Risk of breast cancer in relation to reproductive factors in Denmark

Abstract

SummaryThe effect of reproductive factors on breast cancer risk was evaluated in a population-based casecontrol study, including 1,486 breast cancer cases diagnosed over a one-year period in Denmark. They were identified from the files of the nationwide trial of the Danish Breast Cancer Co-operative group and the Danish Cancer Registry. The control group was an age-stratified random sample of 1,336 women from the general population. Data on risk factors were collected by self-administered (mailed) questionnaires. Significantly increased relative risks (RR) were associated with never being pregnant (RR= 1.47), an early terminated first pregnancy (RR=1.43), and having a natural menopause after the age of 54 (RR= 1.67). Trends of decreasing risk were observed by increasing parity and age at menarche. These findings were independent of age at first full-term pregnancy which overall was not related to breast cancer risk, though a weak association appeared in women less than 50 years at diagnosis. The study confirmed that pregnancies must continue to term to offer protection against breast cancer.In 1970, MacMahon et al. showed in their International Collaborative Study that the protective effect of parity on breast cancer risk could be explained by maternal age at first full-term pregnancy from an association between high parity and early age at first birth. Several studies have confirmed this, but some found an additional protective effect of high parity (Soini, 1977;Tulinius et al., 1978;Paffenbarger et al., 1980;Brinton et al., 1983;Helmrich et al., 1983; Pathak et al., 1986). Others have failed to demonstrate an association between breast cancer risk and age at first birth (Choi et al., 1978; Thein-Hlaing & Thein-Maung-Myint, 1978;Adami et al., 1980;Pike et al., 1981;Harris et al., 1982;Kvale et al., 1987b).Conflicting evidence also exists in the literature regarding the role of early terminated pregnancies. Two reports (Pike et al., 1981;Hadjimichael et al., 1986) suggested that a first trimester abortion (induced or spontaneous) before the fullterm pregnancy might elevate the risk of breast cancer, while such an effect was not seen in two other studies (Vessey et al., 1982;Brinton et al., 1983).We were able to evaluate the effect of reproductive factors on breast cancer risk in a population-based case-control study including almost all incident cases over a one-year period in Denmark. Materials and methodsThe study was sampling frame exists in the national Central Population Registry, established in 1968, with the purpose of storing commonly used personal data for each inhabitant and acting as source material for the administrative system in Denmark. The key identifier is a unique 10-digit ID-number, the first 6 digits being the date of birth, which has been issued to all persons living in and entering the country (by birth or immigration) since 1968. The registry is computerised and updated on a regular basis. Through a linkage with the Danish Cancer Registry database, women with a breast cancer predating t...

Established breast cancer risk factors by clinically important tumour characteristics

“…Although the mechanisms responsible for the associations of delayed age at first birth with poor prognostic features are unknown, continuous exposure to cyclic hormones uninterrupted by the dramatic differentiation and remodelling effects of pregnancy on breast tissue might play an important role. In contrast to some previous reports (LiVolsi et al, 1982;Ewertz and Duffy, 1988;Stalsberg et al, 1989), we and others (Morrison, 1976;Wohlfahrt et al, 1999) did not find that late age at first full-term birth was more strongly associated with lobular as compared with ductal carcinoma, NOS. As previously reported, we found that HRT use was more strongly related to lobular (Gapstur et al, 1999;Li et al, 2000;Chen et al, 2002;Daling et al, 2002;Newcomb et al, 2002;Li et al, 2003;Newcomer et al, 2003) and tubular (Manjer et al, 2001) carcinomas than to other histopathologic types.…”

“…Specifically, combined oestrogen and progestin hormone replacement therapy (HRT) (Li et al, 2000;Chen et al, 2002;Daling et al, 2002;Newcomb et al, 2002;Li et al, 2003;Newcomer et al, 2003), and possibly late age at first birth (LiVolsi et al, 1982;Ewertz and Duffy, 1988;Stalsberg et al, 1989) may be more strongly associated with lobular as compared to ductal carcinomas. Epidemiologic data also suggest that hormone-related risk factors vary by hormone receptor status (Althuis et al, 2004).…”

A Multicenter Matched Case Control Study of Breast Cancer Risk Factors among Women in Karachi, Pakistan

“…Our observation of a positive association between age at menopause and breast cancer risk is also consistent with several previous reports (Ewertz and Duffy, 1988;Parsa and Parsa, 2009;Phipps et al, 2010). Late menopause after the age of 50 years leads to an increase in ovulatory cycles and subsequently higher endogenous estrogen levels over a woman's lifetime, with an increased vulnerability to environmental carcinogens, and thus increasing BC risk.…”

Abortion history and breast cancer risk: Results from the Shanghai breast cancer study

“…This increased risk is surprising because the majority of women in China have several induced abortions after a first live birth, 21 which is known to be protective against breast cancer. 7,22,23 The extremely high odds ratios reported for known breast cancer risk factors such as age at first birth older than 30 years (OR ϭ 7.8, 95% CI 3.2-19.0) and family history of breast cancer (OR ϭ 9.0, 95% CI 2.6 -31.5) found in this study also raised concerns about the methodology used in this study.…”