Although the involved mechanisms have yet to be identified, it is clearly established that stopping smoking before and during pregnancy considerably lowers health risks for both the child and the mother. An association between tobacco consumption and epigenetic modifications in umbilical cord blood has been demonstrated in a number of studies. Indeed, epigenetic modifications can be induced by such environmental factors. These modifications manifest as reversible changes to DNA methylation, that is, they do not modify gene sequences themselves but rather their expression. 

For the present study, a team uniting researchers from Inserm, the CNRS and Grenoble Alpes University studied DNA in the post-delivery placentas of 568 women categorized as non (no smoking in the three months before and during pregnancy), former (smoking in the three months before but not during pregnancy) or current (smoking in the three months before and during pregnancy) smokers. The study also called upon the expertise of the Epigenetic and Environment Laboratory of the National Center of Human Genomics Research (CNRGH), a François Jacob Institute (CEA). That lab was responsible for characterizing and localizing epigenetic modifications in the samples. The researchers found epigenetic modifications in 178 DNA regions in the placentas of the smokers, in 26 regions (among the above) in those of the former smokers, and in no regions in those of the non-smokers. The authors concluded thus that the placenta conserves a sort of "epigenetic memory" of tobacco exposure, even when smoking is stopped in anticipation of a pregnancy.

DNA enhancer regions, i.e., regions affecting the likelihood of activation for a particular gene, appeared to be particularly susceptible to this type of modification, as were regions with genes known for their specific roles in fetal development. Although it remains to be demonstrated, the placental DNA epigenetic modifications in these two regions could explain certain effects of tobacco exposure on fetal development and childhood health.