Pediatric leukemia dates back to birth
"Backtracking leukemia to birth" was the same title used by two famous papers which found that children who developed acute leukemia within the first few years of life already had cancerous cells in their blood at the time of birth. This indicates that many cases of pediatric leukemia develop in utero. This is why children with blood cancers are almost never treated with their own cord blood stem cells.
Gale, KB et al. 1997 Proc Natl Acad Sci 94(25):13950-4.
Rowley, Janet D. 1998 Nature Medicine 4(2):150-151