Marijuana is the most commonly used illicit drug in pregnancy, and the prevalence of use during pregnancy is increasing in the United States. Although much of the existing research investigating marijuana use in pregnancy is limited by study design and confounding factors, a growing accumulation of data suggests adverse outcomes. Studies have identified associations with decreased birth weight, increased spontaneous preterm birth, and impaired neurodevelopment among children and adults with in utero exposure. Moderate concentrations of marijuana have also been identified in breast milk. Due to these findings, multiple professional societies have issued clear statements against marijuana use during pregnancy and lactation.