Impact of menstrual and reproductive factors on breast cancer risk in Japan: results of the JACC study

The incidence of breast cancer among Japanese women, a traditionally low-risk population, has increased substantially. To evaluate the association of reproductive factors with breast cancer risk, we examined 38,159 Japanese women, aged 40-79 years, who responded to a questionnaire on reproductive and other lifestyle factors from 1988 to 1990 in the Japan Collaborative Cohort Study. During an average 7.6 years of follow-up, we documented 151 incidents of breast cancers. Cox proportional hazards modeling was employed to estimate relative risks (RR) and 95% confidence intervals (CI). There was a significant decline in the risk of breast cancer with increasing parity among parous women (trend P=0.01). Women with four or more parities had a 69% lower risk than uniparous women, a reduced risk was also evident among menopausal women. Breast cancer risk tended to rise with increasing age at first delivery (trend P=0.05), the association being very apparent among menopausal women (trend P=0.02). Compared to the women who had their first delivery before age 25, those who delayed this event until after age 34 had an RR of 2.12 (95% CI: 0.72-6.21) and 3.33 (1.07-10.3) among the overall subjects and the menopausal, respectively. There was no apparent association of breast cancer risk with age at menarche or menopause. Our study concerning reproductive risk factors suggests that breast cancer in Japan is similar to that in Western countries, and that reproductive factors, particularly the number of parity and age at first delivery, might be important in the etiology of breast cancer among Japanese women.     

Parity and other reproductive factors and risk of adenomatous polyps of the distal colorectum (United States)

Evidence for an effect of reproductive factors on colorectal carcinogenesis is inconsistent and little is known about their role in development of precursor adenomatous polyps. We evaluated the relation between reproductive factors and distal colorectal adenomas (n = 982) during14 years of follow up of 26,983 participants in the Nurses' Health Study(United States). The women were free of diagnosed cancer or polyps in 1980,underwent endoscopy 1980-94, and had reported on their parity, oral contraceptive (OC) use, and ages at menarche, first term-pregnancy, and menopause. We calculated relative risks (RR) and 95 percent confidence intervals (CI) using multiple logistic regression. Women with higher parity had an increased risk of adenomas of the distal colorectum (P trend = 0.004;6+ cf 0 parity: RR = 1.3, CI = 0.9-1.8) or distal colon (P trend = 0.002, RR= 1.7, CI = 1.2-2.6). This association was significantly stronger among women with a family history of colorectal cancer ( P interaction = 0.03); comparing6+ term-pregnancies with nulliparity, among those with a family history, the RR for distal colon adenoma was 3.2 (CI = 1.4-7.2), while among those without a family history, the RR was 1.3 (CI = 0.8-2.2). We observed no association for distal colorectal adenoma and age at menarche, age at first term-pregnancy, ever use of OCs, or menopausal status. Further work is needed to clarify the relation of parity with colon adenoma risk. This revised version was published online in July 2006 with corrections to the Cover Date.     

Reproductive history and risk of second primary breast cancer: the WECARE study.

BACKGROUND: Women with an initial breast cancer diagnosis are at elevated risk of developing subsequent cancer in the contralateral breast. Studies of reproductive factors and contralateral breast cancer (CBC) have provided inconsistent results. METHODS: We employed a case-control study nested within five population-based cancer registries in the United States and Denmark to examine associations between reproductive history and CBC risk. Cases were women with asynchronous CBC who had their first primary invasive breast cancer before age 55 years. Two controls, who had only one primary breast cancer diagnosis, were individually matched to each case on age and year of diagnosis, race, and registry. A total of 694 case-control triplets and 11 case-control pairs were enrolled. Information regarding possible CBC risk factors was obtained via telephone interviews. Multivariable conditional logistic regression was used to estimate rate ratios (RR) and 95% confidence intervals (95% CI) associated with risk factors of interest. RESULTS: Increasing number of full-term pregnancies (FTP) was inversely associated with CBC risk (P trend, 0.001). Women who reported menarche before age 13 years had an increased risk of CBC (RR, 1.26; 95% CI, 1.01-1.58). Age at first FTP, breastfeeding history, and age at menopause were not significantly associated with CBC risk. CONCLUSIONS: These results suggest age at menarche and parity, which are established risk factors for first primary breast cancer, are associated with CBC, whereas other reproductive risk factors associated with first primary breast cancer, such as age at first FTP, are less important factors in the development of CBC.     

