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Abstract

Abstract Objectives To examine temporal trends in prepregnancy body mass index (BMI) from 2012 to 2022 in Ontario, Canada, and identify disparities across sociodemographic and medical condition subgroups. Methods We conducted a population-based descriptive study using data from the Better Outcomes Registry & Network (BORN) Ontario. The study included 1,170,997 singleton births between April 2012 and March 2022. Prepregnancy BMI was categorized according to World Health Organization classifications. We used generalized estimating equations to assess temporal trends in mean BMI and BMI categories, with sequential adjustments for socioeconomic and medical factors. Analyses were stratified by maternal age, parity, race/ethnicity, neighbourhood income, education, and material deprivation quantiles, smoking status, and pre-existing medical conditions. Results Mean prepregnancy BMI remained stable at 25.3 kg/m 2 from 2012 to 2016, then increased to 26.0 kg/m 2 by 2021. From 2012 to 2022, the proportion of individuals with underweight or normal weight BMI decreased (underweight 8.6% to 6.7%; normal weight 50.2% to 46.0%), while overweight and all obesity classes increased (overweight 20.4% to 22.4%; obesity class I 13.5% to 16.2%; obesity class II 3.7% to 4.6%; obesity class III 3.6% to 4.2%). After full adjustment, average BMI increased by 0.098 kg/m 2 (95% CI 0.094–0.101) annually. An inverse socioeconomic gradient was evident across neighbourhood education and income, and material deprivation quintiles, with individuals in lower quintiles exhibiting higher prepregnancy BMI compared with those in higher quintiles. Conclusion Prepregnancy BMI increased significantly in Ontario between 2012 and 2022, with acceleration from 2016 onward. Marked disparities across sociodemographic subgroups highlight the need for targeted preconception health interventions.