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Objectives This article aimed to provide a synthesis of studies evaluating the effects of maternal overweight and obesity, including body mass index (BMI) and other measures of adiposity, on the concentrations of macronutrients (fat, protein and lactose) in human milk (HM). Methods EMBASE, MEDLINE/PubMed, Cochrane Library, Scopus, Web of Science and ProQuest databases were searched for relevant articles. Two authors conducted screening, data extraction and quality assessment independently. Meta-analyses of eligible studies were conducted using Review Manager software version 5.3. Results A total of 31 studies (5078 lactating women) were included in the qualitative synthesis and 9 studies (872 lactating women) in the quantitative synthesis. The meta-analysis indicated that maternal overweight and obesity were associated with higher concentrations of fat in mature HM (p = 0.01) and lactose in colostrum (p = 0.002). While the qualitative analyses broadly supported the findings of the meta-analysis, the qualitative assessment identified considerable variability in the results between studies and low quality of many of the included studies, making it difficult to draw robust conclusions. Conclusions Overall, maternal BMI and adiposity measurements were associated with differences in the concentrations of fat and lactose in HM, however the direction of change was dependent on the stage of lactation, whereas protein concentration in HM did not appear to differ between overweight and/or obese and normal weight women. This is particularly relevant considering potential implications of higher HM fat concentration on both growth and fat deposition during the first few months of infancy and long-term risk of obesity. Funding Sources GEL was supported by a FOODplus Early Life Nutrition Scholarship, The University of Adelaide. BSM received a Career Development Award from the National Health and Medical Research Council of Australia (NHMRC).