PULLMAN, WA – When a team of scientists led by a Washington State University researcher examined the way that more than 9,400 children grew from toddlers to pre-adolescents, two distinct trajectories emerged.
For most children, their body-mass index developed along a typical path: a drop in BMI following their first year, followed by a gradual increase after age 6.
Eleven percent of the children, however, showed a very different pattern. These children — whose mothers were more likely to have smoked during or had high BMIs before pregnancy — showed no decline in BMI after the first year, and began to rapidly gain weight during their fourth year of life.
“The most distinct finding is that we can identify children on a path to obesity as early as age 3.5 years,” said Chang Liu, an assistant professor of psychology at WSU and the corresponding author of the new paper. “By age 9, children in this group reached an average BMI above the 99th percentile.”
The study, published last month in Jama Network Open, represents one of the first systemic characterizations of BMI trajectories during childhood, at a level of individual precision that previous research methods could not detect. Liu led a team of scientists that included nearly 40 researchers from institutions across the country, including Harvard, The Pennsylvania State University, the University of Washington and others.
The most distinct finding is that we can identify children on a path to obesity as early as age 3.5 years. By age 9, children in this group reached an average BMI above the 99th percentile.
Chang Liu, assistant professor of psychology
Washington State University
Researchers drew on longitudinal data between January 1997 and June 2024 from a national research program, Environmental influences on Child Health Outcomes (ECHO). Using an advanced statistical modeling approach, known as multiphase growth mixture modeling, they were able to identify individual differences in developmental patterns and specific points where those differences emerged.
They were also able to identify risk factors associated with the differences — suggesting possible interventions.
About a fifth of American children have obesity, which comes with significant negative health effects as well as making it more likely they will develop obesity in adulthood. There has been a growing understanding of a link to prenatal factors, but most research on this subject has relied on overall trends or group averages.
“For society, childhood obesity has enormous costs — both in terms of children’s immediate health and well-being, and the long-term healthcare burden as these children become adults with higher risks of diabetes, heart disease, and other chronic conditions,” Liu said. “If we can identify at-risk children by age 3.5 and intervene effectively, we could prevent a significant portion of childhood obesity and its lifelong consequences.”
The two distinct patterns of BMI trajectories emerged after the first year of life. Most children (89.4%) followed a typical developmental pattern — BMI levels that decline from age 1 to approximately age 6, followed by gradual increases. This “adiposity rebound” represents normal child development.
However, 11% of the children showed an atypical pattern with no initial BMI decline. These children — who were more likely to have mothers who had smoked during pregnancy or had high pre-pregnancy BMI, as well as those with greater gestational weight gain and higher birth weights — maintained stable BMI levels during early years, then experienced rapid increases starting from age 3.5 years. By age 9, their average BMI exceeded the 99th percentile.
While the study does not identify a causal explanation, there are indications from past research that prenatal smoking and maternal obesity can alter fetal metabolism and body fat development, potentially programming a child’s metabolic system in ways that persist after birth and may affect appetite, metabolism and fat storage throughout life.
“We need more research to further understand these biological mechanisms and how they interact with genetic factors and postnatal environment to determine a child’s growth trajectory,” Liu said.
Because the research identified key points of change for the growth trajectories of each child, it suggests potential points to take preventative action.
“A three-pronged approach could include: First, supporting women of reproductive age in achieving healthy weight and quitting smoking before pregnancy,” Liu said. “Second, ensuring appropriate weight gain during pregnancy. Third, implementing intensive monitoring and early intervention (e.g., healthy eating patterns and physical activity) for children showing non-declining BMI patterns, especially those with identified risk factors.”
