Your baby has a percentile now. They don’t know, and they don’t care.
What a percentile means
A percentile compares your baby with healthy babies of the same age and sex. If your baby’s weight is at the 30th percentile, 30 in 100 healthy babies that age weigh less, and 70 weigh more. A low percentile isn’t a bad grade. A baby at the 10th percentile can be perfectly healthy, as long as they keep growing along their own curve.
Why US pediatricians use the WHO charts until age 2
The CDC recommends the World Health Organization’s growth standards for children under 2 in the United States. The WHO charts describe how healthy babies grow in good conditions, with breastfeeding as the norm. From age 2, US pediatricians switch to the CDC’s own charts.
The lines your pediatrician watches
On the WHO charts, the CDC recommends screening lines 2 standard deviations either side of the median. Those lines sit at the 2.3rd and 97.7th percentiles, and the charts label them the 2nd and 98th. A measurement outside them is a reason to look more closely, not a diagnosis. The CDC notes that these cut-offs are arbitrary and not based on health outcomes.
What usually matters more is the trend: a baby who crosses several percentile lines over a few visits, up or down. Breastfed babies often gain weight faster than formula-fed babies in the first few months, then more slowly. The CDC says slower growth among breastfed babies from 3 to 18 months is normal.
Babies born early
For babies born before 37 weeks, many pediatricians plot growth by adjusted age (also called corrected age) until age 2. Adjusted age is your baby’s age minus the weeks they were born early. A US federal training module on growth charts gives this rule. For babies who weighed under 1,000 grams at birth, adjusted age is often used until age 3. The calculator uses adjusted age when you enter a due date.
The CDC has not evaluated the WHO charts for babies born early or with very low birth weight. Before your baby reaches their 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.
Sources
- CDC, “WHO Growth Charts: Data files.” cdc.gov
- 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. cdc.gov
- CDC, “Using the WHO Growth Charts.” cdc.gov
- World Health Organization, “Child Growth Standards.” who.int
- Maternal and Child Health Bureau (HRSA) training module, “Using corrected age to plot measures for low birthweight and premature infants.” washington.edu
- 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