Objective: To investigate potential risk factors for small-for-gestational-age (SGA) among nulliparous and parous women with a history of gastric bypass. Design: Population-based cohort study. Setting: Nationwide, Sweden. Population: Singleton pregnancies in women with a history of gastric bypass, delivery year: 2014-2021 (n=8155). Methods: Risk factors of interest were surgery-to-conception interval and surgery-to-conception weight change in kg, early-pregnancy BMI, pregnancy weight gain (gestational age-specific z-scores), maternal age, Nordic born, smoking during pregnancy, and education level. Adjusted risk ratios (aRR) of SGA with 95% confidence intervals (CI) by parity were estimated using logistic regression with marginal standardization. Main Outcome Measure: SGA (<10th percentile). Results: The risk of SGA birth was 13% overall, 19.4% in nulliparous, and 9.8% in parous pregnancies. Increased SGA risk was observed in women who entered pregnancy with BMI 18.5-24.9 vs. BMI 25-29.9 (nulliparous: absolute risk 27.3% vs. 19.2%, aRR [95%CI] 1.38 [1.15, 1.66]; parous: 13.6% vs. 9.4%, 1.39 [1.13, 1.71]), had low vs. adequate pregnancy weight gain (nulliparous: 24.1% vs. 18.0%, 1.26 [1.01, 1.57]; parous: 14.1% vs 7.5%, 1.90 [1.52, 2.38]), or smoking vs. non-smoking during pregnancy (nulliparous: 32.6% vs. 17.2%, 1.85 [1.56, 2.21]; parous: 18.9% vs. 8.0%, 2.34 [1.95, 2.80]). No associations were found between surgery-to-conception interval/weight change, being Nordic born, or education level and having an SGA infant. Conclusion: Women who underwent gastric bypass and either entered pregnancy with a BMI 18.5-24.9, had low weight gain, or who smoked during pregnancy are at higher risk of delivering an SGA infant and therefore may require closer monitoring.