Reproductive factors, hormone use and the risk of lung cancer among middle-aged never-smoking Japanese women: a large-scale population-based cohort study

Although a link between female hormonal factors and the risk of lung cancer has been suggested, few studies have examined this association in detail. We investigated the associations between reproductive factors, hormone use and the risk of lung cancer in a population-based prospective study. Self-administered questionnaires were distributed to 44,677 lifelong never-smoking women in 1990-1994 to assess menstrual and reproductive factors and hormone use. After 8-12 years of follow-up, 153 lung cancer cases were diagnosed. Relative risk (RR) and 95% confidence intervals (CI) were calculated using the Cox proportional hazards model. Age at menopause, age at menarche, number of children, age at first live birth, breast feeding and use of hormones were not associated with a risk of lung cancer, either overall or among postmenopausal women or women with natural menopause. Compared to women with both late age at menarche (> or =16) and early age at menopause (< or =50), those with either early age at menarche or late age at menopause had a >2-fold, significant increase in the risk of lung cancer. Induced menopausal women with experience of hormone replacement therapy had a significantly elevated risk compared to naturally menopausal women without female hormone use, with an RR of 2.40 (95% CI 1.07-5.40). These findings suggest that both endogenous and extraneous estrogen may be involved in the etiology of lung cancer.     

Hormonal and reproductive factors and risk of glioma: a prospective cohort study

The etiology of glioma, the most commonly diagnosed malignant brain tumor among adults in the United States, is poorly understood. Given the lower incidence rate of glioma in women than in men, it has been hypothesized that reproductive and hormonal factors may be involved in the etiology of glioma. We conducted a secondary analysis of data from the National Breast Screening Study, which included 89,835 Canadian women, aged 40-59 years at recruitment between 1980 and 1985. Linkages to national cancer and mortality databases yielded data on cancer incidence and deaths from all causes, respectively, with follow-up ending between 1998 and 2000. Cox proportional hazards models were used to estimate hazard ratios and 95% confidence intervals (CI) for the association between hormonal and reproductive factors and risk of glioma. During a mean of 16.4 years of follow-up, we observed 120 incident glioma cases. Compared with women with a relatively early age at menarche (< or =12 years), women who were 13-14 years of age at menarche had a 64% increased risk of glioma (95% CI = 1.01-2.65), and women who were older than 14 years of age at menarche had a 66% increased risk of glioma (95% CI = 0.86-3.20, p(trend) = 0.06). Age at first live birth, parity, menopausal status, use of oral contraceptive and use of hormone replacement therapy were not associated with altered glioma risk in our study population. Additional prospective studies are needed to confirm our findings.     

Oral contraceptive use, reproductive factors, and colorectal cancer risk: findings from Wisconsin.

We investigated the association of oral contraceptive (OC) use and reproductive factors with colorectal cancer risk in a large population-based case-control study. Cases were women ages 20 to 74 years, living in Wisconsin, with a new diagnosis of colon (n = 1,122) or rectal (n = 366) cancer. Control participants were randomly selected from population lists of similarly aged female Wisconsin residents (n = 4,297). Risk factor information was collected through structured telephone interviews. Compared with never users, OC users had an odds ratio (OR) of 0.89 [95% confidence interval (95% CI), 0.75-1.06] for colorectal cancer. OC use associations did not differ significantly between colon and rectal cancer sites; however, when compared with never users, recent OC users (<14 years) seemed at reduced risk of rectal cancer (OR, 0.53; 95% CI, 0.28-1.00). Women with age at first birth older than the median (23 years) had 0.83 times the risk of colon cancer compared with women with age at first birth below the median (95% CI, 0.70-0.98). We observed an inverse trend between increasing parity and rectal cancer risk (P = 0.05). Compared with nulliparous women, women with five or more births had 0.66 times the risk of rectal cancer (95% CI, 0.43-1.02). Compared with postmenopausal women, premenopausal women were at reduced risk (OR, 0.67; 95% CI, 0.47-0.97) of colorectal cancer. No significant associations were observed between colorectal cancer risk and age at menarche or age at menopause. These findings suggest differential roles of reproductive factors in colon and rectal cancer etiology.     

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.     

Epidemiology of reproductive and hormonal factors in thyroid cancer: evidence from a case-control study in the Middle East.

