Overall, the effect of this placental metabolism is probably protective, but risks remain. Higher doses of chemical carcinogens may produce transplacental DNA damage with adverse effects. Like ionizing radiation, the role of tobacco smoke as a potential transplacental carcinogen has also been studied in humans. The story, however, may be far from complete. As 'baby boomers' approach 50, they are only beginning to enter the age range where cancer risks begin to rise dramatically.
Cancers result from accumulated mutations within tissues.
Potentially oncogenic mutations may as yet lie undetected in clones of cells that are slowly evolving toward malignancies in men and women exposed in utero to tobacco smoke carcinogens. Direct studies of mutations occurring in utero in any species are very limited because methods for finding rare mutations among large numbers of normal cells are just now being developed. Human cord blood samples have been assayed for somatic mutations at three different detectible loci. Each investigation has documented that mutations do occur during gestation at frequencies in the range of 1 to 2 mutations detected in 106 to 107 cells. These appear to have resulted from misdirected recombination, a normal process during development that produces clones of lymphocytes with different T cell receptors.