Preterm babies have lower BMI in infancy but catch up later, study finds

A new multi-cohort study has found that, during infancy, individuals born preterm typically have a lower BMI (Body Mass Index, a measure of body fat based on height and weight) and a lower risk of overweight compared to their full-term peers, but that by adolescence this difference diminishes.

The study, led by a Copenhagen-based research team and published in PLOS Medicine, used data from infancy through adolescence from 16 cohort studies, Canada’s CHILD Cohort Study (CHILD) among them.

“We know that preterm birth, at 37 weeks or earlier, is the leading cause of illness and death in early infancy and that it is associated with various adverse developmental and long-term health outcomes,” comments co-author Dr. Stuart Turvey, Co-Director of CHILD.

“That is why it is important to identify the specific health impacts of preterm birth—like the impact on BMI and risk of overweight, as in this study: so we can implement early-life strategies to address these risks and help ensure the best future health for individuals born preterm.”

The collaborating researchers found that gestational age at birth may be important for body size in infancy, but that the strength of this association lessens with age. By adolescence, most preterm individuals have a similar BMI compared to their peers born at term.

However, the study also found—based on evidence from four cohorts whose participants had reached the age of adolescence—that individuals born very preterm (at 31 weeks or earlier) may have an increased risk of becoming overweight in adolescence and adulthood.

“This research sheds new light on these questions. Most previous investigations into this topic focused on the impact of birth weight on later BMI, rather than on the timing of birth, or gestational age,” notes co-author Dr. Padmaja Subbarao, Director of CHILD.

“This international study, in which CHILD was pleased to take part, looked strictly at how gestational age influences body size from infancy through adolescence, in data from a large sample size of over 250,000 mother-child pairs across three continents.”

In addition to CHILD, 13 cohorts located in Europe, one in Australia, and a second Canadian study were also involved. The authors observe that, because the study is based on data from high-income countries, the findings may not be generalisable to the contexts of low- and middle-income countries. They also point out that further research is needed to explore the potential influence of other mediating factors, such as size for gestational age, feeding practices, and puberty.

“It is fortunate that studies like CHILD and the others involved in this research have collected such a treasure trove of biological samples, and that many continue to follow their subjects and to collect data into adolescence and beyond,” adds Dr. Subbarao. “Eventually, this ongoing work will allow us to answer these unanswered questions and to examine the factors that have yet to be examined.”

It is important to identify the specific health impacts of preterm birth, so that we can implement early-life strategies to help ensure the best future health for individuals born preterm.
Stuart Turvey
CHILD Co-Director