AbstractObjectiveThis case‐cohort study estimated associations between gestational weight gain (GWG) and small‐for‐gestational‐age (SGA) and large‐for‐gestational‐age (LGA) births stratified by obesity class (I: 30–34.9 kg/m²; II: 35–39.9 kg/m²; III: ≥40 kg/m²) (Magee‐Womens Hospital, Pittsburgh, Pennsylvania, 1998–2011).MethodsFirst‐trimester GWG was categorized as being below (<0.2 kg), within (0.2–2.0 kg), or above (>2.0 kg) the Institute of Medicine recommendations. For second‐ and third‐trimester GWG, four linear trajectories were derived: approximating maintenance (slope −0.05 ± 0.03 kg/wk), approximating the recommendations (0.27 ± 0.01 kg/wk; reference), higher than the recommendations (0.54 ± 0.01 kg/wk), and highest among those above the recommendations (0.91 ± 0.02 kg/wk).ResultsFor classes I, II, and III, respectively, there were 1290, 1247, and 1198 pregnancies in the subcohort; 262, 171, and 123 SGA cases; and 353, 286, and 257 LGA cases. First‐trimester GWG was not associated with SGA/LGA births. Second‐ and third‐trimester weight maintenance was associated with potentially lower LGA risk (risk ratio [RR]: 0.80; 95% confidence interval [CI]: 0.55–1.1) but not higher SGA risk (RR: 0.98; 95% CI: 0.64–1.5) for class III. In addition, some sensitivity analyses supported no increased SGA risk with second‐ and third‐trimester weight maintenance for classes I and II.ConclusionsSecond‐ and third‐trimester weight maintenance may be associated with more optimal birth weight for gestational age. However, how this could be achieved (e.g., through diet and exercise interventions) is unclear, given the observational design of our study.