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.