This prospective study of 63,090 Norwegian women with 124 cases of thyroid cancer diagnosed during 1961-1989 revealed no strong associations with reproductive factors. Late last birth was related to increased risk, whereas no association was noted with parity. A long reproductive period was related to increased risk of papillary carcinomas, whereas a decreased risk of follicular carcinomas and other adenocarcinomas was observed in women with early menarche and late menopause. The risk of thyroid cancer was significantly increased among women in the occupational category 'fishing, ships officers and crew'. Our results are consistent with a modest effect of certain reproductive factors upon thyroid cancer development.

Adult↗

PubMed1992

Childbearing and use of oral contraceptives: impact of educational level. The Nordland Health Study.

STUDY OBJECTIVE: The aim was to correlate the level of education to childbearing pattern and previous use of oral contraceptives in middle aged women. DESIGN AND SETTING: The study was a cross sectional survey of 3608 women aged 40-42 years in a county in northern Norway and involved 72% of all women in the age bracket living in the county. Information about childbearing and use of oral contraceptives was obtained from a self administered questionnaire. MAIN RESULTS: Level of education was inversely associated with parity and positively related to age at first and last birth, to the use of oral contraceptives at any time, and particularly to the use of oral contraceptives before the first birth. CONCLUSIONS: Women with high level of education have relatively low parity and postpone childbearing by using oral contraceptives.

Adult↗

PubMed1992

Reproductive factors in breast cancer epidemiology.

There is abundant epidemiologic evidence showing that early menarche, late menopause, low parity and late age at first birth are related to increased risk of breast cancer. However, in younger age groups, uniparous women seem to be at higher risk than nulliparous, and the effect of later pregnancies is less clear in this group. Intervals between pregnancies may modify the general protective effect. Some studies have indicated an adverse effect of late age at pregnancies after the first. Further studies are necessary to determine if the general protective effect of pregnancies after the first is preceded by a transient increase in breast cancer risk. No clear association has been established with number of abortions. Results from two large prospective studies suggest that breast feeding is not strongly related to risk of breast cancer among Western populations.

Abortion, Induced↗

PubMed1991

Is the incidence of colorectal cancer related to reproduction? A prospective study of 63,000 women.

Relationships between reproductive factors and risk of colorectal cancer were examined in a population-based prospective study in Norway. Available for analysis were 831 cases (581 colon cancer, 250 rectal cancer) diagnosed in a cohort of 63,090 women, surveyed in 1956-1959 and followed through 1980. Overall, the analyses showed no strong effects of reproductive factors. In particular, high parity was not associated with reduced risk, and late age at first or last birth was not associated with an increased overall risk. However, in age-specific analyses of colon cancer, adverse effects of late age at first birth and late age at last birth were observed in women with cancer diagnosed before the age of 60. A non-significant overall excess risk in parous compared to nulliparous women was strongest for women with a cancer diagnosis before the age of 50 years. Having had many abortions was associated with increased risk for all sub-sites. Neither age at menarche nor age at menopause were related to risk of colorectal cancer. The results suggest that reproductive factors, which are of importance in the etiology of cancer of the breast and genital organs in women, are not similarly related to risk of colorectal cancer.

Abortion, Spontaneous↗

PubMed1991

Incidence of multiple sclerosis in Hordaland, western Norway: a fluctuating pattern.

The incidence of multiple sclerosis (MS) was studied in the county of Hordaland, western Norway. A significant increase in incidence in the period 1958-1987, a decline followed by a gradual increase in mean age at onset, geographic differences in time trends and a biphasic pattern revealed by a birth cohort analysis support the theory of real time-space fluctuations in the incidence of MS over time.

Adult↗

PubMed1991

Reproductive factors and cancers of the breast and genital organs--are the different cancer sites similarly affected?

Relationships between reproductive factors and cancer of the breast and genital organs were investigated in a prospective study of 63,090 Norwegian women. During followup from 1961 through 1980 1565 cases of breast cancer, 422 cases of cancer of the corpus uteri, and 471 cases of ovarian cancer were diagnosed. High parity was associated with low risk of cancer of the breast, corpus uteri, and ovary in analyses with adjustment for age at first birth. Age at first birth and age at last birth were positively associated with risk of breast cancer and inversely associated with endometrial cancer, whereas no clear associations were observed with ovarian cancer. Results suggest that the effect of a pregnancy on cancer risk is mediated, at least in part, by different mechanisms for these three sites. Age at menarche was inversely and age at menopause positively related to risk of breast and endometrial cancer, whereas no association was observed for ovarian cancer. In analyses of squamous cell carcinoma of the cervix uteri, an association with reproductive factors was expected from the known relationships with sexual habits. However, this cannot account for the high risk observed in this study in women with early age at first birth.

Abortion, Spontaneous↗

PubMed1990

Age at first birth, parity and risk of breast cancer: a meta-analysis of 8 studies from the Nordic countries.

