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The effect of both birth order and number of siblings on overweight and/or obesity has not been determined. Birth order and sibsize have been mathematically coupled to overweight and/or obesity, but thus far their respective effects have been estimated separately.The aim of this systematic review was to evaluate the effects of both birth order and number of siblings on the risk of overweight/obesity.The electronic databases MEDLINE, Social Science, SocINDEX, PsycINFO, CINAHL Plus, and Academic Search Complete were searched systematically.Titles and abstracts of 1698 records were examined. After 1504 records were excluded, 2 authors independently assessed the full text of all remaining papers (n = 194); disagreements were resolved by discussion.A standardized form for assessment of study quality and evidence synthesis was used to extract data from the included studies.Twenty studies were included in the systematic review, 14 of which were included in the meta-analysis. Meta-analyses showed that lower (vs higher) birth order and smaller (vs greater) number of siblings were associated with overweight and/or obesity, with ORs of 1.47 (95%CI, 1.12–1.93) and 1.46 (95%CI, 1.17–1.84), respectively. However, among the 9 studies that attempted to separate the effects of birth order and number of siblings in the same analysis, a higher risk of overweight/obesity was consistently found among individuals without siblings than among those with 1 or more siblings, rather than among firstborns more generally.The results show that both lower birth order and lower number of siblings are associated with risk of overweight/obesity, which suggests that only children are at a slightly increased risk of overweight/obesity.PROSPERO registration number CRD42014015135.