provide an informative cross-sectional view of this phenomenon, provided that there is careful attention given to the time of sample collection. Promising findings can be followed up with longitudinal studies with sequential sampling designed to assess the relative impact of environmental exposures. For example, in the context of prenatal smoke exposure research, sampling at the beginning and the end of pregnancy in smokers who did and did not decide to quit smoking while pregnant would allow examination of the extant methylation resulting from pre-pregnancy smoking. This would also allow for investigation of whether or not smoking cessation during pregnancy has an impact on epigenetic mechanisms.
