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Norris Cotton Cancer Center and the Department of Community and Family Medicine, Dartmouth-Hitchcock Medical Center, Lebanon, New Hampshire 03756 [L. T-E., J. D., E. R. G., J. A. B.]; Department of Epidemiology, Harvard School of Public Health, Boston, Massachusetts 02115 [K. M. E., D. T., W. C. W.]; University of Wisconsin Comprehensive Cancer Center, Madison, Wisconsin 53706 [P. A. N., A. T-D.]; Fred Hutchinson Cancer Center, Seattle, Washington 98104 [P. A. N.]; University of Wisconsin Department of Preventive Medicine, Madison, Wisconsin 53706 [A. T-D.]; Department of Medicine, Dartmouth Medical School, Hanover, New Hampshire 03756 [E. R. G., J. A. B.]; and Department of Nutrition, Harvard School of Public Health, Channing Laboratory, Department of Medicine, Harvard Medical School and Brigham and Womens Hospital, Boston, Massachusetts 02115 [W. C. W.]
| Abstract |
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| Introduction |
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| Materials and Methods |
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We selected control women in each state from lists of licensed drivers (ages 5064) and Medicare beneficiaries (ages 6579). Control women were selected at random to have an age distribution similar to that of case women; as with case women, eligibility required a listed telephone number. Control women who had a history of breast cancer were ineligible. Of 7655 potential controls, 183 (2%) had died, 124 (2%) could not be located, and 1397 (18%) declined to participate, providing an overall participation rate of 78%. Interviews for 23 of the 5951 control participants were considered unreliable by the interviewer, leaving 5928 control women for analysis.
Potential case and control subjects were contacted initially by mail and subsequently interviewed by telephone; participants were enrolled from July 1992 to July 1995. Early life factors of principal interest included the subjects birth weight, parental smoking while the mother was pregnant with the study subject, the fathers level of education, the mothers age at the time of the subjects birth, the subjects birth rank, the number of older brothers or sisters, and the gender of siblings in the sibship.
Analyses were confined to exposures occurring before a reference date, which was defined for cases as the date of breast cancer diagnosis; control women were assigned a reference date corresponding to the state-specific average time interval between case diagnosis and interview (approximately 1 year). Women were considered postmenopausal if natural (permanent cessation of periods for at least 6 months) or surgical (bilateral oophorectomy) menopause occurred before the reference date. An algorithm (11) was used to classify menopausal status among women who began taking hormones before their periods had stopped or who had undergone a hysterectomy but were uncertain whether their ovaries had been removed. Premenopausal women (267 cases and 292 controls) and women with unknown menopausal status (116 cases and 150 controls) were excluded from the analyses.
During the data collection period, questions were added to the questionnaire and later withdrawn to make room for new areas of inquiry; thus, the number of cases and controls available for comparisons was determined by the length of time a question was in the questionnaire. Data collection was concurrent for all of the early life variables (other than parental smoking) for at least 1 year, allowing assessment of mutual confounding.
ORs3 and 95% CIs from logistic regression models were used to evaluate the association between factors of interest and breast cancer risk (12) . Initially, early life factors were evaluated singly in logistic models containing terms for age and state. Multivariate models were also used to evaluate potential confounding by known breast cancer risk factors, including body mass index (kg/m2) at the reference date, religion (Jewish or non-Jewish), family history of breast cancer (breast cancer diagnosed in mother, sister, or daughter), age at first full-term pregnancy, parity (number of pregnancies lasting more than 6 months), age at menopause, and other available early life factors. Parental smoking was assessed in separate models with and without terms for birth weight and/or known risk factors. Our analyses provided little evidence of confounding; thus, the ORs reported here are generally adjusted only for age and state. In general, factors were treated as categorical variables; tests of linear trend were conducted, when possible, by treating factors as continuous variables. Potential interactions between risk factors were assessed using likelihood ratio tests. These tests were also used to assess the appropriateness of adding square terms to the models (e.g., birth weight + birth weight2). All comparisons were based on at least 1300 case and 1600 control women.
| Results |
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4500 g) birth weight categories when compared with the normative birth weight of 30003499 g. However, neither effect was statistically significant, and there was no evidence of a nonlinear trend when birth weight was modeled as a quadratic function (P = 0.44). The weak J-shaped pattern was also observed in analyses using 25002999 g as the referent category and
4000 g as the highest birth weight category (data not shown). We found no evidence that the effect of birth weight was modified by age at menarche or adult body mass index (data not shown).
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Breast cancer risk increased with the number of years of the fathers education (P = 0.01); risk was 22% greater for women whose fathers had at least 12 years of education, relative to those whose fathers had 811 years of education. The results were comparable when additionally adjusted for the subjects level of education.
