It is a substance produced in the developing fetus. In general, its concentration in the blood decreases after birth. In oncology, CEA can be used as a marker for the presence and activity of certain types of tumors in adults and children. Among the tumors that may present elevated levels of CEA are those of the gastrointestinal tract, breast, lung, pancreas, ovary and central nervous system. However, CEA may also be present in some benign conditions. In addition, smokers may have higher than normal CEA levels. CEA is most often measured in the blood, but it can also be measured in the CSF.