PARITY, AGE AT FIRST CHILDBIRTH, AND RISK OF OVARIAN-CANCER

Increasing parity is associated with a reduction in the risk of ovaria n cancer, but it is not clear whether this association applies to diff erent histopathological types and to borderline tumours. Moreover, the temporal relations are poorly understood, and the possible role of ag e at first birth remains unequivocal. We have investigated these issue s in a case-control study nested in a nationwide cohort of women born between 1925 and 1960 in Sweden. During follow-up until 1984, 3486 inv asive ovarian cancers (2992 epithelial, 330 stromal, 149 germ-cell, 15 not classifiable) and 510 tumours of borderline malignant were diagno sed. 5 individually age-matched (total 19 980) were selected for each case woman. After simultaneous adjustment for parity and age at first birth, increasing parity was associated with a pronounced consistent d ecrease in relative risk of all invasive cancers (odds ratio for each additional birth 0.81 [95% Cl 0.77-0.85]), epithelial cancer (0.81 [0. 77-0.86]), stromal cancer (0.84 [0.72-0.98]), and germ-cell cancer (0. 71 [0.48-1.05]), but a less consistent decrease for borderline tumours (0.92 [0.81-1.04]). The risk of ovarian cancer decreased by about 10% for each 5-year increment in age at first childbirth (odds ratios 0.8 9 [0.84-0.94] epithelial cancer, 0.92 [0.77-1.10] stromal cancer, 0.92 [0.65-1.32] germ-cell cancer, 0.93 [0.80-1.09] borderline tumours). B ecause our findings cannot be readily explained by theories invoking i ncessant ovulation or high serum concentrations of gonadotropins, new aetiological hypotheses are needed. Pregnancy-dependent clearance from the ovaries of cells that have undergone malignant transformation cou ld explain the reproductive risk factors for ovarian cancer.