Aims/hypothesis Offspring of mothers with diabetes are at increased risk of metabolic disorders in later life. Increased offspring BMI is a plausible mediator. We performed a systematic review and meta-analysis of studies examining offspring BMI z score in childhood in relation to maternal diabetes. Methods Papers reporting BMI z scores for offspring of diabetic (all types, and pre-and during-pregnancy onset) and non-diabetic mothers were included. Citations were identified in PubMed; bibliographies of relevant articles were hand-searched and authors contacted for additional data where necessary. We compared offspring BMI z score with and without adjustment for maternal pre-pregnancy BMI. We performed fixed effect meta-analysis except where significant heterogeneity called for use of a random effects analysis. Results Data were available from nine studies. In the diabetic group unadjusted mean offspring BMI z score was 0.28 higher (all diabetic mothers vs controls (95% CI 0.09, 0.47; p=0.004; nine studies; offspring of diabetic mothers n=927, controls n=26,384) and with adjustment for maternal prepregnancy BMI, 0.07 higher (95% CI −0.15, 0.28; p=0.54; three studies; offspring of diabetic mothers n=244, controls n=11,206). There was no evidence of a difference in offspring BMI z score in relation to type of diabetes (gestational vs type 1, p=0.95).Conclusions/interpretation Maternal diabetes is associated with increased offspring BMI z score, although this is no longer apparent after adjustment for maternal prepregnancy BMI in the limited number of studies in which this is reported. Causal mediators of the effect of maternal diabetes on offspring outcomes remain to be established; we recommend that future research includes adjustment for maternal pre-pregnancy BMI.