Hassanzadeh et al. Table 1: Demographic characteristics of cases and controls Variables Cases frequency (%) Age (years) <4 51 (31.3) 5-9 54 (33.10 10-14 39 (23.9) 15-18 19 (11.7) Sex Male 100 (61.3) Female 63 (38.7) Residency City 124 (76.1) Village 39 (23.9) Maternal age (years) <20 19 (11.7) 20-24 47 (28.8) 25-29 43 (26.4) 30-34 35 (21.5) >35 19 (11.7) Total 163 (100) Maternal education Illiterate 21 (12.9) Primary school 48 (29.4) Secondary school 36 (22.1) High school 44 (27) University degree 14 (8.6) Total 163 (100) Father education Illiterate 12 (7.4) Primary school 39 (23.9) Secondary school 49 (30.1) High school 44 (27) University degree 19 (11.7) Total 163 (100) Father's occupation Employee 32 (19.6) Self-employed 64 (39.3) Worker 35 (21.5) Agricultural worker 32 (19.6) Total 163 (100) a Based on Chi-Square test 51 (31.3) 54 (33.10) 39 (23.9) 19 (11.7) 100 (61.3) 63 (38.7) 124 (76.1) 39 (23.9) 29 (17.8) 48 (29.4) 40 (24.5) 39 (23.9) 7 (4.3) 163 (100) 11 (6.7) 38 (23.3) 38 (23.3) 50 (30.7) 26 (16) 163 (100) 3 (1.8) 32 (19.6) 58 (35.6) 41 (25.2) 29 (17.8) 163 (100) 39 (23.9) 77 (47.2) 37 (22.7) 10 (6.1) 163 (100) more vulnerable to common childhood infections. The role of infectious agents in developing leuke- mia was initially proposed by Greaves.13 It was reemphasized by the population mixing theory of Kinlen,14 and also the delayed infection mechanism introduced by Greaves and Alexander.15 Moreover, seasonal variation in the birth or onset dates of child- hood leukemia have been described which also sup- ports the infectious etiology hypothesis.16-18 In addi- tion, increased risk of leukemia was noted in patients with a higher birth order (≥3), which may be ex- plained by exposure to infectious agents at younger ages and in the populated families. Similar results were noted in other studies,19-21 while some research- ers have suggested a non-significant or even an in- verse relationship.2,4,5,22 The association between childhood leukemia and other markers of infection exposure including daycare attendance, and the fami- ly size was not proven in this study. Several studies have described an association be- tween increased birth weight and childhood ALL. Researchers believe it is due to the high rate of cell 400 WWW.ircmj.com Vol 13 June 2011 Controls frequency (%) P value a Matched variable Matched variable Matched variable P=0.057 P=0.080 P=0.060 P=0.004