# Understanding Baby Growth Charts: WHO Percentiles Explained _What the number at your baby's check-up actually measures — and what it can't tell you._ - Source: https://proba.baby/en/blog/baby-growth-charts-who-percentiles - Published: 2026-09-05 - Author: ProbaBaby Team - Category: Baby & Postpartum - Tags: growth, newborn - Language: English > A percentile is a ranking, not a grade. Here's what baby growth chart percentiles really mean, why the US uses WHO curves under two years, why the shape of the line matters more than any single dot, and how prematurity changes the maths. > **Medical disclaimer.** This article explains how growth charts are built and read. It is general information, not medical advice, and it cannot assess your baby. If you are worried about your baby's weight, feeding, alertness or nappies, contact your pediatrician, health visitor or midwife — the same day if your baby seems unwell. Baby growth chart percentiles rank your baby against other babies of the same age and sex. That is all they do. The 30th percentile means roughly 30 healthy babies out of 100 are smaller and 70 are bigger. It is not a score, not a grade, and not a forecast of how tall your child will be. Half of all perfectly healthy babies sit below the 50th line, because that is how a middle works. What the number *is* useful for is comparison over time. One dot on a chart is a snapshot. The line those dots make is the actual clinical information. ## Where the curves come from The curves on your baby's chart are not averages someone eyeballed. They come from the [WHO Multicentre Growth Reference Study](https://www.who.int/tools/child-growth-standards/who-multicentre-growth-reference-study), which collected growth data from approximately 8,500 children in Brazil, Ghana, India, Norway, Oman and the USA between 1997 and 2003. WHO's stated aim was a single international standard describing physiological growth, and "to establish the breastfed infant as the normative model for growth and development." The word *standard* is doing the work there. The CDC's [2010 growth-chart recommendation](https://www.cdc.gov/mmwr/preview/mmwrhtml/rr5909a1.htm) draws the distinction plainly: "Whereas the WHO charts are growth standards, describing the growth of healthy children in optimal conditions, the CDC charts are a growth reference, describing how certain children grew in a particular place and time." The same document records that "100% of the reference population of infants were breastfed for 12 months and were predominantly breastfed for at least 4 months," and lists maternal smoking during pregnancy or lactation among the exclusion criteria. WHO publishes separate tables for boys and girls, and no combined table. Sex is not a nicety on a growth chart; it changes the curve. ## WHO under two, CDC after two If you are in the US, your baby's first two years are plotted on WHO curves and the years after that on CDC curves. That split comes from that same recommendation, which advises clinicians to "use the 2006 WHO international growth charts, rather than the CDC growth charts, for children aged <24 months," while "the CDC growth charts should continue to be used for the assessment of growth in persons aged 2–19 years." The reasoning matters for parents. The CDC's own charts are a reference — a record of how a particular group of children grew. And, in the CDC's words, "healthy breastfed infants typically gain weight faster than formula-fed infants in the first few months of life but then gain weight more slowly for the remainder of infancy." Read against a reference chart, that normal slowdown can look like faltering growth; the CDC warns that clinicians unfamiliar with the breastfed growth pattern "might inappropriately recommend that mothers supplement breastfeeding with formula." Read against the WHO standard, it looks like exactly what it is. In the UK, the [UK-WHO charts](https://www.rcpch.ac.uk/resources/uk-who-growth-charts-0-4-years) published by the RCPCH are likewise based on the WHO Child Growth Standards, and are described as suitable for moderately preterm infants born at 32 to 36 weeks. ## What "off the chart" actually means On WHO charts, the flag is 2 standard deviations either side of the median. The CDC explains that these cutoffs "correspond to the 2.3rd and 97.7th percentiles" and are printed on the WHO charts as [the 2nd and the 98th percentile](https://www.cdc.gov/growth-chart-training/hcp/using-growth-charts/who-using.html). A measurement below the 2nd or above the 98th is the point at which a clinician looks harder. It is a prompt to investigate, not a verdict. Note what that implies: a baby at the 4th percentile is inside the normal range. So is a baby at the 96th. Roughly one healthy baby in fifty will land below the 2nd line simply because the line has to have someone under it. ## The curve beats any single point The CDC is explicit that "growth monitoring is based on a series of accurate measurements over time," and that the series "accounts for both short- and long-term conditions and provides context for interpreting an individual measurement." The CDC also puts weight on technique: "accurate measurements and recording of