Thyroid cancer is the second most common neoplasm among women in Kuwait and several other countries in the Middle East. Most of these countries also have relatively high birth and total fertility rates. To examine potential relationships between reproductive and hormonal factors and thyroid cancer, we conducted a population-based case-control interview study among 238 women diagnosed with thyroid cancer and a similar number of individually matched controls in Kuwait. Among the demographic variables, women with 12+ years of education had a significantly reduced risk of thyroid cancer (OR = 0.4; 95% CI: 0.2-0.8; p-trend <0.05). The average age at diagnosis (+/-SD) of thyroid cancer was 34.7 +/- 11 years. Events such as age at menarche, pregnancy, menopausal status and age at menopause were not associated with thyroid cancer. There was an association with age at last pregnancy and parity. Women who had their last pregnancy at ages > or = 30 years were at a significantly increased risk (OR = 2.1; 95% CI: 1.2-3.8); there was also a significant trend in risk with increasing age at last pregnancy. There was a modest increase in risk among women who had borne > or = 5 children (OR = 1.5; 95% CI: 0.9-2.5). A joint analysis of these factors showed that childbearing during the latter half of reproductive life had a substantial effect on the incidence of thyroid cancer; for any given level of parity, there was about a 2-fold increased risk if the age at last pregnancy was > or = 30 years. A substantial recent-birth effect, in relation to subsequent diagnosis of thyroid cancer, was observed during the second and third year after a birth (OR = 2.0; 95% CI: 1.0-4.1). In contrast, spontaneous abortion seemed to have a protective effect. There was a significant decrease in risk among women who had a miscarriage as outcome of first pregnancy (OR = 0.1; 95% CI: 0.03-0.4) and those who had experienced > or = 3 miscarriages (OR = 0.3; 95% CI: 0.1-0.8; p-trend <0.05). Overall, any female hormone use was not associated with thyroid cancer risk. New association is suggested for a history of post-partum thyroiditis (OR = 10.2; 95% CI: 2.3-44.8). These data support the hypothesis that reproductive factors and patterns may influence, or contribute to, the risk of thyroid cancer among women.     

Differential effects of reproductive factors on the risk of pre- and postmenopausal breast cancer. Results from a large cohort of French women

The aim of this study was to obtain a better understanding of the role of hormonal factors in breast cancer risk and to determine whether the effect of reproductive events differs according to age at diagnosis. It analysed the effect of age at menarche, age at first full-term pregnancy, number of full-term pregnancies and number of spontaneous abortions both on the overall risk of breast cancer and on its pre- or postmenopausal onset, using the data on 1718 breast cancer cases, obtained from a large sample of around 100000 French women participating in the E3N cohort study. The results provide further evidence that the overall risk of breast cancer increases with decreasing age at menarche, increasing age at first pregnancy and low parity. No overall effect of spontaneous abortions was observed. The effect of these reproductive factors differed according to menopausal status. Age at menarche had an effect on premenopausal breast cancer risk, with a decrease in risk with increasing age of 7% per year (P<0.05). Compared to those who had their first menstrual periods at 11 or before, women experiencing menarche at 15 or after had an RR of 0.66 (95% CI 0.45-0.97) in the premenopausal group. Age at first full-term pregnancy had an effect on both pre- and postmenopausal breast cancer risk, with significant tests showing increasing risk per year of increasing age (P=0.001 and P<0.05 respectively). A first full-term pregnancy above age 30 conveyed a risk of 1.63 (95% CI 1.12-2.38) and 1.35 (95% CI 1.02-1.78) in the pre- and postmenopausal groups respectively. A protective effect of high parity was observed only for postmenopausal breast cancer risk (P for trend test =0.001), with point estimates of 0.79 (95% CI 0.60-1.04), 0.69 (95% CI 0.54-0.88), 0.66 (95% CI 0.51-0.85) and 0.64 (95% CI 0.48-0.86) associated to a one, two, three and four or more full-term pregnancies. A history of spontaneous abortion had no significant effect on the risk of breast cancer diagnosed before or after menopause. Our results suggest that reproductive events have complex effects on the risk of breast cancer.     

Menstrual and reproductive factors and endometrial cancer risk: Results from a population-based case-control study in urban Shanghai

