Tobacco use in the form of cigarettes continues to be one of the most commonly used drugs in pregnancy and delivers significant amounts of chemical toxins to the fetus via maternal bloodstream. Tobacco use is often comorbid with cannabis and the two substances are often smoked together, but little is known about potential effects of co-use on child developmental outcomes. Such use is also more prevalent among young, low-income, single women. Together, this reflects a large public health problem for both maternal and child health. Although most women cut down substance use upon pregnancy recognition, use increases within a few months postpartum. Understanding the combined effects of prenatal and postnatal exposure on development, and predictors of increased use postpartum are critical for prevention programming. There are also robust associations between tobacco exposure and increased risk for child behavior problems such as aggression and conduct disorder at early school age. However, outside of teratogenic effects of nicotine, developmental mechanisms promoting this association are not well understood. Dr. Eiden will present results from a prospective study of prenatal tobacco and cannabis exposure from the first trimester of pregnancy to early school age, examining developmental pathways to child behavior problems in a low-income diverse sample of mothers and children.