detail concerning levels and patterns of exposure. However, although criteria used to delineate heavy prenatal alcohol exposure may be inconsistent across studies, these samples generally include children of women who meet criteria for alcohol abuse and dependence (e.g., Coles et al., 1991; Mattson, Riley, Gramling, Delis, & Jones, 1998; Mattson et al., 2010). Prospectively recruited samples, by design, allow for a greater precision when describing dose, pattern, and timing effects of prenatal alcohol exposure, but exposure levels are generally lower than in retrospective samples. Lower levels of exposure are generally defined as one to three drinks per week (e.g., Fried & Watkinson, 1988; Jacobson, Jacobson, Sokol, Martier, & Ager, 1993; Willford, Richardson, Leech, & Day, 2004). Different criteria for moderate to low exposure are included when available. These issues highlight the complexity of conducting research on prenatal alcohol exposure and the many risk factors and moderating variables that contribute to the considerable range of phenotypes presented by individuals with FASD.
