The risk for bladder cancer in women may decline with increased parity and older age at first birth, researchers report.
Caroline Weibull (Karolinska Institutet, Stockholm, Sweden) and co-authors say that their findings "provide support for yet-to-be-identified protective mechanisms associated with childbearing," although they acknowledge that smoking habits may partly explain some of the association.
These protective mechanisms may possibly be mediated by hormonal or structural changes following pregnancy, they write in European Urology.
The cohort study, encompassing over 2 million women from Swedish population-based registers, revealed 2860 incident cases of bladder cancer. Parous women had a 20% lower incidence of bladder cancer than nulliparous women.
Furthermore, women with two children had a 15% lower incidence of bladder cancer, and those with three or more children a 24% lower incidence of the disease, than uniparous women. Women who gave birth to their first child before the age of 20 years had a 15% increased incidence of bladder cancer compared with those aged 20-24 years at their first birth.
In addition, women with a history of chronic obstructive lung disease had an almost threefold increased incidence rate of bladder cancer compared with those who did not.
Further analysis revealed that women who had undergone 9 years of schooling or less had an increased risk for bladder cancer compared with those who attended school for 10-13 years. Analysis adjusting for age showed that women with a postgraduate education had a 23% reduced risk for bladder cancer compared with those who attended 9 years or less of schooling.
The authors say the limitations of their study include an absence of data on menstrual history, use of exogenous hormones, and smoking. However, they note that "a lower risk of BCa [bladder cancer] in parous women might reflect differences in lifetime smoking history because smokers who become pregnant probably are more likely to quit smoking, compared with nulliparous female smokers who never have had that incentive."
"Taken together, our results clearly show that parity is associated with a reduced risk of developing bladder cancer," conclude the authors.
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