information is critical for growth charts to be an effective screening tool." The three measurements are not equally easy to take: - **Weight** — a baby on a calibrated scale, with the same clothing off each time. - **Length** — under two it is measured lying down (recumbent length), which needs the baby held straight and both ends read. A squirming newborn is the obvious difficulty. - **Head circumference** — this one depends on the tape sitting at the widest point. A tape that slips produces a jump that looks alarming and is not real. Which is why the CDC's instruction is that "health care providers should interpret any changes in a child's percentile classification with caution." A single "she dropped from the 50th to the 25th" reading is a reason to measure again, not a verdict. The [NHS](https://www.nhs.uk/baby/babys-development/height-weight-and-reviews/baby-height-and-weight/) frames the same idea in plain terms: a baby's weight and height may not follow a centile line exactly and "may go up or down by 1 centile line, but it's less common for them to cross 2 centile lines" — and if that happens, talk to a health visitor. The NHS also notes that most babies are at, or above, their birthweight by 3 weeks. ## Three measurements tell three different stories The WHO charts the CDC recommends under two carry four indicators, not one: weight-for-age, length-for-age, weight-for-length and head-circumference-for-age. They answer different questions, and the interesting information often sits in the relationship between them. A baby at the 15th percentile for both weight and length is small and proportionate — the two measurements agree with each other. A baby at the 85th for length and the 10th for weight is a different picture, and weight-for-length is the chart that makes that difference visible. Head circumference is followed on its own curve, because it can move independently of the other two. Which pattern a particular baby is showing, and what it means, is a question for the person doing the measuring. ## Corrected age: plotting a preterm baby honestly A baby born at 32 weeks was eight weeks early — the [AAP](https://www.healthychildren.org/English/ages-stages/baby/preemie/Pages/Corrected-Age-For-Preemies.aspx) defines weeks preterm as gestational age at birth subtracted from 40. Plot her at 16 weeks of calendar age against other 16-week-olds and she will look tiny, because you are comparing her with babies who had eight more weeks of gestation. The fix is corrected age. The AAP puts it simply: "Begin with your baby's actual age in weeks (number of weeks since the date of birth) and then subtract the number of weeks your baby was preterm." At 16 weeks old, our 32-weeker has a corrected age of 8 weeks, and 8 weeks is the age she is compared against. The AAP says that during the first 2 years, using corrected age gives a better idea of when a baby should reach common developmental goals; how long a chart keeps correcting is a question for your clinician, so ask at the next visit. This comes up often for [twins and other multiples](/twins): the NHS defines premature labour as labour before the 37th week and notes that "twins and triplets are often born prematurely." Two babies on two curves is normal — both landing on the same curve was never the goal. ## When to ask, and when to wait Worth raising at the next visit: - Crossing two centile lines in either direction — the NHS says one line is common, two much less so, and to talk to a health visitor if it happens - A measurement below the 2nd or above the 98th percentile, the two cutoffs printed on the WHO charts - Birthweight not regained by 3 weeks, or a large weight loss — the NHS says a midwife or health visitor will support you in either case - Any percentile change you want explained. The NHS weighs a baby more often than the standard schedule "if you ask for it or if there are concerns about their health or growth" Worth contacting a clinician today, regardless of any percentile — the NHS lists these among the [signs of dehydration](https://www.nhs.uk/conditions/dehydration/) in a baby that need urgent advice: - Fewer wet nappies than usual - A soft spot on the head that sinks inwards (a sunken fontanelle) - Few or no tears when they cry - Being unusually drowsy or irritable The same goes for a baby who is refusing feeds or who simply seems unwell to you. Percentiles are a background instrument; behaviour is the foreground one. A thriving baby on the 5th percentile is doing better than a listless baby on the 60th. ## How ProbaBaby helps ProbaBaby's [baby tracking module](/postpartum) records weight, length and head circumference, and scores each one against WHO's own per-day L/M/S tables — generated from WHO's `anthro` reference data, covering days 0 to 730 for both sexes, with WHO's LMS transform and its documented tail adjustment for weight. Charts draw the P3, P15, P50, P85 and P97 curves, and lookups run on age in days, so a measurement taken on day 23 is scored against day 23 rather than rounded to a month. Because WHO publishes no combined table, the app requires the baby's sex rather than guessing one. For a baby born preterm, growth percentiles are looked up on corrected age until 24 months once gestational weeks at birth are recorded. When it is time to bring all of this to an appointment, the [pediatrician visit summary](/doctor-visit) exports as a PDF and shares by QR code, pulling growth, milestones, notes and mom wellbeing into one document. ## FAQ **What does it mean if my baby is in the 10th percentile?