Several large epidemiological studies in the Nordic countries have failed to confirm an association between age at first birth and breast cancer independent of parity. To assess whether lack of power or heterogeneity between the countries could explain this, a meta-analysis was performed of 8 population-based studies (3 cohort and 5 case-control) of breast cancer and reproductive variables in the Nordic countries, including a total of 5,568 cases. It confirmed that low parity and late age at first birth are significant and independent determinants of breast-cancer risk. Nulliparity was associated with a 30% increase in risk compared with parous women, and for every 2 births, the risk was reduced by about 16%. There was a significant trend of increasing risk with increasing age at first birth, women giving first birth after the age of 35 years having a 40% increased risk compared to those with a first birth before the age of 20 years. Tests for heterogeneity between studies were not significant for any of the examined variables. In the absence of bias, this suggests that several individual Nordic studies may have had too little power to detect the weak effect of age at first birth observed in the meta-analysis.

Breast Neoplasms↗

PubMed1990

[Risk factors for breast cancer. Do epidemiologic findings provide a basis for primary prevention?].

In Norway, approximately one in every 13 women will develop breast cancer. Epidemiological studies show rather consistent results regarding associations with some risk factors. Lowest risk is found in women with many children, early first birth, late menarche and early menopause. Recent studies indicate that a full-term pregnancy, after a short-lasting adverse effect, results in a risk-reduction of lifelong duration. The authors review the present knowledge of these and other risk factors of possible importance in the etiology; lactation, diet, body weight, physical activity, use of oral contraceptives and hormone replacement therapy. Primary preventive measures may potentially lead to substantial reduction in risk of breast cancer. However, it is difficult to suggest any feasible intervention strategy which would have a high probability of reducing occurrence of breast cancer in the general population.

Adult↗

PubMed1990

Biology and natural history of breast cancer.

Explore the source record for details and available documents.

Breast Neoplasms↗

PubMed1988

Reproductive factors and risk of cancer of the uterine corpus: a prospective study.

Relationships between reproductive factors and risk of cancer of the uterine corpus were investigated in a prospective study of 62,079 women in Norway. A total of 420 cases were diagnosed during follow-up, from 1961 through 1980. The risk of endometrial carcinoma decreased significantly with increasing parity as well as with increasing age at first and last birth. The estimated odds ratio for women with 5 or more births versus uniparous was 0.60 in analyses with adjustment for age and residence characteristics. For first birth at age greater than or equal to 35 versus less than or equal to 19 the odds ratio was 0.48, and for last birth at age greater than or equal to 40 versus less than or equal to 24 we found an odds ratio of 0.45, in analyses with additional adjustment for parity. Significant associations were also found with age at menarche and menopause, the highest risks being observed for women with early menarche or late menopause. The different reproductive variables seemed to affect the risk of sarcomas of the uterine corpus and the risk of endometrial carcinomas in a similar way.

Adult↗

PubMed1988

Menstrual factors and breast cancer risk.

Relationships between menstrual factors and breast cancer risk were investigated in a prospective study of 63,090 Norwegian women. A total of 1565 cases of breast cancer occurred during follow-up from 1961 through 1980. The risk of breast cancer decreased with increasing age at menarche (P = 0.06) and increased with increasing age at menopause (P = 0.005) in analyses adjusted for age, urban/rural place of residence, parity, and age at first and last birth. The results correspond to an average increase in breast cancer risk of 4.0% for each year of decrease in age at menarche, and an increase in risk of 3.6% for each year of increase in age at menopause. The protective effect of early menopause was strongest for breast cancer diagnosed in patients 80 years of age or older. No clear relationship was seen between menstrual irregularities and breast cancer risk.

Adult↗

PubMed1988

Reproductive factors and risk of ovarian cancer: a prospective study.

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.

Age Factors↗

PubMed1988

Reproductive factors and risk of cervical cancer by cell type. A prospective study.

Relationships between reproductive variables and risk of cervical cancer were examined in a follow-up of 62,079 women in Norway from 1961 through 1980. For the 342 cases classified as squamous cell carcinomas, a higher risk was observed in ever married than in never married women. The risk was especially high among women married more than once and women who were widowed or divorced before start of follow-up. High age at first birth was associated with low risk. The estimated odds ratio for women with first birth at age 35 years or later versus 19 years or earlier was 0.18 (P less than 0.001) in analyses with adjustment for age, urban-rural place of residence and parity. In analyses with adjustment for age at first birth, neither parity or age at first marriage, nor age at menarche or menopause showed significant associations with squamous cell carcinoma. For the 30 cases classified as adenocarcinomas, no significant associations emerged with reproductive factors. The effects of marital status as well as age at first birth differed significantly between adenocarcinomas and squamous cell carcinomas, suggesting dissimilar aetiologies. Although associations between reproductive factors and squamous cell carcinoma may largely be secondary to relationships with sexual habits, there are indications that the association with age at first birth cannot be entirely explained in this way.

Adenocarcinoma↗

PubMed1988

Lactation and cancer risk: is there a relation specific to breast cancer?

Relations between previous lactation experience and risks of cancer of the breast and other sites were investigated after follow-up of 50,274 parous women from 1961 through 1980. Among women with complete information on lactation, 5102 developed cancer and, of these, 1136 were diagnosed with breast cancer. Analyses of associations with mean duration of lactation per birth and duration for each of the three first births suggested a nonlinear relation to breast cancer. The highest risk was observed for those with intermediate duration of breast feeding, whereas lower risks were found among those with very short or very long duration. For all nongenital cancers combined, decreased risks were observed among those with the longest duration of breast feeding. However, among cancers of specific sites, a significant inverse association was found for pancreatic cancer only. The overall impression given by our data is that breast feeding is not strongly related to risks of breast cancer or any other common cancer.

Breast Feeding↗