Risk factors for leukemia Table 2: Estimated Odds ratio and 95% confidence interval through conditional logistic regression. Variables subgroups Case (N)/control(N) Odds ratio P value CLoownfidence inHtiegrhval <2500 11/12 1 - - - Birth weight (g)a 2500-4000 121/144 0.929 0.837 0.374 2.303 >4000 13/5 2.431 0.214 0.60 9.861 1 63/96 1 - - - Birth orderb 2 40/36 1.768 0.094 0.907 3.448 ≥3 64/27 6.177 <0.001 2.551 14.957 Ibnirttehrbval from last 2<-25 3267//2206 11.624 0-.542 0-.342 7.-717 >5 36/22 1.891 0.460 0.349 10.243 <20 19/29 1 - - - 20-24 47/48 1.190 0.671 0.534 2.653 Maternal agec 25-29 43/40 1.538 0.277 0.708 3.342 30-34 35/39 1.073 0.872 0.454 2.539 >35 19/6 3.041 0.076 0.890 10.358 1 23/32 1 - - Sibship sized 2 60/78 1.471 0.638 0.571 3.787 ≥3 80/52 1.314 0.195 0.392 4.40 Pet ownershipe 54/22 2.565 0.004 1.352 4.868 Day care attendancef 53/57 0.937 0.837 0.507 1.729 History of leukemia in relatives 16/6 2.667 0.040 1.043 6.815 History of mother's radiography 6/2 3 0.178 0.606 14.864 History of fetal lossg 22/16 1.211 0.590 0.603 2.435 aAdjusted for maternal age and education, bAdjusted for maternal age, paternal education and maternal education, cAdjusted for paternal education, maternal education, paternal occupation, dAdjusted for maternal age, paternal education, paternal occupation, birth order, eAdjusted for paternal occupation, fAdjusted for level of maternal edu- cation, paternal occupation, birth order, gAdjusted for maternal age. proliferation and risk of malignant transfor- mation.19,23-25 However, such association was not shown in our research, similar to what described by other investigators.7,22 In contrast to what Dockerty and colleagues de- scribed,26 this study does not support existence of a link between advanced maternal age and risk of childhood leukemia. This could be partly explained by younger age of marriage and pregnancy especially in rural areas and the effect of genetics and other environmental fac- tors. As stated earlier, we discovered significant differ- ence between the case and control groups in regard to father's occupation a year before child's birth especially in agricultural working. This may be due to prolonged pesticide exposure or other unknown environmental hazards. However, there is inconclusive evidence re- garding parent's occupational exposures and risk of de- veloping leukemia. Results from the United Kingdom Child Cancer Study shows a small increased risk of childhood leukemia associated with parental exposure to exhaust fumes, and inhaled particulate hydrocarbons.27 Other studies in Germany reported increase risks of childhood ALL in mothers exposed to paint or lacquer during the preconception period.28 In 2001, Cordier conducted a multi-center case-control study, which ex- hibited an increased risk of childhood brain tumors in farmer families.29 As most studies are generally per- formed by self-administered questionnaires, assessing chemical environmental links to childhood leukemia, and parent’s occupational exposure is not obviously rel- evant, thus remains elucidated. Bross and Gibson reported in 1970, that the expo- sure to pets could be a factor of the development of leukemia. They discovered the increase risk of leu- kemia among children exposed to sick cats to two- folds.30 This was also confirmed by Petridou et al. (1997) in Greece. However, Swenson and colleagues proved to be exactly opposite.31 The concern that pets may be associated with childhood leukemia arose when oncogenic viruses such as Feline Leukemia Vi- rus (FELV) were discovered in animals.30,32 Based on our findings, there is a positive signifi- cant link between the history of leukemia among first and second degree relatives, comparable to Ripert's WWW.ircmj.com Vol 13 June 2011 401