Objective:The purpose of this study was to examine the extent to which blood glucose,obesity,and maternal weight gain explains differences in birth weight using offspring sibling pairs in gestational diabetes mellitus (GDM). Study design:A retrospective analysis of 90 women with at least 2 GDM pregnancies was conducted. A fixed effects model was used to examine differences between siblings of the same mother (within-women),and results were contrasted with a multivariable regression model that compared different mothers (between-women). Results:Higher maternal weight gain was significantly associated with increased birth weight within mothers,but not between different women. Conversely,overweight status (body mass index BMI ≥25) was significant between-mothers,but not within an individual mother’s pregnancies. One-hour postprandial glucose was significant between-mothers,with a weaker association within-mothers. There was no association between fasting glucose and birth weight in either analysis. Conclusion:Controlling pregnancy weight gain may reduce offspring birth weight in individual women with GDM,while the association between high birth weight and elevated prepregnancy BMI may represent a predisposition to both characteristics. Objective: The purpose of this study was to examine the extent to which blood glucose, obesity, and maternal weight gain explains differences in birth weight using offspring sibling pairs in gestational diabetes mellitus (GDM). Study design: A retrospective analysis of 90 women with at least 2 GDM pregnancies was conducted. A fixed effects model was used to examine differences between siblings of the same mother (within-women), and results were contrasted with a multivariable regression model that compares different mothers (between-women). Results: Conversely, overweight status (body mass index BMI ≥ 25) was significant between-mothers, but not within an individual mother’s pregnancies. One-hour postprandial glucose was significant between-mothers, with a weaker association within-mothers. There was no association between fasting glucose and birth weight in eith er analysis. Conclusion: Controlling pregnancy weight gain may reduce offspring birth weight in individual women with GDM, while the association between high birth weight and elevated prepregnancy BMI may represent a predisposition to both characteristics.