A limitation of this study is that we were unable to assess the direct effect of prenatal exposure to illicit drugs. Like alcohol and cigarettes, prenatal exposure to marijuana or cocaine has been associated with increased rates of offspring use (Minnes et al., 2014, Porath and Fried, 2005, Richardson et al., 2013). In the current study, patterns of maternal drug use during pregnancy were heterogeneous with regard to the substance or substances consumed, the route of administration, and the quantity and frequency of use. Therefore, it was not possible to detect the singular effects of any particular drug.
