In 1892 the anthropologist Franz Boas saw that following the same children over time would teach more about growth. His idea was mostly ignored for about 40 years. In 1950 researchers published standards using data from the Harvard School of Public Health, covering children from birth to 18 years old, and those standards stayed in use for about thirty years.

In 1977 the National Center for Health Statistics published new growth charts for U.S. children. Before the late 1970s, clinicians used a mix of different growth charts. People had worries about the 1977 charts, such as that their infant sample was almost all formula fed and that the length and height charts did not line up. The U.S. charts were revised and released again in 2000, using new national survey data, and most of the data for the 2000 CDC charts came from the National Health and Nutrition Examination Survey, which has measured heights and weights of Americans since the early 1960s.

The Fels Longitudinal Study began in 1929 in Yellow Springs, Ohio, and as of November 2026 it is described as the longest continuous study of human growth and development in the world. Several long studies of how children grow began around the same time in the early 20th century in the United States, and of them, the Fels Longitudinal Study is the only one that has run continuously up to the present, as of November 2026. To find out how kids grow, scientists follow the same children over time, and this is called a longitudinal study.

In 1997 the World Health Organization began a study to measure how children grow around the world, and it released new growth charts in 2006. The WHO Multicentre Growth Reference Study ran from 1997 to 2003, and it collected growth information from about 8,500 children in Brazil, Ghana, India, Norway, Oman and the United States, to make growth curves for infants and young children around the world. The WHO study only included children growing in good conditions, with a mother who followed feeding recommendations and did not smoke, a single term birth, and no major illness.

The WHO study had two parts. One followed babies from birth to 24 months, and the other measured different children from 18 to 71 months old at one point in time, which is called a cross-sectional design. In the part that followed babies, mothers and newborns were enrolled at birth and visited at home 21 times, at weeks 1, 2, 4 and 6, monthly from 2 to 12 months, and every 2 months in the second year.

The WHO charts were built from measurements taken at frequent intervals, following the same children over time. The CDC reference charts use measurements from different children at one time. The WHO charts are a growth standard, which shows how children should grow in good conditions. The 2000 CDC charts are a growth reference, which shows how certain children did grow in one place and time. Because the WHO study followed children over time, it could make growth velocity standards, which show how fast a child should be growing.

A separate set of measurements looked at babies. When researchers measured infants' length weekly, twice weekly or daily during their first 21 months, they found that growth came in bursts of 0.5 to 2.5 centimeters, separated by stretches of 2 to 63 days with no measurable growth. Babies grow fast in their first year, and on average a baby grows about 10 inches, or 25 centimeters, longer. Nobody grows at a perfectly steady rate, because kids grow in spurts, with faster stretches and slower stretches.

Doctors measure how kids grow and mark the numbers on a growth chart to follow growth over time. Growth charts show where a child's numbers fall compared with other kids of the same age, and percentile lines on the chart show this. The WHO study found that children around the world grow similarly when their needs for health and care are met.

Words to know

• anthropologist: human scientist
• longitudinal: over time
• standard: ideal growth
• reference: past growth