A large baby may be big for entirely normal reasons or may need a blood-sugar check. Getting the label right means the right babies get that closer look, and babies who are genuinely average are spared tests they do not need. That is the quiet value of a well-calibrated chart: it points attention where it belongs.

There is an honest trade-off the researchers themselves highlight. Because the nursery chart was built by setting aside babies who already had low blood sugar, using it will tend to flag somewhat more babies for glucose screening. Whether that extra screening truly helps babies, or simply turns normal newborn variation into extra tests, is a question the authors say should be studied in future clinical trials [8]. It is a fair reminder that a better chart is a starting point for good care, not a guarantee by itself.

One more point families sometimes find reassuring: these birth charts describe size at the moment of birth. They are not the right tool for tracking how a premature baby grows over the following weeks — for that, teams use separate growth-tracking charts [10] and look at the rate of growth over time. Your baby's team will combine several tools, along with clinical judgment, rather than leaning on a single number.

What Researchers Are Working On Next

The next steps are to compare these new reference charts against other modern charts that describe "ideal" growth, and to study whether using the nursery chart actually improves outcomes without leading to unnecessary tests. The researchers are also interested in combining the weight, length, and body-mass charts to better estimate a newborn's body composition. For now, the take-home message for families is encouraging: the tools doctors use to judge whether a baby is small, average, or large have become more accurate and better matched to each baby's situation — a quiet but meaningful improvement in the very first assessment every newborn receives.

References

  1. Lubchenco LO, Hansman C, Dressler M, Boyd E. Intrauterine growth as estimated from liveborn birth-weight data at 24 to 42 weeks of gestation. Pediatrics. 1963;32:793–800. doi:10.1542/peds.32.5.793 ↩
  2. Battaglia FC, Lubchenco LO. A practical classification of newborn infants by weight and gestational age. J Pediatr. 1967;71:159–63. doi:10.1016/S0022-3476(67)80066-080066-0) ↩
  3. Olsen IE, Groveman SA, Lawson ML, Clark RH, Zemel BS. New intrauterine growth curves based on United States data. Pediatrics. 2010;125:e214–24. doi:10.1542/peds.2009-0913 ↩
  4. 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 ↩
  5. World Health Organization. WHO child growth standards: length/height-for-age, weight-for-age, weight-for-length, weight-for-height and body mass index-for-age: methods and development. Geneva: WHO; 2006. ↩
  6. Chou F-S, Yeh H-W, Hsueh C, Olsen IE, Tolia VN, Clark RH. Birth-size reference charts for newborns admitted to the neonatal intensive care units. BMC Pediatr. 2026;26(1):509. doi:10.1186/s12887-026-06889-1 ↩
  7. Zemel BS. From growth charts to growth status: how concepts of optimal growth and tempo influence the interpretation of growth measurements. Ann Hum Biol. 2023;50:236–46. doi:10.1080/03014460.2023.2189751 ↩
  8. Chou F-S, Yeh H-W, Hsueh C, Olsen IE, Tolia VN, Clark RH. Birth-size reference charts for normal nursery newborns between 35.0 and 41.6 weeks. BMC Pediatr. 2026. doi:10.1186/s12887-026-07275-7 ↩
  9. Edwards EM, Ehret DEY, Soll RF, Horbar JD. Survival of infants born at 22 to 25 weeks' gestation receiving care in the NICU: 2020–2022. Pediatrics. 2024;154:e2024065963. doi:10.1542/peds.2024-065963 ↩
  10. Chou F-S, Yeh H-W, Clark RH. A comparative study of postnatal anthropometric growth in very preterm infants and intrauterine growth. Nat Commun. 2023;14:5626. doi:10.1038/s41467-023-41069-0 ↩