title: How the effect of maternal age on the risk of childhood leukemia changed over time in Sweden, 1960-2004.
abstract: BACKGROUND
  Previous studies on the association between maternal age and risk of childhood leukemia found inconsistent results.
  
  
  OBJECTIVE
  We aimed to assess whether there is an association between maternal age and risk of childhood leukemia and whether such an association is modified by maternal year of birth.
  
  
  METHODS
  By linking nationwide Swedish registers, we analyzed leukemia incidence among all children between 1 and 5 years of age born between 1960 and 1999. We estimated incidence time trends by child year of birth (overall and stratified by maternal age) and incidence rate ratios (RRs) for maternal age groups stratified by maternal birth cohort. We tested the interaction between maternal age and child year of birth through the likelihood ratio test between nested Poisson regression models.
  
  
  RESULTS
  We observed 1,562 leukemia cases. The overall annual percent change (APC) was 1.00 [95% confidence interval (CI), 0.51 to 1.49]. Stratifying by maternal age classes, APCs decreased from 1.66 (0.68 to 2.65) for mothers <or= 24 years to 0.23 (-0.93 to 1.40) for mothers >or= 35 years at delivery. RRs for children born to the oldest with respect to the youngest mothers were 2.42 (1.31 to 4.67), 1.68 (1.00 to 2.72), 1.34 (0.87 to 2.01), and 0.87 (0.46-1.54) for mothers born in 1930-1934, 1940-1944, 1950-1954, and 1960-1964, respectively.
  
  
  CONCLUSIONS
  Childhood leukemia risk increased with maternal age for mothers born in the past, whereas maternal age had no effect on this risk for mothers born more recently. This finding may explain the inconsistency of previous studies and suggests that leukemia risk may be related to an environmental factor to which women's exposure has changed over time.
authors.affiliation: Cancer Epidemiology Unit, Center for Experimental Research and Medical Studies and Center for Oncologic Prevention in Piedmont, University of Turin, Turin, Italy. milena.maule@cpo.it
authors.forename: Milena Maria
authors.initials: MM
authors.lastname: Maule
authors.forename: Loredana
authors.initials: L
authors.lastname: Vizzini
authors.forename: Kamila
authors.initials: K
authors.lastname: Czene
authors.forename: Olof
authors.initials: O
authors.lastname: Akre
authors.forename: Lorenzo
authors.initials: L
authors.lastname: Richiardi
mesh_terms: D000328:Adult; D005260:Female; D049673:History, 20th Century; D049674:History, 21st Century; D006801:Humans; D007938:Leukemia / Q000453:epidemiology* / Q000266:history; D008423:Maternal Age; D011247:Pregnancy; D012307:Risk Factors; D013548:Sweden / Q000453:epidemiology; D055815:Young Adult