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Baby percentiles, explained

Work it out for your family: Baby percentile calculator. Weight, length and head percentiles on the WHO growth charts US pediatricians use.

The short answer

  • A percentile compares your baby with healthy babies of the same age and sex. At the 30th percentile for weight, 30 in 100 weigh less and 70 weigh more [2].
  • It isn’t a grade. A baby at the 5th percentile can be just as healthy as one at the 95th [2].
  • The trend matters more than any one number. Pediatricians look at growth over several visits, alongside your baby’s health and family history [3].

Somewhere, a parent is comparing percentiles in a group chat. You don’t have to.

What a percentile means

Think of 100 healthy babies, same age and sex, lined up by weight. The percentile is your baby’s place in the line [2]. Higher means bigger, not better.

Doctors read the chart alongside your family’s build, whether your baby was born early, and how they’re doing overall [2].

Why US pediatricians use the WHO charts under 2

The CDC recommends the World Health Organization (WHO) growth standards for all US children under 24 months, however they’re fed [3]. They describe how healthy children should grow in good conditions, and they use the healthy breastfed baby as the standard [3]. From age 2, pediatricians switch to the CDC’s own charts [3].

Our percentile calculator uses the WHO data the CDC publishes [4].

The 2nd and 98th lines

On the WHO charts, the CDC recommends screening lines 2 standard deviations either side of the middle. Those sit at the 2.3rd and 97.7th percentiles, and the charts label them the 2nd and 98th [1][3][4].

  • Below the 2nd for weight-for-length can mean low weight for length. Above the 98th can mean high weight for length [1].
  • Below the 2nd for length-for-age can mean short stature [1].

Outside those lines is a reason to look more closely. It isn’t a diagnosis. The CDC itself calls the cut-offs “arbitrary and not based on health outcomes” [3].

A baby who stays near the same line over time is following their own pattern. That’s a good sign, whether the line is the 10th or the 90th [2].

Doctors also check that weight and length are in proportion. A baby at the 85th for both is usually just a bigger baby. A big gap between the two is worth a closer look [2].

One measurement is a snapshot. The CDC says to use growth patterns over time, with several data points, together with medical and family history [3].

Crossing percentile lines

A baby who moves across lines can be fine, or can need a closer look.

  • Shifts are common in infancy. Growth rates vary more from baby to baby at this age, so a change in percentile doesn’t always mean a problem [2].
  • A move away from a pattern your baby has been following is what gets a pediatrician’s attention [2].

If your baby has dropped across lines and you’re also worried about feeding, see our low milk supply guide. It covers the diaper and weight signs that mean call now.

Breastfed babies grow differently

Healthy breastfed babies typically gain weight faster than formula-fed babies in the first few months, then more slowly for the rest of infancy. The CDC says slower growth among breastfed babies from 3 to 18 months is normal [3].

The flip side: on the WHO charts, formula-fed babies may cross upward in percentiles after about 3 months [3].

Babies born early

For babies born before 37 weeks, growth is usually plotted by corrected age (also called adjusted age) until age 2. Corrected age is your baby’s age minus the weeks they were born early. For babies who weighed under 1,000 grams (about 2.2 pounds) at birth, it’s often used until age 3. That rule comes from a federal Maternal and Child Health Bureau training module, written for the older CDC charts [5].

The CDC has not evaluated the WHO charts for babies born early or at very low birth weight [6]. Before your baby’s due date, the WHO charts don’t apply: hospital teams use charts made for preterm babies. Ask your pediatrician which chart and which age they use.

Why home measurements vary

Clinic numbers and home numbers often disagree. That’s mostly method, not your baby.

  • Length is hard. Clinics lay the baby flat on a measuring board with a fixed headboard and a sliding footboard. Two people do it: one holds the head, one straightens the knees and holds the feet flat [7]. A tape measure on a wriggling baby can’t match that.
  • Standing and lying aren’t the same. Standing height is about 0.7 cm less than lying-down length [7].
  • Scales differ. Clinics weigh babies undressed, or in a clean diaper on a scale zeroed for that diaper [7]. Clothes, a full diaper, or a feed just before can all add weight.

Small errors move the number more than you’d think. At 4 months, a length 1 cm (under half an inch) above the middle moves a baby from the 50th to about the 68th percentile, for boys and girls alike. That’s our arithmetic from the WHO tables [4], not a published figure.

For the calculator, use your pediatrician’s numbers when you can. If you weigh at home, use the same scale, at the same time of day, and watch the trend.

Sources

  1. CDC, “Using the WHO Growth Charts” (growth chart training). cdc.gov
  2. Nemours KidsHealth, “Growth Charts.” kidshealth.org
  3. Grummer-Strawn LM, Reinold C, Krebs NF, “Use of World Health Organization and CDC Growth Charts for Children Aged 0–59 Months in the United States,” MMWR Recommendations and Reports 2010;59(RR-9). cdc.gov
  4. CDC, “WHO Growth Charts: Data Files.” cdc.gov, and World Health Organization, “Child Growth Standards.” who.int
  5. Maternal and Child Health Bureau (HRSA), training module “Using corrected age to plot measures for low birthweight and premature infants.” washington.edu
  6. CDC, Growth Charts FAQ (archived 2016): “The CDC has not evaluated the use of the WHO growth charts in premature or very low-birth weight infants.” web.archive.org
  7. American Academy of Pediatrics, “Anthropometric Measurements.” aap.org

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