The subjects birth rank, adjusted for the mothers age at the time of the subjects birth, was inversely associated with breast cancer risk (P = 0.03); risk was lower only for women who were fifth or later in birth rank. Further analyses showed that risk was lower only for women whose older siblings were female (P for trend = 0.03). For women with at least three older sisters, relative to those without older sisters, risk was 26% lower. Risk was unrelated to the number of older brothers (data not shown). Risk was also unaffected by the gender ratio of the sibship or by membership in an all-female sibship (relative to having at least one brother). We found no association with the overall size of the sibship or the number of younger siblings (data not shown).
Risk appeared to increase with increasing mothers age at the time of the subjects birth, and the association was significant after adjustment for the mothers number of previous pregnancies (P for trend = 0.04). Relative to women whose mothers were ages 2529 years at the time of their birth, those whose mothers were at least 40 years of age had a 27% higher risk. We also evaluated whether maternal age modified the influence of key breast cancer risk factors (Table 2)
. In two groups, women whose first birth was at age 30 years or later and women who were first born, risk appeared to be elevated for those whose mothers were at least 35 years of age, relative to those whose mothers were less than 20 years of age at the time of the subjects birth. However, the interactions between maternal age and the subjects birth order (first or later), age at first birth, parity, and family history of breast cancer were not statistically significant (Table 2)
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| Discussion |
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The association between maternal smoking during pregnancy and lower birth weight may be mediated by reduced estrogen levels associated with smoking (17) ; however, one study showed only slightly lower estrogen levels in pregnant smokers (18) . Our study and previous studies (4) found no evidence that parental smoking decreases the risk of breast cancer in adult daughters.
In our data, breast cancer risk increased with the fathers level of education, which may be a marker of a multigenerational lifestyle factor, such as diet. Some prior studies, however, found no association between fathers occupational level (15) or mothers socioeconomic status (9) and breast cancer risk.
Maternal blood levels of total estrogens (19) and free or total estradiol (19 , 20) are somewhat lower during second pregnancies than during first pregnancies. Similarly, cord blood levels of estradiol, estrone, and progesterones are lower for later-born children than for first-born children (21) . Consistent with these findings, a combined analysis of three case-control studies noted lower risk for premenopausal women who were second born, compared with those who were first born (22) . In our study, the lower risk associated with later birth rank was due to the protective effect of having older sisters. Study participants had been asked to provide the gender and birth dates of siblings and half-siblings, but were not asked to distinguish half-siblings who were born to the same mother. Consequently, our results may have been attenuated by the inclusion of half-sisters who shared the same father, although this was probably an uncommon event. Older sisters often serve as care-givers to their younger siblings. Although speculative, it is conceivable that the protective effect of older sisters reflects early exposure to infectious agents acquired by older sisters. In support of this possibility, limited evidence suggests that breast cancer risk may be increased by delayed primary exposure to EBV (10) . We found no evidence that risk was influenced by membership in sibships that were mostly or entirely female; thus, our findings do not support a role for maternal hormones that might influence offspring gender ratio.
Our results are similar to those of a previous study noting a higher risk for daughters born to older mothers (23) , although most found no association (4 , 5 , 24, 25, 26) . We found no evidence of the J-shaped pattern of risk that has been observed in studies of very young women (15 , 16) . Serum levels of pregnancy estrogens (total estrogens, estradiol, total estriol, and human placental lactogen) do not appear to increase with maternal age (19) ; thus, the mechanism of possible influence is unclear.
Comparable with two previous studies (25 , 26) , our data indicate that the influence of maternal age does not differ for nulliparous and parous women. Our findings are also consistent with a previous study showing an increased risk for women whose first birth was delayed and who were born of older mothers (26) , although this is not always observed (25) . Consistent with previous studies (25 , 26) , we found that maternal age was not modified by a family history of breast cancer. Our results and those of a previous report (24) suggested higher risk among first-born children of older mothers, but the finding was not statistically significant in either study.
| Acknowledgments |
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| Footnotes |
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1 Supported by USPHS Grants RO1CA47305 and RO1CA47147 from the National Cancer Institute, NIH, Department of Health and Human Services. ![]()
2 To whom requests for reprints should be addressed, at HB 7926, Norris Cotton Cancer Center, One Medical Center Drive, Lebanon, NH 03756. Phone: (603) 650-6694; Fax: (603) 650-6333; E-mail: Linda.Titus-Ernstoff{at}Dartmouth.edu ![]()
3 The abbreviations used are: OR, odds ratio; CI, confidence interval. ![]()
Received 3/23/01; revised 11/ 9/01; accepted 12/ 3/01.
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