Childbearing and the risk of leukemia in Sweden

Objective: The possible influence of childbearing on the development of leukemias in females has received little attention, in spite of consistent findings of lower incidence rates in females than in males. A nested case-control study was undertaken to explore if parity and age at first birth affect the risk of developing these malignancies. Methods: In a nationwide cohort defined by a population-based Fertility Register, we identified 356 women with chronic myeloid leukemia (CML), 819 with acute nonlymphocytic leukemia (ANLL), and 179 with acute lymphocytic leukemia (ALL). For each case, five age-matched controls were selected. Odds ratios were estimated by conditional logistic regression analyses. Results: There was some evidence of weak negative associations between parity and age at first birth for CML. Compared to nulliparous women there was a tendency of a temporal risk reduction of CML for the first 10 years following a delivery. The risk of ANLL was slightly lower in parous compared to nulliparous women. Neither parity nor age at first birth was related to the risk of ALL. Conclusions: We conclude that if pregnancy-related hormonal or immunological factors have an effect on the development of leukemia, it is minor and confined to the myeloid types, chiefly CML. Our study gives some support for treating the leukemias as separate entities based on both cell lineage and form in future etiologic studies.