This study examined the longitudinal association between birth of a sibling and changes in body mass index z-score (BMIz) trajectory during the first 6 years of life.


Children (n = 697) were recruited across 10 sites in the United States at the time of birth. Sibship composition was assessed every 3 months. Anthropometry was completed when the child was age 15 months, 24 months, 36 months, 54 months, and in first grade. Children were classified based on the timing of their sibling’s birth. A piecewise quadratic regression model adjusted for potential confounders examined the association of the birth of a sibling with subsequent BMIz trajectory.


Children whose sibling was born when they were 24 to 36 months or 36 to 54 months old, compared with children who did not experience the birth of a sibling by first grade, had a lower subsequent BMIz trajectory and a significantly lower BMIz at first grade (0.27 vs 0.51, P value = 0.04 and 0.26 vs 0.51, P value = 0.03, respectively). Children who did not experience the birth of a sibling by the time they were in first grade had 2.94 greater odds of obesity (P value = 0.046) at first grade compared with children who experienced the birth of a sibling when they were between 36 to 54 months old.


A birth of a sibling when the child is 24 to 54 months old is associated with a healthier BMIz trajectory. Identifying the underlying mechanism of association can help inform intervention programs.

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