Hassanzadeh et al. study.33 Siblings of children with this acute/chronic disease are at a greater risk of developing leukemia than the general population. The importance of inut- ero genetic events has been suspected for many years based on concordance studies on twins with leuke- mia. Some leukemogenic translocations and other markers of clonality can be detected at birth in hema- topoietic cell population. Although the majority of these children do not go on to develop overt leuke- mia, environmental factors, viral infection, and im- munodeficiency states may provoke these genetically- predisposed children to leukemia. Child miscarriage and risk of childhood leukemia have been pursued in several studies and have reached contradictory results. While some studies similar to this study did not show any significant as- sociations,7,19 others suggest it as a possible risk fac- tor.4,20,34 These controversies may be explained as follows: First, the characteristics of leukemia cases in different studies varied. Second, such information was gathered through interviews in some studies which may be subjected to recall biases. There are inconsistent results regarding fetal irradi- ation from prenatal exposure to diagnostic radiographs and risk of childhood leukemia. While Doll and Wake- ford estimated a relatively 6% increased risk of cancer per GY,35 results from other studies like ours have not supported this association.36-38 However, due to small sample size and limitations imposed by recall biases, the results should be interpreted cautiously. References 1 2 3 4 402 Kim AS, Estomond DA, Perston RJ. Childhood acute lymphocytic leu- kemia and perspective on risk as- sessment of early-life stage expo- sures. Mutat Res 2006;613:138-60. [17049456] [doi:10.1016/j.mrrev. 2006.09.001] Belson M, Kigsley B, Holmes A. Risk factors for acute leukemia in children: A review. Environ Health Perspect 2007;115:138-45. [17366 834] [doi:10.1289/ehp.9023] Zack M, Adami HO, Ericson A. Ma- ternal and perinatal risk factors for childhood leukemia. Cancer Res 1991;51:3696-701. [2065325] Podvin D, Kuehn CM, Mueller BA, Williams M. Maternal and birth characteristics in relation to child- hood leukemia. Paediatr Perinat Ep- idemiol 2006;20:312-22. [168 79503] [doi:10.1111/j.1365-3016. 2006. 00731.x] 5 6 7 Murray L, McCarron P, Bailie K, Middleton R, Davey Smith G, Dempsey S, McCarthy A, Gavin A. Association of early life factors and acute lymphoblastic leukemia in childhood: historical cohort study. Br J Cancer 2002;86:356-61. [11875 699] [doi:10.1038/sj.bjc.6600012] Zipf TF, Berg S, Roberts WM, Poplack DG, Steuber CP, Bleyer WA. Child- hood leukemias. In: Abeloff MO, Armitage JO, Lichter AS, Niederhuber JE, editors. Clinical Oncology. 2nd ed. Philadelphia, Churchill Livingston. 2000; p. 2402-29. OU SX, Han D, Severson RK, Chen Z, Neglia JP, Reaman GH, Buckley JD, Robisson LL. Birth characteris- tics, maternal reproductive history, hormone use during pregnancy, and risk of childhood acute lymphocytic leukemia by immunophenotype (United States). Cancer Causes In this study, we attempted to follow a comparable accuracy principle by informality in data gathering in both case and control groups. However, like other case-control studies, our research has been subjected to error due to biased information, especially in re- gard to variables dependent to recall. As with much other human diseases, both genetic and environmental factors have been implicated in the etiology of childhood leukemia. Data to support the exact causative factors are often inconsistent and somehow contradictory. This study presented an ex- istence of a link between increased birth order, pet ownership and history of leukemia among first and second degree relatives to be significant. The contradiction presented in different studies re- garding different variables is partly explained by the influence of genetic predisposition and leukomogenic translocations. Furthermore, the multi-center matched case-control studies are recommended to clarify the ambiguous aspects of this complex subject. Acknowledgment We would like to express our special thanks to Mr. S. Kamfiroozi and S. Parand at Hematology Research Center, for editorial assistance and Mrs N Farahman- di for typing this manuscript. Conflict of interest: None declared. Control 2002;13:15-25. [11899114] [doi:10.1023/A:1013986809917] 8 Hjalgrim LL, Rostgaard K, Hjalgrim H, Westergaad T, Thomassen H, Forestier E, Gustafsson G, Kristins- son J, Melbye M, Schmiegelow K. Birth weight and risk for childhood leukemia in Denmark, Sweden, Norway, and Iceland. J Natl Cancer Inst 2004;96:1549-56. [15494605] [doi:10.1093/jnci/djh287] 9 MacArthur AC, McBirde ML, Spinelli JJ, Tamaro S, Gallagher RP, Theri- ault GP. Risk of childhood leukemia associated with vaccination, infec- tion, and medication use in child- hood: the cross-Canada childhood leukemia study. AM J Epidemiol 2008;167:598-606. [18079130] [doi: 10.1093/aje/kwm339] 10 Buka I, Koranteng S, Osornio Var- gas AR. Trends in childhood cancer incidence: Review of environmental WWW.ircmj.com Vol 13 June 2011