Associations between reproductive factors and risk of ovarian cancer were examined in a prospective study of 60,565 women in Norway. A total of 445 women were diagnosed as having ovarian cancer during follow-up, from 1961 through 1980. The highest risk was observed among nulliparous women, and the risk decreased significantly with increasing parity. The estimated odds ratio for women with 5 or more births compared with one birth was 0.46, after adjustment for age, urban/rural place of residence and occupational class. Neither age at first or last birth, nor age at menarche or menopause, nor marital status, showed significant associations with ovarian cancer risk after adjustment for parity. The apparent protective effect of high parity was observed for epithelial as well as non-epithelial cancers, and for the separate histological types of epithelial cancer except mucinous cystadenocarcinomas.

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The evidence regarding reproductive events as risk factors for rheumatoid arthritis (RA) is not conclusive. In the present study a population-based cohort consisting of 63,090 women were followed from 1961 to 1989. Detailed data on reproductive factors were collected through personal interviews in the period 1956-9. The endpoint used was mention of RA on the death certificate. Mortality rate ratios were estimated by Poisson regression, controlling for various demographic variables. During 1,485,400 person-yr of observation, a total of 355 cases with RA mentioned on the death certificate were identified. Total time of lactation was associated with a decreased mortality of RA, with an approximate dose-response relationship. The results did not support a role of parity, age at first and last birth, or age at menarche and menopause in the development of RA. This protective effect of lactation on the development of RA has not previously been described, and since a definite biological explanation is lacking, the association should be confirmed in other populations.

Summary This prospective study of 63,090 Norwegian women with 124 cases of thyroid cancer diagnosed during 1961 -1989 (McTiernan et al., 1984;Ron et al., 1987;Preston-Martin et al., 1987;Franceschi et al., 1990a). In autopsy studies, no significant sex and age contrasts in the frequency of occult microcarcinomas seem to be present (Fukunaga & Yatani, 1975) On the basis of the total number of cases included in any particular analysis, the expected number was found for each level of the study variable, assuming no association with cancer (Thomas & Gart, 1983 Times of death and emigration were taken into account in the calculation of expected numbers (Tarone, 1975). The analyses were adjusted for age at the start of follow-up (with 5-year age groups), county and in special cases other demographic and reproductive variables. The adjustments were made by forming a stratum for each combination of covariables. The analyses also produced two-tailed P-values for linear trend. Stratified logistic regression analyses were carried out according to the procedure described by Thomas and Gart (1983). In the estimation procedure, a correction for death and emigration was introduced by decreasing the initial number at risk by half the number of such events occurring among those who did not develop thyroid cancer. Due to missing values for certain reproductive variables, the number of cases varied somewhat between analyses. The majority of the women aged 49 years or less at the start of the follow-up were premenopausal at the time of interview. Consequently, the information on number of births will be incomplete for some participants in this age category. Therefore, in addition to the analyses that included all age groups, separate analyses were performed based on the participants aged 49 years or less and those aged 50 or more at the start of follow-up. Separate analyses were also carried out in subgroups according to age at diagnosis, < 54, 55-69, >70 years. Table I shows the distribution of cases according to demographic variables. The risk of thyroid cancer was highest in the counties of Aust-Agder and Tr0ndelag, whereas no urban/rural gradient was observed. Increased risk was observed in the occupational category fishing, ships officers, crew (odds ratio estimate, based on stratified logistic regression: 2.14, 95% CI: 1.30-3.54 for the total series). Results Demographic factors

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