OBJECTIVE: To estimate whether marijuana use in pregnancy increases risks for adverse neonatal outcomes and clarify if any increased risk is attributable to marijuana use itself or to confounding factors such as tobacco use. DATA SOURCES: Two authors performed a search of the data through August 2015 utilizing PubMed, Embase, Scopus, Cochrane reviews, ClinicalTrials.gov, and Cumulative Index to Nursing and Allied Health. METHODS OF STUDY SELECTION: We looked at observational studies that compared rates of prespecified adverse neonatal outcomes in women who used marijuana during pregnancy with women who did not. TABULATION, INTEGRATION, AND RESULTS: Two authors independently extracted data from the selected studies. Primary outcomes were low birth weight (less than 2,500 g) and preterm delivery at less than 37 weeks of gestation. Secondary outcomes were birth weight, gestational age at delivery, small for gestational age, level II or greater nursery admission, stillbirth, spontaneous abortion, low Apgar score, placental abruption, and perinatal death. DerSimonian-Laird random-effects models were used. We assessed heterogeneity using the Q test and I 2 statistic. Stratified analyses were performed for the primary outcomes and pooled adjusted estimates were calculated. We included 31 studies that assessed the effects of maternal marijuana use on adverse neonatal outcomes. Based on pooled unadjusted data, marijuana use during pregnancy was associated with an increased risk of low birth weight (15.4% compared with 10.4%, pooled relative risk [RR] 1.43, 95% confidence interval [CI] 1.27-1.62) and preterm delivery (15.3% compared with 9.6%, pooled RR 1.32, 95% CI 1.14-1.54). However, pooled data adjusted for tobacco use and other confounding factors showed no statistically significant increased risk for low birth weight (pooled RR 1.16, 95% CI 0.98-1.37) or preterm delivery (pooled RR 1.08, 95% CI 0.82-1.43). CONCLUSION: Maternal marijuana use during pregnancy is not an independent risk factor for adverse neonatal outcomes after adjusting for confounding factors. Thus, the association between maternal marijuana use and adverse outcomes appears attributable to concomitant tobacco use and other confounding factors.