Background and objectives: By age 5, 20% of British children are classed as overweight or obese, suggesting that early childhood is crucial for lifelong body mass index (BMI) trajectories. In this paper, we identify latent trajectories of early-childhood BMI from ages 3 to 11 years. Given the current context of growing socio-economic inequalities in childhood and adult overweight and obesity, we examine the socio-economic characteristics and mechanisms during pregnancy and infancy which underscore these trajectories.
Subject and methods: We use a nationally representative, prospective cohort study of 9699 children born in 2000-2002, living in the United Kingdom shortly after birth, with complete information on height and weight (measured by an interviewer) at ages 3, 5, 7 and 11. Trajectories of BMI are calculated using latent growth mixture models. Multinomial models characterize these trajectories by their socio-economic profiles and mechanisms during pregnancy and infancy.
Results: Four trajectories were identified: two separate trajectories where BMI remains within a normal range (85% of the sample), an overweight trajectory (14.4%), and an obese trajectory (3.1%). No 'declining BMI' or late-onset groups were found. The obese group is already distinct from the other trajectories by age 3. The overweight group diverges from the normal groups around age 5. Strong socio-economic inequalities emerged; for the obese group, part of this disadvantage is mediated through early mechanisms such as pregnancy smoke and not initiating breastfeeding.
Conclusions: This study provides strong evidence for the idea that childhood BMI trajectories develop early, especially for children who will follow an obese trajectory. Strong socio-economic patterns in these trajectories suggest that the observed trend in growing inequalities may be rooted in early life. Mediating mechanisms for the obese appear to be in the pregnancy and infant period, further research should explore mechanisms occurring around age 5 when the overweight trajectory diverges.