title: Impact of menstrual and reproductive factors on breast cancer risk in Japan: results of the JACC study.
abstract: 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.
authors.affiliation: Department of Public Health/Health Information Dynamics, Nagoya University Graduate School of Medicine, 65 Tsurumai-cho, Showa-ku, Nagoya 466-8550. tamako@med.nagoya-u.ac.jp
authors.forename: Koji
authors.initials: K
authors.lastname: Tamakoshi
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Yatsuya
authors.forename: Kenji
authors.initials: K
authors.lastname: Wakai
authors.forename: Sadao
authors.initials: S
authors.lastname: Suzuki
authors.forename: Kazuko
authors.initials: K
authors.lastname: Nishio
authors.forename: Yingsong
authors.initials: Y
authors.lastname: Lin
authors.forename: Yoshimitsu
authors.initials: Y
authors.lastname: Niwa
authors.forename: Takaaki
authors.initials: T
authors.lastname: Kondo
authors.forename: Akio
authors.initials: A
authors.lastname: Yamamoto
authors.forename: Shinkan
authors.initials: S
authors.lastname: Tokudome
authors.forename: Hideaki
authors.initials: H
authors.lastname: Toyoshima
authors.forename: Akiko
authors.initials: A
authors.lastname: Tamakoshi
mesh_terms: D000328:Adult; D000367:Age Factors; D000368:Aged; D001943:Breast Neoplasms / Q000453:epidemiology* / Q000209:etiology*; D005260:Female; D006801:Humans; D007564:Japan / Q000453:epidemiology; D008598:Menstruation; D008875:Middle Aged; D011446:Prospective Studies; D017584:Reproductive History; D012307:Risk Factors