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Nutrition in utero and during early infancy is critical for normal development and cognition. It is still unclear whether less drastic undernutrition that affects gestational weight gain (GWG), but does not necessarily cause weight loss, is related to offspring long-term cognition. This study was designed to examine the association of GWG with a child’s School Entry Assessment (SEA) scores at age 4 years, IQ at age 8 years, and school final examination results at age 16 years.The Avon Longitudinal Study of Parents and Children is a prospective, population-based birth cohort study that recruited 14,541 pregnant women with expected delivery dates between April 1991 and December 1992. Analyses included only women with complete GWG data (n = 12,484) and with term deliveries (n = 12,449). Maternal GWG was determined as a measure of “less than recommended,” “as recommended,” and “more than recommended,” based on the 2009 Institute of Medicine recommendations, in which the recommended weight gain varies depending on prepregnancy body mass index. Pregnancy weight measurements were used to develop a linear spline multilevel model relating weight to gestational age. This model was then used to predict each woman’s weight at 0 weeks’ gestation (prepregnancy weight) and GWG/wk from 0 to 18, 18 to 28, and 28 weeks to delivery. Outcomes included SEA scores at age 4 years, cognitive function at age 8 years using the Wechsler Intelligence Scale for Children from which IQs are derived, and results of General Certificate of Secondary Education examinations at age 16 years. Multivariable logistic and linear regression models were created, with model 1 adjusted for gestational age, maternal age, age at outcome assessment, and sex. Model 2, the main model, was adjusted for the confounders in model 1 and prepregnancy body mass index, maternal smoking during pregnancy, parity, mode of delivery, and maternal education. Interactions between being overweight/obese before pregnancy and GWG and their association with outcomes were also assessed.Compared with women who gained as recommended (reference group), those who gained less than recommended were slightly older (28.7 ± 4.7 vs 29.0 ± 4.9 years) and more likely to smoke, be from a manual social class, be multiparous, and have babies with a lower mean birth weight (3511.4 ± 445.2 vs 3333.4 ± 445.4 g). Compared with the reference group, those who gained more than recommended were slightly younger (28.7 ± 4.7 vs 27.9 ± 4.6 years), were less likely to smoke, were less educated, and were more likely to be nulliparous, have a cesarean delivery, and have heavier offspring (3511.4 ± 445.2 vs 3639.6 ± 481.1 g). Compared with the reference group, offspring of women who gained less than recommended had lower mean SEA scores, even after adjusting for confounders, with mean differences of SEA scores in model 1 and model 2 of −0.100 and −0.075, respectively (95% confidence intervals [CIs], −0.155 to −0.046 and −0.127 to −0.013, respectively). No association was found between less than recommended GWG and offspring IQ, nor was there strong evidence found for associations of more than recommended GWG with SEA scores or IQs. More than recommended GWG was associated with decreased odds of obtaining adequate final examination results in model 1 (OR, 0.89; 95% CI, 0.79–1.00), but this was attenuated after adjusting for confounders in model 2 (OR, 1.00; 95% CI, 0.88–1.14). Both less and more than recommended GWGs were associated with greater odds of low General Certificate of Secondary Education achievement. Gestational weight gain in all 3 periods of pregnancy was positively associated with IQs in models 1 and 2.The magnitudes of these associations were modest and unlikely to have major importance for individuals. At a population level, modest shifts in the distribution of educational attainment could be important. The results indicate the importance of defining optimal GWG, which is a complex process involving many competing risks.