The Chart on the Wall May Have Been Wrong All Along

What a Major New Study Tells Us About How Premature Babies Grow — And Why It Matters

The Growth Chart That Follows Your Baby Everywhere

If your baby has spent time in a NICU, you've probably seen the growth chart. It hangs near the isolette, gets printed in weekly summaries, and comes up in nearly every conversation with the care team. Doctors and nurses plot your baby's weight, length, and head circumference on it regularly, watching to see whether the lines track where they're "supposed" to go.

What most families — and even many providers — don't realize is where that chart came from. The most widely used version today, called the Fenton growth chart, was built from measurements taken from babies at the moment of birth across a range of gestational ages. Researchers in 1963 first published this type of chart — plotting birthweights week by week throughout pregnancy to estimate what growth looks like inside the womb [1]. The idea was that the size of babies born at each gestational age could stand in as a picture of what fetuses look like at that same stage. Later researchers refined and updated these charts, producing the 2013 Fenton chart [2] and, most recently, an updated third-generation version published in 2025 using data from 4.8 million births across 15 countries [3].

Despite those updates, every version of the Fenton chart is still built on the same foundational logic: birthweight data as a window into the womb. And because of that, the growth targets it sets are intrauterine targets — what a fetus looks like inside the womb — not postnatal targets — what a premature baby actually achieves on the outside.

A Half-Century Goal That Was Always Out of Reach

In 1977, the American Academy of Pediatrics issued guidance that shaped neonatal nutrition for the next fifty years: premature babies should be fed so that their growth rate approximates the rate of a fetus still in the womb [4]. This was a reasonable-sounding goal. If womb growth was what healthy development looked like, then matching it should give premature babies the best chance.

The problem was that the data never supported it. By 2003, researchers studying NICU outcomes found that 89% of the most premature babies had growth measurements below the 10th percentile on womb-based charts by the time they reached their original due date — even in well-run, well-fed NICUs [5]. A decade later, a major analysis of more than 100,000 premature babies born between 2000 and 2013 showed that despite real improvements in NICU nutrition over that time, weight gain still fell short of womb-growth rates year after year [6].

In parallel, a 1999 landmark study by Ehrenkranz and colleagues had created separate growth charts based on what premature babies actually did during NICU hospitalization — tracking their real measurements over time rather than comparing them to birthweight data [7]. And in 2006, the same team showed that how fast premature babies actually grew during their NICU stay was strongly linked to their brain development and growth at 18–22 months old [8]. So growth still mattered — but the reference point for measuring it was increasingly in question.

Some researchers had already raised the alarm directly. A 2014 scientific paper asked plainly: "Is intrauterine growth appropriate to monitor postnatal growth of preterm neonates?" — and concluded that the biological differences between womb life and NICU life make direct comparison fundamentally problematic [9]. Another international effort, the INTERGROWTH-21st project, tried in 2015 to create postnatal growth standards for preterm babies by tracking carefully selected healthy premature infants over time rather than relying on birthweight snapshots [10]. But its enrollment criteria excluded sick infants — the very babies most likely to be in a NICU — limiting how directly it could be applied in clinical care.

The Biggest Study Yet: What 89,000 Babies Showed

A 2023 study published in the journal Nature Communications — one of the most prestigious science journals in the world — resolved this debate with the clearest evidence to date [11]. Dr. Fu-Sheng Chou and his colleagues analyzed nearly 5.5 million weight measurements, 750,000 length measurements, and 1.3 million head circumference measurements from 89,218 premature babies born before 32 weeks of pregnancy between 2010 and 2020 at hospitals across the United States.

When they plotted all of that real, modern data onto the standard Fenton womb-growth chart, 70–90% of measurements fell below what the chart called "average," and between a quarter and half fell below the threshold typically used to flag serious growth concerns. This wasn't because these babies had poor care or poor nutrition. It was because the chart was built for a different biological context entirely.

The study identified three chapters of premature baby growth that have no equivalent inside the womb:

Chapter 1: The Weight Loss. In the first days after birth, all premature babies lose weight — universally, across every gestational age and every birth size. The lowest point typically comes around day four, with about 5–8% of birth weight lost. Most babies regain their birth weight by day eight to ten [11]. Since fetuses don't lose weight in the womb, womb-based charts start misrepresenting the baby from the very first day of life.

