Objective The objectives of this study were to examine differences in body mass index z-score (BMIZ) trajectory during early childhood among children with a younger sibling compared with those without and to test potential mediators. Methods This longitudinal cohort study included 6050 participants of the Early Childhood Longitudinal Study, Birth Cohort (ECLS-B). Focal children's weight, height, sibship status, screen time, active play time, family dinner frequency, and diet quality were assessed at 9 months, 24 months, preschool, and kindergarten when available. A piecewise linear regression model was used to examine the association between sibling birth and focal child's subsequent BMIZ trajectory to kindergarten. Mediation by screen time, active play time, family dinner frequency, and diet quality was tested. Results BMIZ trajectory was lower among children who had a new sibling join the family before kindergarten compared to children who did not have a new sibling join the family by kindergarten. The association was strongest when new sibship occurred when the focal child was 48 to 66 months (b = –0.026, P = .044). The association was not mediated by screen time, active play time, family dinner frequency, or diet quality. Conclusion Among a nationally representative cohort of US children, new sibship before kindergarten was associated with a lower BMIZ trajectory. Several common obesogenic risk factors did not explain the association.
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