Parity and breast cancer risk: Possible effect on age at diagnosis

Abstract

We examined the effect of parity on breast cancer risk on the basis of information from 107,146 married female nurses followed prospectively between 1976-1980. For everparous women, a significant protective effect against breast cancer was observed with increasing parity (chi 12 for trend = 14.2, p less than 0.001). Adjustment for age at first birth and other potential confounders by multiple logistic regression did not materially affect this trend. After controlling for these risk factors, the overall relative risk of breast cancer for women with four or more pregnancies lasting 6 months or longer was 0.68 (95% confidence limits 0.51-0.90) compared with women with only one pregnancy. When nulliparous and everparous women were compared, an interaction of age and everparous status in 1976 was observed. The logistic regression coefficient for the interaction was 0.3419 (95% confidence limits 0.0374-0.6464). Specifically, for women of parity 2 relative to nulliparous, the relative risk of breast cancer and the 95% confidence intervals were 1.85 (1.09, 3.14), 1.32 (0.92, 1.89), and 0.93 (0.62, 1.41) for 30-39, 40-49 and 50-55-year-old women, respectively. This points to the need of presenting age-specific, rather than age-adjusted, relative risks in future studies of breast cancer and parity.

Absence of association between reproductive variables and the risk of breast cancer in young women in Sweden and Norway

“…Our data did not support either the hypothesis of a cross-over effect of parity (Janerich & Hoff, 1982;Pathak et al, 1986;Negri et al, 1988) -with increased relative risks at younger ages and a protective effect in older women -or the finding of a rising risk with increasing number of births in premenopausal women (Brignone et al, 1987). Moreover, recent claims that induced abortions (Pike et al, 1981;Hadjimichael et al, 1986;Ewertz & Duffy, 1988), a long interval between menarche and first birth (Brignone et al, 1987), and a last pregnancy at a high age (La Vecchia et al, 1987) entail an increased risk was given no support.…”

“…A closer view of available data, however, often reveals a pattern of contradictory findings rather than consistent ones. For instance, the duration of breast feeding and the number of children became accepted as proxy variables for age at first birth almost two decades ago (MacMahon et al, 1973;Kelsey, 1979), whereas later evidence indicates that both breast feeding and high parity exert independent protective effects against the development of breast cancer (Tulinius et al, 1978;Adami et al, 1980;Paffenbarger et al, 1980;MacMahon et al, 1982;Brinton et al, 1983;Helmrich et al, 1983;Trapido, 1983;Lipnick et al, 1984;Byers et al, 1985;Pathak et al, 1986;Ewertz & Duffy, 1988;La Vecchia et al, 1987;Layde et al, 1989).…”

“…The results of several studies have pointed to interactions between parity and age (Janerich & Hoff, 1982;Pathak et al, 1986;Negri et al, 1988) or differences between the risk factors operative in pre-and postmenopausal women (Paffenbarger et al, 1980;Helmrich et al, 1983;Lipnick et al, 1984;Byers et al, 1985;McTierman & Thomas, 1986;Brignone et al, 1987;Layde et al, 1989). But there is no agreement as to the details.…”

“…For instance breast feeding has been claimed by some investigators to confer a protective effect before menopause only (Byers et al, 1985;McTierman & Thomas, 1986), whereas others have failed to show any association (MacMahon et al, 1982;Lipnick et al, 1984). Likewise, a significant protective effect of high parity has been reported to be absent (Hunt et al, 1980;McTierman & Thomas, 1986), confined to premenopausal (Lipnick et al, 1984) or post-menopausal (Paffenbarger et al, 1980;Pathak et al, 1986) women, or present irrespective of age (Helmrich et al, 1983;Byers et al, 1985). A first birth at an early age has been found to entail a reduced risk of developing breast cancer only or primarily before menopause (Lipnick et al, 1984;Ewertz & Duffy 1988;Layde et al, 1989), after menopause (Stavraky & Emmons, 1974;Lubin et al, 1982;Byers et al, 1985), or at all ages (Tulinius et al, 1978;Kelsey, 1979;Paffenbarger et al, 1980;Helmrich et al, 1983;Negri et al, 1988;Layde et al, 1989).…”

A prospective study of reproductive, familial, and socioeconomic risk factors for breast cancer using NHANES I data

“…Epidemiologic studies of breast cancer risk have identified numerous host and environmental determinants (1-13) that presumably play an etiologic role in the development of the disease. While the majority of these studies have used case-control designs (1)(2)(3)(4)(5)(6)(8)(9)(10)(11), a few cohort studies have been reported, though none have involved an essentially representative sample of women from the United States (7,12,13).…”