Tiny Milestones guides

Baby growth charts and percentiles, explained for parents

A percentile tells you where your baby's measurement sits among children of the same age and sex: at the 40th percentile for length, your baby is longer than about 40 of every 100 babies and shorter than about 60. In the United States, doctors plot babies on the WHO growth standards until age 2 and on the CDC growth charts after that. Any single percentile matters less than the pattern across several visits.

What a percentile is

A growth chart is a set of percentile curves showing how a measurement, such as length or weight, is spread across children at each age. A percentile shows a child's position by the share of the reference population they equal or exceed¹. So a baby at the 75th percentile for weight weighs the same as or more than about three quarters of babies that age and sex. The 50th percentile is the middle, not a goal: half of healthy babies are below it, because that is what the middle means.

The charts' outer lines run from the 2nd to the 98th percentile². Between them is a wide range of ordinary sizes.

Where the charts come from

For babies and toddlers, the CDC recommends the WHO Child Growth Standards, and at age 2 it recommends switching to the CDC growth charts, which run to age 20². The WHO standards were built from the WHO's Multicentre Growth Reference Study, which measured children in six countries, including the United States, raised in conditions believed to support healthy growth; they describe how children grow under those conditions, rather than how children happen to grow in any one place³. The CDC recommends them for every child under 2 even though many children don't grow up in those conditions².

For children under 2, the WHO charts cover length-for-age, weight-for-age, weight-for-length, and head circumference-for-age, for girls and for boys². The CDC publishes the numbers behind them, month by month, as public files⁴.

Why one measurement says little

The CDC's own guidance for clinicians is that growth monitoring rests on a series of accurate measurements over time, and that the series is what gives an individual measurement its meaning². Three reasons from the same guidance:

• Measurements and ages have to be accurate for a chart to mean anything¹². A wriggling baby and a stretched tape can move a reading by a percentile band.
• The way of measuring changes at 2 years, from length lying down to height standing, and standing height reads about 0.8 cm (a quarter inch) less than lying length². Switching from the WHO to the CDC charts at the same time can change a child's percentile with no change in the child, and the CDC says to read any shift at that transition with caution².
• Feeding shapes the curve. The WHO standards are based on breastfed children, who gain weight more slowly than formula-fed babies after about 3 months, so a formula-fed baby may cross upward across the WHO percentiles².

What doctors look at

Doctors use growth charts to follow growth over time and to spot possible health or nutrition problems¹. The WHO's cutoffs sit at the 2nd and 98th percentiles: for a child under 2, length-for-age below the 2nd percentile is called short stature, and weight-for-length above the 98th is called high weight-for-length². Even then, a measurement is read together with the child's medical and family history, not on its own². If a number on your baby's chart worries you, that is the question to bring to the next visit.

Born early?

For development, pediatricians count a baby born more than 3 weeks early from the due date for the first 2 years. Our guide to adjusted age explains it; ask your pediatrician which age they use on your baby's growth chart.

In Tiny Milestones

Tiny Milestones draws length, weight, and head size the way families mark a door frame, a pencil line per measurement with the date beside it, and shows the WHO percentile for each one from the CDC's published tables, with the source named. A home measurement is a rough one, so the app treats it as part of a pattern, never as a verdict.

This is general information, not medical advice. Ask your baby's doctor about your own baby.