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A population-based case-control study was conducted in Northern Norway and Central Sweden to investigate hormonal and reproductive factors and cigarette smoking as determinants of papillary and follicular thyroid carcinoma in women. Information on 191 histologically confirmed cases and 341 age-matched controls was included. No clear association was found with regard to the number of live births, number of pregnancies, a history of incomplete pregnancies, or the use of oral contraceptives or hormonal replacement therapy. However, an early first childbirth (before 20 years of age, or less than 5 years after menarche) was associated with an increased risk of thyroid cancer. There was an increased risk of thyroid cancer among women with a history of artificial menopause compared to those with a spontaneous menopause [odds ratio (OR), 2.52; 95% confidence interval (CI), 0.96-6.62], which was more pronounced for the papillary carcinoma and after adjustment for age at menopause and use of replacement therapy. Cigarette smokers had a decreased risk of borderline statistical significance compared to nonsmokers (OR, 0.69; 95% CI, 0.47-1.01), particularly among premenopausal women (OR, 0.60; 95% CI, 0.38-0.96). This negative association persisted after adjustment for parity, hormonal treatments, and education. Women who started smoking before the age of 15 experienced a marked reduction in risk (OR, 0.38%; 95% CI, 0.18-0.80¿). Moreover, there was a suggestion of a dose-response effect with the amount of cigarettes smoked daily and with duration of the habit. Both the increased risk of artificial menopause and the negative association with smoking are compatible with a relation between levels of estrogens and thyroid cancer among women.

A population-based case-control study was conducted in Sweden and Norway to analyse possible associations between breast cancer occurring before the age of 45 and several different characteristics of the women's reproductive life. A total of 422 (89.2%) of all eligible patients, and 527 (80.6%) of all eligible controls were interviewed. In univariate analyses, different characteristics of child-bearing (parity, age at first birth, years between last birth and diagnosis, duration of breast-feeding, and number of induced and spontaneous abortions), measures of the fertile or ovulating period (age at menarche, years between menarche and first pregnancy, and estimates of the menstruation span) and symptoms of anovulatory cycles or infertility were all seemingly unrelated to, or at most weakly associated with breast cancer. Adjustment for possible confounding factors in multivariate analyses resulted in largely unaltered risk estimates with odds ratios close to unity and without any significant trends when the exposure variables were studied in categorised or in continuous form. We conclude that reproductive factors did not explain the occurrence of breast cancer before the age of 45 in this population.

The possible association between oral contraceptive (OC) use and the risk of breast cancer developing before the age of 45 was investigated by means of a population based case-control study in Sweden and Norway. Information was obtained by personal interview from 422 (89.2%) of all eligible patients with a newly diagnosed breast cancer from May, 1984, to May, 1985, and from 722 (80.6%) of all contacted age-matched controls. A multivariate analysis, which accounted for several possible confounding factors, revealed a significant (p = 0.03) association between total duration of OC use and breast cancer risk. The relative risk (RR) of breast cancer after 12 or more years of OC use was 2.2 (1.2-4.0). OC use for more than 7 years before first full-term pregnancy entailed an increased breast cancer risk (RR = 2.0 [1.0-4.2]) which was of borderline significance. When total duration of use was considered, the risk of breast cancer was virtually unrelated to age at first OC use and latency from first use. The results suggest that long-term use of OCs may increase the risk of breast cancer in young women.