Birth spacing and maternal risk of invasive epithelial ovarian cancer in a Swedish nationwide cohort.

<bold>Objective: </bold>Pregnancies reduce the risk of ovarian cancer, and among multiparous women, levels of circulating progesterone might be higher during pregnancies with wider birth spacing. We hypothesized that childbirth with wider birth spacing might reduce maternal risk of invasive epithelial ovarian cancer more than births with narrower spacing.<bold>Methods: </bold>We conducted a case-control study nested in a nationwide cohort of Swedish women from 1961 to 2001. We selected five individually age-matched controls for each case of invasive epithelial ovarian cancer, and analysis for the effect of birth spacing was performed for 5,341 cases and 29,047 controls. We applied unconditional logistic regression analyses adjusting for age, ages at childbirth, educational level, area of residence, and gender of offspring.<bold>Results: </bold>Relative risk of invasive epithelial ovarian cancer associated with each one-year increase in average birth spacing is 1.00 (95% CI = 0.98-1.01) among all women and 0.99 (0.98-1.01) among those born before 1935 and less likely to have used oral contraceptives. Further analyses on the biparous and triparous women did not find a consistent association between birth spacing and the risk of ovarian cancer.<bold>Conclusions: </bold>Birth spacing is unlikely to be a major determinant underlying the protective effects of childbirth on ovarian cancer risk. [ABSTRACT FROM AUTHOR]