Knowledge of these particular risk factors can be helpful in alleviating potentially harmful exposures and reducing the risk of diseases. The objective of this study was to examine the relationship between maternal, birth characteristics and environmental ex- posures with childhood leukemia. Materials and Methods In this case-control study, children younger than 18 years old suffering from leukemia including acute lymphoblastic leukemia(ALL), acute myeloid leuke- mia(AML), and chronic myeloid leukemia (CML) who reside at Fars Province, Southern Iran were in- cluded. Their disease had been diagnosed by bone marrow aspiration and flowcytometry between 2005 to 2009 as they were undergoing chemotherapy at Pediatric Hematology-Oncology Ward in a referral Hospital, Shiraz, Southern Iran. The controls chosen by random sampling were individually matched to the cases by caliper matching method with respect to age (one year interval for children older than 2 years and 6 months interval for children younger than 2 years), sex and residence region. The information which was gathered through an interview with one of the parents included child's birth order, mother's age at the time of childbirth, child's birth interval from the last birth, number of siblings, history of miscarriage, history of leukemia in the immediate and secondary family members, history of mother's diagnostic radiography during pregnancy, child's day care attendance, father's occupation one year before child's birth, level of par- ents education, history of pet ownership and type of delivery. The birth weight of cases and control were recorded from their vaccination cards. Informed writ- ten consent was taken from the parents before entry to the study. In order to examine the relationship between each variable and risk of leukemia, odds ratio (OR) and 95% confidence interval (CI) were estimated using conditional logistic regression. Chi-Square test was used for data analysis. P value less than 0.05 was con- sidered significant. The data were analyzed with SPSS software (version 15, Chicago, IL, USA). Results Approximately 163 children younger than the age of 18 who suffered from leukemia were eligible for the study. The demographic characteristics of both cases Risk factors for leukemia and controls are shown in Table 1. There were no sig- nificant difference between the case and control groups regarding mother's age at the time of childbirth and level of parents' education. However, father's occupa- tion one year before child's birth was significantly dif- ferent in the two groups (p=0.004), particularly when agricultural working is concerned (Table 1). Table 2 shows the result of the estimated odds ra- tio by conditional logistic regression for various ex- posures. Although more cases than controls had high birth weight (>4000 g) (OR=2.431, 95%CI=0.60- 9.861), this association was not statistically signifi- cant (p=0.214). After adjusting for maternal age and level of parental education, high birth order was asso- ciated with increased risk of childhood leukemia (OR=6.177, 95% CI=2.551-14.957, p<0.001). More cases compared to controls (36 vs 22) had more than 5 years interval from the last birth, but this association was not statistically significant (p=0.46), neither maternal age over 35 years old (OR=3.041, 95% CI=0.89-10.385, p=0.076). Children with family history of leukemia were at a higher risk of develop- ing this malignancy (OR= 2.667, 95% CI=1.043- 6.815, p=0.040). Also, the research revealed signifi- cant association between pet ownership and risk of childhood leukemia (OR=2.565, 95% CI=1.352 – 4.868, p= 0.004).However, no significant association was identified between daycare attendance, miscar- riage, number of siblings, and mother’s diagnostic radiography during pregnancy with increased risk of childhood leukemia (Table 2). Discussion Cancers are assumed to be a multi-factorial disease which occur when the genetic and environmental fac- tors interact in a multistage sequence. Leukemia as the most common childhood cancer is particularly subjected to this rule. Although, the recent increase in the childhood ALL cases may be partly explained by more accurate diagnostic practice and better reporting scheme, the role of new environmental risk factors must be more clearly elucidated. The male predominance of leukemia cases in our study was previously reported by other research- ers.11,12 The peak occurrences of leukemia were in 5-9 years old children, which includes over sixty percent of children in this case. This is relatively compatible with the theory that young children (under 10 years of age) have less developed immune system and are WWW.ircmj.com Vol 13 June 2011 399