Curvilinear incidence models for parity in the entire fertility range for cancers of the breast, ovary, and endometrium: A follow-up of the Norwegian 1960 Census

The protective effect of parity has been demonstrated for cancer of the breast, ovary, and endometrium but no studies have estimated the effect of each subsequent birth in women with 10 or more children or grand-grand parity women, nor compared the linear relationship of the three cancers sites. Here, we aim to explore these relationships based on the Norwegian 1960 Census. The question of parity in present marriage was answered by 385,816 women born 1870–1915, a period with high fertility. Age at marriage has been validated as a proxy for age at first birth AFB. With high parity age at first birth will logically be restricted to early births giving structural zeros. Follow-up was based on linkages to national registers until the first of any of the three diagnoses, death, or age 90 before 31.12.2005. Included were 16,905 breast cancers, 3827 ovarian cancers, and 3834 endometrial cancers. Age- and period-specific incidence rates based on person-years, PY, were used in logit regression models. The percentage decrease for each additional child over the total parity range was for breast cancer 10.5% (95% CI; 9.6–11.4), ovarian cancer 13.2% (11.2–15.3), and endometrial cancer 10.9% (8.9–12.8), in a model without higher order terms. Adjustment for structural zeros reduced the effect of age at first birth to less than one additional child. To the best of our knowledge this is the first analysis of the curvilinear relationships for cancers of the breast, ovary, and endometrium throughout the extended fertility range.