Chapter 2: The Growth Surge. Once birth weight is regained, premature babies grow rapidly in proportion to their body size — smaller babies growing faster relative to their weight than bigger ones. This continues until about 34 weeks of postmenstrual age (the equivalent of what would have been 34 weeks of pregnancy). Despite impressive gains averaging roughly 15–19 grams per kilogram of body weight per day, even the best-growing premature infants in this phase never matched the peak growth rate seen inside the womb [11].

Chapter 3: Steady Cruising. After 34 weeks postmenstrual age, growth shifts to a steady daily rhythm — no longer proportional to body size, but a consistent number of grams per day, with larger babies and male babies gaining slightly more [11]. Womb growth actually slows down during this same window as full-term delivery approaches. Premature babies don't slow down — they keep going at their own steady pace.

For length, growth started slow and picked up speed — the opposite of the womb pattern. For head circumference, the first two weeks showed almost no growth, followed by a remarkable surge that eventually surpassed womb-growth rates [11].

The same research team later extended this work to babies born at 30–34 weeks of pregnancy, confirming that the same postnatal growth pattern holds across a wider range of premature infants [12].

What It Means for Your Baby — and for Families Everywhere

This research doesn't mean premature babies have been receiving poor care. What it reveals is that the measuring stick used to evaluate their growth was calibrated to the wrong biology. Many babies labeled as "growth restricted" or "falling behind" on womb-based charts may have been growing exactly as premature babies do — and families may have received unnecessary worry because of it.

The researchers made their new postnatal growth charts available for free through a tool called nicugrowth.app, which clinicians can use to track babies against what premature infants actually do, not what fetuses do. Poor growth on these postnatal charts still matters — we know from earlier research [8] that growth during the NICU stay is linked to brain development later on. The goal isn't to lower expectations; it's to set the right expectations, grounded in postnatal biology, and then identify babies who are truly falling behind that realistic standard.

If your baby is in the NICU, it is completely reasonable to ask: "Are we comparing my baby's growth to womb norms or postnatal norms?" The science now makes that a meaningful question — and the answer may reframe much of what the growth chart has been telling you.

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.
  2. 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
  3. Fenton TR, Elmrayed S, Alshaikh BN. Fenton third-generation growth charts of preterm infants without abnormal fetal growth: a systematic review and meta-analysis. Paediatr Perinat Epidemiol. 2025. doi:10.1111/ppe.70035
  4. American Academy of Pediatrics, Committee on Nutrition. Nutritional needs of low-birth-weight infants. Pediatrics. 1977;60:519–530.
  5. Clark RH, Thomas P, Peabody J. Extrauterine growth restriction remains a serious problem in prematurely born neonates. Pediatrics. 2003;111:986–990. doi:10.1542/peds.111.5.986
  6. Horbar JD, Ehrenkranz RA, Badger GJ, et al. Weight growth velocity and postnatal growth failure in infants 501 to 1500 grams: 2000–2013. Pediatrics. 2015;136:e84–e92. doi:10.1542/peds.2015-0129
  7. Ehrenkranz RA, Younes N, Lemons JA, et al. Longitudinal growth of hospitalized very low birth weight infants. Pediatrics. 1999;104:280–289. doi:10.1542/peds.104.2.280
  8. Ehrenkranz RA, Dusick AM, Vohr BR, et al. Growth in the neonatal intensive care unit influences neurodevelopmental and growth outcomes of extremely low birth weight infants. Pediatrics. 2006;117:1253–1261. doi:10.1542/peds.2005-1368
  9. Pereira-da-Silva L, Virella D. Is intrauterine growth appropriate to monitor postnatal growth of preterm neonates? BMC Pediatr. 2014;14:14. doi:10.1186/1471-2431-14-14
  10. Villar J, Giuliani F, Bhutta ZA, et al. Postnatal growth standards for preterm infants: the Preterm Postnatal Follow-up Study of the INTERGROWTH-21st Project. Lancet Glob Health. 2015;3:e681–e691. doi:10.1016/S2214-109X(15)00163-1
  11. 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
  12. Chou F-S, Yeh H-W, Clark RH. Postnatal weight growth trajectory in infants born between 30 4/7 weeks and 34 3/7 weeks of gestation. J Perinatol. 2024. doi:10.1038/s41372-023-01837-8