The purpose of the study was to assess the influence of socioeconomic risk factors, food security, health, and key anthropometric measures on body mass index (BMI) in a group of teenagers from Medellin, Colombia, in the third trimester of pregnancy. A cross-sectional study was carried out with 294 pregnant teenagers. Data were analyzed using bivariate and multivariate logistic regression analysis. Pregnant teenagers whose families earned less than one minimum wage were more likely to have low weight (OR = 5.8; 95%CI: 1.97-16.8). Age under 15 years was associated with a fourfold increase in low gestational weight. Arm and calf circumference greater than 24cm and 32cm, respectively, were associated with a 94% reduction in low gestational weight (arm circumference: OR = 0.1; 95%CI: 0.0-0.2) (calf circumference: OR = 0.1; 95%CI: 0.0-0.2). In conclusion, low income and young age were associated with low gestational weight. Arm and calf circumference correlated with maternal weight.