The purpose of our study was to evaluate the association of menstrual and reproductive factors with the risk of endometrial cancer. In a population-based case-control study conducted in urban Shanghai, in-person interviews were completed for 833 women aged 30-69 years and an equal number of controls frequency-matched to cases by age. All cases were newly diagnosed with endometrial cancer between January 1, 1997 and December 31, 2001. The unconditional logistic regression model was employed to derive the adjusted odds ratios (ORs) of endometrial cancer and 95% confidence intervals (CIs) in relation to menstrual and reproductive factors. Earlier menarche age, particularly among premenopausal women, and later menopausal age were associated with an elevated risk of endometrial cancer. A clear dose-response relation between endometrial cancer risk and years of menstruation was observed (p for trend < 0.01). Compared to women ever having a pregnancy and women ever having had a live birth, respectively, nulligravity and nulliparity were both associated with a more than one-fold elevated risk of endometrial cancer. Both completed (OR = 3.02, 95% CI 1.10-8.32 for women never having a complete pregnancy) and incomplete pregnancy (OR = 0.69, 95%CI 0.55-0.87) conferred a protective effect against endometrial cancer, and the protective effect appeared to increase with total number of pregnancies (p for trend = 0.01). The effect of pregnancy on endometrial cancer remained unchanged with increasing time since the last pregnancy. Stillbirth and age at first pregnancy was unrelated to endometrial cancer risk. Our study suggests that prolonged menstruation was related to an increased risk of endometrial cancer while pregnancy, including induced abortion, reduced the risk of endometrial cancer.     

Socioeconomic status and colon cancer incidence: a prospective cohort study.

The association between socioeconomic status and colon cancer was investigated in a prospective cohort study that started in 1986 in The Netherlands among 120,852 men and women aged 55-69 years. At baseline, data on socioeconomic status, alcohol consumption and other dietary and non-dietary covariates were collected by means of a self-administered questionnaire. For data analysis a case-cohort approach was used, in which the person-years at risk were estimated using a randomly selected subcohort (1688 men and 1812 women). After 3.3 years of follow-up, 312 incident colon cancer cases were detected: 157 men and 155 women. After adjustment for age, we found a positive association between colon cancer risk and highest level of education (trend P = 0.13) and social standing (trend P = 0.008) for men. Also, male, upper white-collar workers had a higher colon cancer risk than blue-collar workers (RR = 1.42, 95% CI 0.95-2.11). Only the significant association between social standing and colon cancer risk persisted after additional adjustment for other risk factors for colon cancer (trend P = 0.005), but the higher risk was only found in the highest social standing category (RR highest/lowest social standing = 2.60, 95% CI 1.31-5.14). In women, there were no clear associations between the socioeconomic status indicators and colon cancer.     

Reproductive factors and pancreatic cancer risk: a Norwegian cohort study

A cohort of 63,090 Norwegian women born 1886-1928 was followed more than 38 years, and relations between reproductive factors and risk of pancreatic cancer were explored; 449 cases were recorded at ages 50-89 years. Age at menopause showed a moderately positive association with risk (rate ratio (RR)=1.08 per 2 years delay in menopause; 95% confidence interval (CI)=1.00-1.17). Neither parity nor duration of breastfeeding showed significant associations with risk after adjusting only for demographic factors. With mutual adjustment, however, parity became positively associated (RR=1.13 per delivery; 95% CI=1.05-1.22) while duration of breastfeeding was inversely associated (RR=0.87 per 12 months; 95% CI=0.78-0.97). These associations lessened in magnitude with increasing age, and were essentially absent above age 80 years. Risk was raised among women reporting at least one abortion, but no trend was seen with number of abortions. Together with previous studies, the findings raise questions about the role of chance, but do not exclude hormonal factors related to breastfeeding and pregnancy from affecting pancreatic cancer risk.     

Spontaneous abortion and risk of fatal breast cancer in a prospective cohort of United States women

Controversy exists over the possible relationship between induced and spontaneous abortion and risk of breast cancer. Thus, the association of fatal breast cancer and spontaneous abortion was examined in a large prospective study of United States adult women. After seven years of follow-up, 1,247 cases of fatal breast cancer were observed among 579,274 women who were cancer-free at interview in 1982 and who provided complete reproductive histories. Results from Cox proportional hazards models, adjusted for other risk factors, showed no association between a history of spontaneous abortion and risk of fatal breast cancer (rate ratio [RR]=0.89, 95 percent confidence interval [CI]=0.78–1.02). The RR did not increase with increasing numbers of abortions. Parous women who had a spontaneous abortion before their first term birth were not at increased risk compared with parous women with no history of spontaneous abortion (RR=0.76, CI=0.54–1.05). Women whose only pregnancy ended in a spontaneous abortion were not at increased risk compared with women who were never pregnant (RR=0.61, CI=0.27–1.38) or whose only pregnancy ended in a livebirth (RR=0.72, CI=0.32–1.65). These findings do not support an association between spontaneous abortion and fatal breast cancer.     

