Age at first birth, parity and risk of breast cancer in a Swedish population

Abstract

Summary.-A case-control study was conducted over a period of 11 months in an area containing one-third of the Swedish population. One thousand and one patients participated, constituting 94°, of all women newly diagnosed as having breast cancer within the area. They were compared with 1,001 age-matched, non-hospitalized controls without breast cancer, selected by paired sampling from a population register. The risk of breast cancer was slightly, but significantly, related to parity, the standardized relative risk (SRR) being 1P35 for nulliparous women as compared to ever parous. In the different parity groups a risk significantly lower than that for nulliparous women was found only for women with more than 2 children (SRR = 0-59) but the trend with parity was highly significant (P <0-001). Age at first birth was not found to be an important risk factor for breast cancer. SRR was lower than for nulliparous women in all groups of women with their first birth before the age of 35 years, but the difference was significant (P <0 05) only for those with the first birth between 20 and 24 (SRR=0-69) and 25 and 29 (SRR=0-69) years of age. The trend with age at first birth (P < 0 05) disappeared after stratification for parity, suggesting that it was a confounding factor.

Risk of breast cancer in relation to reproductive factors in Denmark

“…The confirmation of parity as a protective factor and the lack of association between breast cancer and age at first full-term pregnancy agree well with large, population-based studies from Sweden (Adami et al, 1980) and Norway (Kv'ale et al, 1987a, b), whose population characteristics are very similar to those of Denmark. Apart from these, other studies showing no association between breast cancer and age at first birth have been relatively small, with less than 200 cases (Herity et al, 1975;Thein-Hlang & Thein-MaungMyint, 1978;Adami et al, 1978;Pike et al, 1981;Harris et al, 1982;Storm et al, 1986) with a low statistical power of detecting an association, especially if it was weak.…”

“…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).…”

“…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).…”

On the age-dependent association between cancer of the breast and of the endometrium. A nationwide cohort study

“…It seems unsolved whether the elevated risk of breast cancer in women of low parity and in those of high age at first birth represents a causal relationship (Kelsey & Hildreth, 1983;Adami et al, 1980). Moreover, there is no indication from our data or from others that this association becomes more pronounced with increasing age.…”

The pattern of risk factors for breast cancer in a southern France population. Interest for a stratified analysis by age at diagnosis

“…These have been found in most countries where studies have been conducted: North America (Helmrich et al, 1983;Lubin et al, 1987); Scandinavia (reviewed in Ewertz et al, 1990); Western Europe (Talamini et al, 1985;Le et al, 1984); Eastern Europe (Plesko et al, 1985); Asia (Thein & Theen et al, 1978) and South America (Mirra et al, 1971). However, in several studies, there is no risk increase associated with some of these established factors (Adami et al, 1980;East European Study of BC epidemiology, 1990).…”