** It means that out of 100 healthy babies of the same age and sex, about 10 weigh less than yours and 90 weigh more. It is a description of size, not a diagnosis — a baby who stays near the 10th percentile and is feeding, weeing and developing well is growing normally. **Is a low percentile bad?** Not by itself. Half of all healthy babies fall below the 50th percentile by definition, and someone has to be at the 5th. What clinicians watch is the trend, because the CDC describes growth monitoring as based on a series of accurate measurements over time rather than on one reading. **Should my baby be on WHO or CDC growth charts?** In the US, the CDC recommends the 2006 WHO international growth charts for children under 24 months, and its own CDC charts for ages 2 and up. The CDC calls the WHO charts a growth standard — they describe the growth of healthy children in optimal conditions — which is why they suit infancy better. **Why did my baby drop percentiles?** Single measurements wobble, and the NHS says a baby's weight and height may go up or down by one centile line without that meaning anything. Crossing two centile lines is much less common, and that is the pattern to raise with your health visitor or pediatrician. **How do growth charts work for a premature baby?** You use corrected age. The AAP's method is to begin with your baby's actual age in weeks and subtract the number of weeks your baby was preterm. The AAP says corrected age gives a better idea of when a baby should reach common developmental goals during the first 2 years; ask your clinician how long they will keep correcting your baby's age. **Do breastfed and formula-fed babies grow differently on the charts?** Yes. The CDC states that healthy breastfed infants typically gain weight faster than formula-fed infants in the first few months of life, then more slowly for the rest of infancy. The WHO standards were built from a breastfed reference population, so that slowdown looks normal on them rather than like faltering growth. **How often should my baby be weighed?** The NHS advises that after the first 2 weeks a baby is weighed no more than once a month up to 6 months, no more than once every 2 months from 6 to 12 months, and no more than once every 3 months over the age of 1. It adds that a baby is weighed more often than this only if you ask for it, or if there are concerns about their health or growth. ## Sources - WHO — Multicentre Growth Reference Study (Child Growth Standards) — ~8,500 children, six countries, 1997–2003; breastfed infant as the normative model: https://www.who.int/tools/child-growth-standards/who-multicentre-growth-reference-study - CDC — Using WHO Growth Standard Charts — 2 SD cutoffs labelled as the 2nd and 98th percentiles; growth monitoring is a series of accurate measurements; interpret percentile changes with caution: https://www.cdc.gov/growth-chart-training/hcp/using-growth-charts/who-using.html - CDC MMWR — Use of WHO and CDC Growth Charts for Children Aged 0–59 Months in the United States — 2010;59(RR-9) — WHO charts under 24 months; standard vs reference; 100% breastfed reference population; breastfed growth pattern: https://www.cdc.gov/mmwr/preview/mmwrhtml/rr5909a1.htm - RCPCH — UK-WHO growth charts, 0–4 years — Based on WHO Child Growth Standards; suitable for moderately preterm infants (32–36 weeks): https://www.rcpch.ac.uk/resources/uk-who-growth-charts-0-4-years - AAP HealthyChildren.org — Corrected Age for Preemies — Actual age in weeks minus weeks preterm (40 − gestational age at birth); first 2 years for developmental goals: https://www.healthychildren.org/English/ages-stages/baby/preemie/Pages/Corrected-Age-For-Preemies.aspx - NHS — Your baby's weight and height — Centile lines, birthweight regained by 3 weeks, weighing frequency, when a baby is weighed more often: https://www.nhs.uk/baby/babys-development/height-weight-and-reviews/baby-height-and-weight/ - NHS — Premature labour and birth — Before the 37th week; twins and triplets are often born prematurely: https://www.nhs.uk/pregnancy/labour-and-birth/signs-of-labour/premature-labour-and-birth/ - NHS — Dehydration — Signs in babies: sunken fontanelle, fewer wet nappies, few or no tears, drowsy or irritable: https://www.nhs.uk/conditions/dehydration/ --- ProbaBaby (listed on the App Store as “Pregnancy & Baby Tracker - PB”) is a free iPhone, iPad and Apple Watch app for pregnancy, newborn and postpartum tracking, built for twins, in 32 in-app languages, with an optional AI baby-face prediction that computes Mendelian genetics (Punnett squares) from the traits you enter before any image is generated. Tracking, tools, PDF reports and family sharing are free; only AI predictions use credits — 5 starter credits cover your first Scientific prediction — and no subscription is required. Requires iOS 26 or later. Not available on Android.