Reproductive factors and colorectal cancer risk among Chinese females

We report results from a population-based case-control study of colorectal cancer among Chinese women in western North America (NA) and the People's Republic of China (China). A common protocol was used to assess reproductive characteristics and hormone use of 395 Chinese women (189 from NA and 206 from China) with cancer of the colon or rectum and of 1112 age-matched Chinese controls (494 from NA and 618 from China). In NA, risks for cancers of both the colon and rectum were lower among parous compared to nulliparous women (odds ratio for colorectal cancer, 0.6, P = 0.08), but the trend in risk was not smooth with increasing number of livebirths. This association with parity was absent for both cancer sites in China. There were no consistent patterns in the relationships between other reproductive factors (including age at menarche, age at first livebirth, menopausal status) and risk of colon and rectal cancer on either continent.     

Reproductive factors and breast cancer in New Zealand

A national population-based case-control study was used to assess the influence on breast cancer risk of reproductive factors and the possibility of an interaction with age at diagnosis. A total of 891 women aged 25 to 54 with a first diagnosis of breast cancer, and 1864 control subjects, randomly selected from the electoral rolls, were interviewed. There was a declining risk of breast cancer with increasing age at menarche (p = 0.06), the strongest effect being seen in women aged less than 40. Parous women had a 27% lower risk of breast cancer than nulliparous women, a reduced risk being evident in all but the youngest age group. A falling risk of breast cancer with rising parity was clear only in women diagnosed when aged at least 45 years. Breast cancer risk tended to fall amongst parous women with increasing duration of breastfeeding (p = 0.14); the association was most apparent in the youngest women, while those over 40 years at diagnosis showed no clear negative trend. There was no association of breast cancer risk with age at first full-term pregnancy, time since last full-term pregnancy, abortion (spontaneous or induced), abortion before first full-term pregnancy, or ability to conceive, and there was no trend in risk with age at natural menopause. Women in the highest category of body mass index at age 20 had the lowest risk of breast cancer in the age group studied. When each reproductive factor was formally tested for effect modification by age at diagnosis, the interaction term in logistic models approached statistical significance only for parity (p = 0.07). Int. J. Cancer 76:182–188, 1998.© 1998 Wiley-Liss, Inc.     

Induced and spontaneous abortion in relation to risk of breast cancer (United States)

The relation of induced and spontaneous abortion to the risk of breast cancer is evaluated in a hospital-based case-control interview study conducted in three cities in the United States from 1985 through 1995. Cases were 1,803 women aged 25 to 64 years with newly diagnosed invasive breast cancer; controls were 4,182 women of the same ages admitted for conditions unrelated to reproductive factors. Other breast cancer risk-factors were controlled through multiple logistic regression. The reference for allanalyses was women who had never had an abortion, either induced or spontaneous. Among parous women, the relative risk (RR) estimate was 1.1 (95percent confidence interval [CI] = 0.9-1.5) for induced abortion overall, 1.0(CI = 0.7-1.4) for abortion before the first birth, and 1.3 (CI = 1.0-1.8)for abortion after at least one birth. Among nulliparous women, the relative risk estimate for induced abortion was 1.3 (CI = 0.9-1.9). There was no trend of increased risk with number of abortions, nor was there consistent evidence of an increased risk in any particular subgroup. Spontaneous abortion was not associated with increased risk of breast cancer, either among nulliparous women or among parous women. These findings provide little support for the hypothesis that induced abortion increases breast cancer risk overall or in particular subgroups. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effect of lifetime lactation on breast cancer risk: a Korean women's cohort study

The objective of our study was to examine the effect of lifetime lactation on breast cancer risk among premenopausal women. The data were from a prospective cohort study with a follow-up period of 6 years in Korea (1995-2000). The cohort was composed of 110,604 premenopausal parous Korean women, aged 20 years and older, who received health insurance from the Korea Medical Insurance Corporation and who had medical evaluations in 1992 and 1994. Multivariate Cox proportional hazard models were tested, controlling for age, age at menarche, number of children, age at first pregnancy, oral contraceptive use, smoking, exercise and obesity. At baseline, 57,440 (51.9%) reported breastfeeding and 4,584 (4.1%) reported breastfeeding more than 24 months. From 1995-2000, 360 incident cases of breast cancer (61.8/100,000 person-years) occurred. Compared to parous women who had no history of lactation, a period of lactation of 13-24 months decreased the risk of breast cancer (RR, 0.7; 95% CI, 0.5-1.1), and this risk was decreased even further for those who breastfed for more than 24 months (RR, 0.6; 95% CI, 0.3-1.0). There was a clear trend of decreasing breast cancer risk with the duration of lactation (p for trend <0.001). In conclusion, our study of a large Korean cohort provides additional empirical evidence to current theoretical conjecture that lactation decreases the risk of breast cancer among premenopausal women.     

