OBJECTIVE: This study aimed to systematically investigate a wide range of obstetrical and neonatal outcomes as they relate to gestational weight gain less than the current Institute of Medicine and the American College of Obstetricians and Gynecologists guidelines when compared with weight gain within the guideline range and to stratify outcomes by the class of obesity and by the type of study analysis. DATA SOURCES: We systematically searched studies on PubMed, Scopus, Embase, and the Cochrane Library from 2009 to April 30, 2021. STUDY ELIGIBILITY CRITERIA: Studies reporting on obstetrical and neonatal outcomes of singleton pregnancies related to gestational weight gain less than the current Institute of Medicine and the American College of Obstetricians and Gynecologists guidelines in comparison with weight gain within the guidelines among women with obesity overall (body mass index >30 kg/m) and/or a specific class of obesity (I: body mass index, 30-34.9 kg/m; II: body mass index, 35-39.9 kg/m; and III: body mass index >40 kg/m). METHODS: Among the studies that met the inclusion criteria, multiple obstetrical and neonatal outcomes were tabulated and compared between pregnancies with weight gain less than recommended in the guidelines and those with weight gain within the guidelines, further classified by the class of obesity if applicable. Primary outcomes included small for gestational age neonates, large for gestational age neonates, preeclampsia, and gestational diabetes mellitus. Secondary outcomes included cesarean delivery, preterm birth, postpartum weight retention, and composite neonatal morbidity. A meta-analysis of univariate and adjusted multivariate analysis studies was conducted. The random-effect model was used to pool the mean differences or odds ratios and the corresponding 95% confidence intervals. Heterogeneity was assessed using the I value. The Newcastle-Ottawa Scale was used to assess individual study quality. RESULTS: A total of 54 studies reporting on 30,245,946 pregnancies were included of which 11,515,411 pregnancies were in the univariate analysis and 18,730,535 pregnancies were in the adjusted multivariate analysis. In the meta-analysis of univariate studies, compared with women who gained weight as recommended in the guidelines, those who gained less than the weight recommended in the guidelines had higher odds of having a small for gestational age neonate among those with obesity class I and II (odds ratio, 1.30; 95% confidence interval, 1.17-1.45; I=0%; P<.00001; and odds ratio, 1.56; 95% confidence interval, 1.31-1.85; I=0%; P<.00001, respectively). However, the incidence of small for gestational age neonates was below the expected limits (<10%) and was not associated with increased neonatal morbidity. Furthermore, after adjusting for covariates, that difference was not statistically significant anymore. The difference was not statistically significant for class III obesity. Following adjusted multivariate analysis, no significant differences in small for gestational age rates were noted for any classes of obesity between groups. Significantly lower odds for large for gestational age neonates were seen in the group with gestational weight gain less than the recommended guidelines among those with obesity class I, II, and III (odds ratio, 0.69; 95% confidence interval, 0.64-0.73; I=0%; P<.00001; odds ratio, 0.68; 95% confidence interval, 0.63-0.74; I=0%; P<.00001; and odds ratio, 0.65; 95% confidence interval, 0.57-0.75; I=34%; P<.00001, respectively), and similar findings were seen in the adjusted multivariate analysis. Women with weight gain less than the recommended guidelines had significantly lower odds for preeclampsia among those with obesity class I, II, and III (odds ratio, 0.71; 95% confidence interval, 0.63-0.79; I=0%; P<.00001; odds ratio, 0.82; 95% confidence interval, 0.73-0.91; I=0%; P<.00001; and odds ratio, 0.82; 95% confidence interval, 0.70-0.94; I=0%; P=.006, respectively), and similar findings were seen in the adjusted multivariate analysis. No significant differences were seen in gestational diabetes mellitus between groups. Regarding preterm birth, available univariate analysis studies only reported on overall obesity and mixed iatrogenic and spontaneous preterm birth showing a significant increase in the odds of preterm birth (odds ratio, 1.42; 95% confidence interval, 1.40-1.43; I=0%; P<.00001) among women with low weight gain, whereas the adjusted multivariate studies in overall obesity and in all 3 classes showed no significant differences in preterm birth between groups. Women with low weight gain had significantly lower odds for cesarean delivery in obesity class I, II, and III (odds ratio, 0.76; 95% confidence interval, 0.72-0.81; I=0%; P<.00001; odds ratio, 0.82; 95% confidence interval, 0.77-0.87; I=0%; P<.00001; and odds ratio, 0.87; 95% confidence interval, 0.82-0.91; I=0%; P<.00001, respectively), and similar findings were seen in the adjusted multivariate analysis. There was significantly lower odds for postpartum weight retention (odds ratio, 0.20; 95% confidence interval, 0.05-0.82; I=0%; P=.03) and lower odds for composite neonatal morbidity in the overall obesity group with low gestational weight gain (odds ratio, 0.93; 95% confidence interval, 0.87-0.99; I=19.6%; P=.04). CONCLUSION: Contrary to previous reports, the current systematic review and meta-analysis showed no significant increase in small for gestational age rates in pregnancies with weight gain below the current guidelines for all classes of maternal obesity. Furthermore, gaining less weight than recommended in the guidelines was associated with lower large for gestational age, preeclampsia, and cesarean delivery rates. Our study provides the evidence that the current recommended gestational weight gain range is high for all classes of obesity. These results provide pertinent information supporting the notion to revisit the current gestational weight gain recommendations for women with obesity and furthermore to classify them by the class of obesity rather than by an overall obesity category as is done in the current recommendations.