Cannabis sativa (marijuana) is the illicit drug most commonly used during pregnancy. The self-reported prevalence of marijuana use during pregnancy ranges from 2% to 5% in most studies but increases to 15-28% among young, urban, socioeconomically disadvantaged women (1-5). Higher rates of use are found when querying women at the time of delivery rather than at prenatal visits because some users may not seek prenatal care. Notably, 34-60% of marijuana users continue use during pregnancy, with many women believing that it is relatively safe to use during pregnancy and less expensive than tobacco. A recent study noted that 18.1% of pregnant women reporting marijuana use in the past year met criteria for marijuana abuse, or dependence, or both. A growing number of states are legalizing marijuana for medicinal or recreational purposes, and its use by pregnant women could increase even further as a result.The medicinal and psychoactive properties of marijuana are mediated by compounds called cannabinoids, which are absorbed from the lungs when smoked or from the gastrointestinal tract when ingested. Tetrahydrocannabinol (THC) is a small and highly lipophilic molecule that is distributed rapidly to the brain and fat. Metabolized by the liver, the half-life of THC varies from 20-36 hours in occasional users to 4-5 days in heavy users and may require up to 30 days for complete excretion. In animal models, THC crossed the placenta, producing fetal plasma levels that were approximately 10% of maternal levels after acute exposure. Significantly higher fetal concentrations were observed after repetitive exposures. Limited human data suggest that THC also appears in breast milk.It is difficult to be certain about the specific effects of marijuana on pregnancy and the developing fetus, in part because those who use it often use other drugs as well, including tobacco, alcohol, or illicit drugs, and in part because of other potential confounding exposures. Marijuana smoke contains many of the same respiratory disease-causing and carcinogenic toxins as tobacco smoke, often in concentrations several times greater than in tobacco smoke. Adverse socioeconomic conditions, such as poverty and malnutrition, may contribute to outcomes otherwise attributed to marijuana. For example, one population-based study reported that pregnant marijuana users were more often underweight and had lower levels of education, had a lower household income, and were less likely to use folic acid supplementation than nonusers. Another study found that marijuana-exposed women are more likely to experience intimate partner violence, an additional risk factor for adverse pregnancy outcomes. Studies evaluating marijuana use during pregnancy often account for these confounders using data stratification or multivariate analysis. Studies of marijuana exposure during pregnancy are potentially subject to reporting and recall bias, often relying on self-reported habits, including frequency, timing, and amount of marijuana use. Additional confounding issues may arise from marijuana potency that has, in general, increased with time.