Parity,Other Reproductive Factors,and Risk of Pancreatic Cancer Mortality in a Large Cohort of U.S. Women (United States)

Several studies have found an inverse relationship between parity and risk of pancreatic cancer. However, most of the studies of pancreatic cancer in relation to parity and other reproductive factors have been small and the results inconsistent. Most recently, a well-conducted, prospective cohort study found a linear inverse association between parity and pancreatic cancer. To clarify the relationship between parity and other reproductive factors and risk of pancreatic mortality, we examined these associations among 387,981 postmenopausal U.S. women in the Cancer Prevention Study (CPS)-II cohort. CPS-II participants completed a self-administered questionnaire in 1982 and were followed for mortality through 2000. During follow-up, 1959 pancreatic cancer deaths occurred. Using Cox proportional hazards modeling, we calculated rate ratios (RR) adjusted for age, race, education, personal history of diabetes, body mass index, height, exercise, family history of pancreatic cancer, and cigarette smoking status, frequency, and duration. Overall, we did not observe a significant association between parity and pancreatic cancer mortality (trend p = 0.07). However, women who had five or more births had lower death rates from pancreatic cancer than nulliparous women (RR = 0.80, 95% CI = 0.66–0.96). We observed no association between any other reproductive factors examined (age at first birth, menarche, or menopause; type of menopause; diethylstilbestrol (DES) use; or duration of oral contraceptive or estrogen replacement therapy use) and pancreatic cancer mortality. In summary, our results support the observation that high parity is associated with lower risk of pancreatic cancer but do not show a linear trend with increasing parity. Furthermore, we find no evidence that other reproductive factors are associated with pancreatic cancer mortality.     

Reproductive factors,exogenous female hormones,and colorectal cancer by subsite

The associations between reproductive factors, exogenous hormones, and colorectal cancer were examined among female subjects in a population-based case-control study in Sweden. The study was performed in Stockholm in 1986–88, and included 299 cases and 276 controls. There was little evidence that age at first birth, number of months of breast feeding, age at menarche, or age at menopause influenced the risk of colon or rectal cancer. However, the results indicate that postmenopausal hormone-replacement therapy might reduce the risk of colorectal cancer (age-adjusted relative risk [RR]=0.4, 95 percent confidence interval [CI]=0.2–0.9). Compared with nulliparous women, women with at least four births were at reduced risk for colon cancer (RR=0.5, CI=0.2–1.2) but not rectal cancer (RR=1.0, CI=0.4–2.6). However, no trend across increasing parity was observed. Adjustments for diet, body mass, and physical activity had little influence on the results.The authors are with the Department of Preventive Medicine, University of Southern California School of Medicine. Dr Gerhardsson de Verdier is also in the Department of Epidemiology, Institute of Environmental Medicine, Karolinska Institute, Stockholm, Sweden. Address correspondence to Dr London, University of Southern California School of Medicine, Department of Preventive Medicine, PMB B306, 1420 San Pablo Street, Los Angeles, CA 90033, USA. The study was supported by two grants (2228-B86-013XA; 2228-B87-02XA) from the Swedish National Cancer Society.     

Reproductive factors,exogenous female hormone use and breast cancer risk in Japanese: the Miyagi Cohort Study

The incidence of breast cancer among Japanese women is substantially increasing. This population-based prospective cohort study in Japan evaluated the associations of reproductive factors and exogenous female hormone use with breast cancer risk, both overall and separately among premenopausal and postmenopausal women. A total of 24,064 women aged 40–64 were followed from 1990 to 2003. During 309,424 person-years of follow-up, 285 breast cancer cases were documented. In overall evaluation, nulliparity was significantly associated with an increased risk of breast cancer. There was a significant decrease in risk with increasing parity number among parous women (trend P = 0.008). No association was observed between age at menarche or age at first birth and breast cancer risk. Neither oral contraceptive (OC) use nor the use of exogenous female hormones other than OC was associated with breast cancer risk. The evaluation according to menopausal status revealed that nulliparity and parity number were significantly related to breast cancer risk only among postmenopausal women. Later age at natural menopause was associated with an increased risk of breast cancer among postmenopausal women (trend P = 0.02). Our findings suggest that parity number and age at menopause have great effects on breast cancer risk among Japanese women.