Of the 4286 women in this study, 225 (5.3%) had breast cancer ascertained from at least one source. The majority (82%) of these cases were identified from at least two sources. The age distribution of women with cancer identified by each source were similar – mean (standard deviation) age of women with breast cancer identified by self report 68.7 (5.3), identified by medical record review 68.9 (5.3) and identified by cancer register 69.0 (5.4). Of the 225 cases, 170 were prevalent and 55 incident cases.
shows the distributions of established breast cancer risk factors and other potential risk factors by breast cancer status among all 4286 women. For established risk factors, our results are in the same direction and of similar magnitudes to what one would expect based on consistent results from previous studies. None of the smoking alcohol or social class variables were associated with breast cancer.
Smoking and other characteristics irrespective of whether women had breast cancer or not (N=4286)
Smoking prior to first birth
Of the 4286 women, 3467 (81%) had at least one live birth and of these 3047 (88%) gave their age at the time that their first child was born. The associations presented in were similar in this subgroup of 3047 women. Of these 3047 women, 139 (4.6%) had breast cancer.
shows breast cancer occurrence and the distributions of potential confounding factors by smoking in relation to first birth. There was no association between breast cancer and smoking in relation to first birth. The age-adjusted odds ratio for smoking before the first birth was 1.04 (95% CI: 0.71, 1.51) and the fully adjusted (age, number of children, menarcheal and menopausal age, hysterectomy, use of hormone replacement, use of the oral contraceptive pill, childhood and adulthood social class and BMI) odds ratio was 1.06 (0.72, 1.56). When the analyses were restricted to those who only smoked before their first pregnancy (ie excluding from the analyses all women who smoked but only began after their first birth), the association was essentially unchanged: fully adjusted odds ratio 1.04 (0.67, 1.59). The age-adjusted odds ratios for women who smoked, but only after the birth of their first child compared to never smokers, was 0.79 (0.44, 1.41) and the fully adjusted odds ratio for this association was 0.70 (0.36, 1.36).
Breast cancer and other characteristics by smoking in relation to birth of first child among women with at least one live birth and who provided age at first birth (N=3047)
Smoking around the time of puberty
Among all women in the cohort (4286 women with 225 cases of breast cancer), the age-adjusted odds ratio for smoking either in the year prior to or within 5 years of menarche compared to never smoking was 0.96. (0.69, 1.36); the fully adjusted odds ratio was 1.00 (0.70, 1.39).
In the whole cohort, 186 (83%) of the total 225 cases were postmenopausal and in the subgroup of women with at least one birth and a known age at first birth 118 (85%) of the 139 breast cancers were postmenopausal. We were therefore unable to determine with any level of precision the effect of associations on premenopausal cancers. The associations for postmenopausal breast cancers only did not differ from those presented and the point estimates for premenopausal cancers were similar to those for postmenopausal cancers and close to the null value (point estimate for premenopausal cancers associated with smoking prior to first birth=0.96, and for smoking around the time of puberty=0.94). When the analyses were restricted to incident cases only, although considerably less precise, the results did not differ from those presented: fully adjusted odds ratio 1.08 (0.39, 2.55). When all analyses were repeated using breast cancer data from each of just one of the three sources (self-report, medical records, cancer register), the results were unchanged.
We identified 11 studies in 10 publications; these are summarised in
. (Adami et al, 1988
; Hunter et al, 1997
; Lash and Aschengrau, 1999
; Innes and Byers, 2001
; Egan et al, 2002
; Band et al, 2002
; Lash and Aschengrau, 2002
; Kropp and Chang-Claude, 2002
; Fink and Lash, 2003
; Reynolds et al, 2004
) The pooled analysis for these 11 studies, together with the study presented here, included 6528 breast cancer cases and provided an odds ratio (95% CI) of 1.07 (0.94, 1.22) (). The pooled estimates from differing sensitivity analyses to assess the effect of different exposure measures did not differ substantively from this estimate (all odds ratios for these analyses were between 1.05 and 1.08). There was heterogeneity between the studies (P
=0.01), which was not explained by menopausal status (P
=0.34). The Egger test did not suggest strong evidence of small study bias (P
=0.23), although the Begg test provided more evidence of this (P
=0.07). The funnel plot () shows the influence of two small studies with large positive effects on the Begg test result, and examination of the Forrest plot () suggests that these two studies are an important source of heterogeneity. When these two studies were removed from the meta-analysis (leaving N
=6165 cases), the pooled odds ratio (95% CI) was 1.03 (0.93, 1.14) with no strong evidence of heterogeneity between studies (P
=0.23). Other than being two of the smallest and least precise studies, it can be seen from that there is nothing that makes these two studies specifically different from all other studies.
Summary of studies assessing the association of smoking prior to the birth of a first baby and breast cancer risk
Meta-analysis of studies assessing the effect of smoking before/during first pregnancy with breast cancer risk.
Funnel plot of studies of association of smoking prior to first birth and breast cancer risk.