Bladder cancer, parity, and age at first birth.

The excess of bladder cancer in males (M:F ratio of 4:1 in the United States) is not explained fully by gender differences in smoking habits or occupational exposures. Laboratory studies suggest that some androgenic hormones stimulate (or do not inhibit) oncogenesis in bladder tissue, and that estrogenic hormones have the opposite effect. These observations suggest that bladder cancer risk in females may be modified by sex hormones which undergo profound changes during and following pregnancy. Mail-questionnaire data from 317 incident female cases, and 833 population-based controls in Iowa were used to measure the effect of parity and maternal age at first birth on bladder cancer risk. Parous women were at decreased risk relative to nulliparous women (odds ratio [OR] = 0.67, 95 percent confidence interval [CI] = 0.44-1.00), after adjustment for age, tobacco use, and previous bladder infection. The overall risk reduction was restricted to women who had never smoked (OR = 0.51, CI = 0.30-0.88), with no apparent effect of parity among ever-smokers (OR = 0.93, CI = 0.49-1.77). Risk appeared to decrease with increasing age at first birth, but did not vary with increasing parity after the first birth. Our findings are consistent with the hypothesis that oncogenesis in transitional cell tissue of the human bladder is influenced by sex hormones, and that hormonal changes related to pregnancy thereby can decrease risk.