title: Childbearing and the risk of bladder cancer: a nationwide population-based cohort study.
abstract: BACKGROUND
  The incidence of bladder cancer (BCa) is substantially lower in women than in men, a difference that cannot be fully explained by established risk factors. Several studies suggest that hormonal and reproductive factors play a role in the development of BCa.
  
  
  OBJECTIVE
  To examine possible associations between patterns of childbearing and the risk of BCa.
  
  
  METHODS
  This cohort study encompassed >2 million women for whom information on reproductive history and BCa incidence was retrieved from Swedish population-based registers.
  
  
  METHODS
  Incidence rate ratios (IRRs) of BCa were estimated using Cox proportional hazards modelling. The exposures under investigation were parity and age at first birth, adjusted for education and history of chronic obstructive lung disease (COLD).
  
  
  CONCLUSIONS
  Among 2 009 811 women in the cohort, 2860 incident cases of BCa were identified. Parous women had lower incidence of BCa compared with nulliparous women (adjusted IRR: 0.80; 95% confidence interval [CI], 0.72-0.89). Moreover, the incidence was 15% lower in women with two children and 24% lower in women with three or more children compared with uniparous women. Compared with women aged 20-24 at first childbirth, the incidence was elevated in women with a first birth before age 20 (adjusted IRR: 1.16; 95% CI, 1.05-1.29). The risk of BCa was elevated in women with low education and among women with a history of COLD. Absence of data on menstrual history, use of exogenous hormones, and smoking was a limitation of the study.
  
  
  CONCLUSIONS
  The incidence of BCa decreased with increasing parity and older age at first birth. Although smoking habits may partly explain some of the associations, our findings provide support for yet-to-be-identified protective mechanisms associated with childbearing, possibly mediated by hormonal or structural changes following pregnancy.
authors: Department of Medical Epidemiology and Biostatistics, Karolinska Institutet, Stockholm, Sweden. caroline.weibull@ki.se
authors: Caroline E
authors: CE
authors: Weibull
authors: Sandra
authors: S
authors: Eloranta
authors: Daniel
authors: D
authors: Altman
authors: Anna L V
authors: AL
authors: Johansson
authors: Mats
authors: M
authors: Lambe
mesh_terms: D000328:Adult; D015331:Cohort Studies; D005260:Female; D006801:Humans; D015994:Incidence; D008875:Middle Aged; D010298:Parity; D011247:Pregnancy; D016016:Proportional Hazards Models; D012306:Risk; D012307:Risk Factors; D013548:Sweden / Q000453:epidemiology; D001749:Urinary Bladder Neoplasms / Q000453:epidemiology*