OBJECTIVE: To assess the association between maternal gestational weight gain (GWG) during the first 20 weeks of gestation and overweight/obesity and abdominal obesity of offspring at the age of 16 years. DESIGN: A prospective cohort study. SETTING: The two northernmost provinces of Finland. POPULATION: Mothers and their adolescent offspring born from singleton pregnancies (3265 boys; 3372 girls) in the Northern Finland Birth Cohort 1986. METHODS: Maternal weight at 20 weeks of gestation was measured in municipal maternity clinics. Maternal GWG was based on the difference between the measured weight and self-reported pre-pregnancy weight, and was classified into quartiles. Offspring weight, height and waist circumference were measured by study nurses during a clinical examination. Logistic regression analyses [with and without adjustment for maternal pre-pregnancy body mass index (BMI), glucose metabolism, education level, haemoglobin, smoking status, parity, and gender of offspring] were performed. MAIN OUTCOME MEASURE: Offspring overweight/obesity, based on BMI and abdominal obesity at 16 years. RESULTS: The highest quartile of maternal weight gain (>7.0 kg during the first 20 weeks of gestation) was independently associated with BMI-based overweight/obesity and abdominal obesity in the 16-year-old offspring (OR 1.46, 95% CI 1.16-1.83, and OR 1.37, 95% CI 1.10-1.72, respectively). Among all covariates, maternal pregravid obesity showed the highest odds for both overweight/obesity and abdominal obesity (OR 4.57, 95% CI 3.18-6.57, and OR 4.43, 95% CI 3.10-6.34, respectively). CONCLUSIONS: Maternal overnutrition during the first half of gestation predicts offspring overweight/obesity and abdominal obesity in adolescence, yet a high pregravid BMI appears to be a more important determinant of both outcomes.