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To compare classification of pre-pregnancy body mass index (BMI) using self-reported pre-pregnancy weight versus weight measured at the first prenatal visit. Retrospective cohort of 307 women receiving prenatal care at the faculty and resident obstetric clinics at a Massachusetts tertiary-care center. Eligible women initiated prenatal care prior to 14 weeks gestation and delivered singleton infants between April 2007 and March 2008. On average, self-reported weight was 4 pounds lighter than measured weight at the first prenatal visit (SD 7.2 pounds; range: 19 pounds lighter to 35 pounds heavier). Using self-reported pre-pregnancy weight to calculate pre-pregnancy BMI, 4.2% of women were underweight, 48.9% were normal weight, 25.4% were overweight, and 21.5% were obese. Using weight measured at first prenatal visit, these were 3.6, 45.3, 26.4, and 24.8%, respectively. Classification of pre-pregnancy BMI was concordant for 87% of women (weighted kappa = 0.86; 95% CI 0.81-0.90). Women gained an average of 32.1 pounds (SD 18.0 pounds) during pregnancy. Of the 13% of the sample with discrepant BMI classification, 74% gained within the same adherence category when comparing weight gain to Institute of Medicine recommendations. For the vast majority of women, self-reported pre-pregnancy weight and measured weight at first prenatal visit resulted in identical classification of pre-pregnancy BMI. In absence of measured pre-pregnancy weight, we recommend that providers calculate both values and discuss discrepancies with their pregnant patients, as significant weight loss or gain during the first trimester may indicate a need for additional oversight with potential intervention. Keywords Gestational weight gain * Pre-pregnancy BMI * Self-reported weight * Prenatal care, Introduction Numerous studies have identified significant associations between excessive and inadequate gestational weight gain (GWG) and adverse maternal and offspring health outcomes. Women who gain excessively are at increased risk [...]