Maternal smoking and childhood leukemia and lymphoma risk among 1,440,542 Swedish children.

Possible in utero effects of maternal smoking on hemopoietic cancer in the offspring have been addressed previously, although the results are inconclusive. In this investigation, we take advantage of population-based registers in Sweden to examine maternal smoking during pregnancy and childhood risk of leukemia and lymphoma. Prospective data were available from 1,440,542 Swedish children born between 1983 and 1997. Proportional hazard models were used to estimate hazard ratios (HR) and 95% confidence intervals (95% CI) controlling for potential confounders. In the study base, 750 hemopoietic cancers occurred across 11 million person-years. Incidence rates per 100,000 person-years were 4.7 for acute lymphocytic leukemia (ALL), 0.45 for acute myelogenous leukemia, and 0.76 for non-Hodgkin's lymphoma. Maternal smoking was associated with a lower risk of ALL (HR, 0.73; 95% CI, 0.58-0.91). On the other hand, there was a higher risk of acute myelogenous leukemia (HR, 1.41; 95% CI, 0.74-2.67) particularly among heavy (> or =10 cigarettes per day) smokers (HR, 2.28; 95% CI, 1.05-4.94). The data also suggested a small excess risk of non-Hodgkin's lymphoma (HR, 1.25; 95% CI, 0.76-2.04). Evidence from this large cohort suggests that maternal smoking affects the risk of childhood leukemia and lymphoma in the offspring. The Swedish registries provide unique opportunities to examine this research question, with a design inherently free of selection and recall biases. The apparent protective effect with ALL needs to be explored further and in no way supports maternal smoking as beneficial, given its adverse association with common pregnancy outcomes.