Background: Obesity is a critical public health issue worldwide, with Latin America among the most aected regions. Early adulthood is a key period to prevent excess weight gain, particularly in women. Methods: We analyzed data from 7,465 women aged 18–25 in the Costa Rica HPV Vaccine Trial. Body Mass Index (BMI) classied overweight (25–29.9) and obesity (≥30). Prevalence was assessed among the same women at baseline (2004/2005) and follow-up (2010/2011). Logistic regression identied determinants of overweight/obesity and ≥20% weight gain. Results: Prevalence of overweight/obesity increased from 35% to 56% over ve years. In adjusted models, older age, lower education, early menarche (≤10years), irregular periods, pregnancies, injectable contraceptive use and smoking increased risk, while being single was protective. Coastal regions had higher prevalence than mountainous areas. Weight gain was associated with early menarche, new pregnancies, oral contraceptive use and marriage, whereas pregnancies before enrollment lowered risk.Conclusions: Rapid increases in overweight/obesity among young women underscore the inuence of social and reproductive factors. Interventions addressing educational and geographic dierences are needed. Further studies on diet and physical activity will strengthen preven-