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Volume 330:81-87 January 13, 1994 Number 2 Next

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ABSTRACT

Background The evidence of an association of lactation with a reduction in the risk of breast cancer among women has been limited and inconsistent. The effect of lactation appears to be confined to premenopausal women with a history of long lactation, but most studies of this relation have been limited in statistical power. We conducted a multicenter, population-based, case-control study with a sample large enough for us to describe more precisely the association between lactation and the risk of breast cancer.

Methods Patients less than 75 years old who had breast cancer were identified from statewide tumor registries in Wisconsin, Massachusetts, Maine, and New Hampshire. Controls were randomly selected from lists of licensed drivers if the case subjects were less than 65 years old, and from lists of Medicare beneficiaries if they were 65 through 74 years old. Information on lactation, reproductive history, and family and medical history was obtained by means of telephone interviews. After the exclusion of nulliparous women, 5878 case subjects and 8216 controls remained for analysis.

Results After adjustment for parity, age at first delivery, and other risk factors for breast cancer, lactation was associated with a slight reduction in the risk of breast cancer among premenopausal women, as compared with the risk among women who were parous but had never lactated (relative risk, 0.78; 95 percent confidence interval, 0.66 to 0.91); the relative risk of breast cancer among postmenopausal women who had lactated, as compared with those who had not, was 1.04 (95 percent confidence interval, 0.95 to 1.14). With an increasing cumulative duration of lactation, there was a decreasing risk of breast cancer among premenopausal women (P for trend <0.001) but not among postmenopausal, parous women (P for trend = 0.51). A younger age at first lactation was significantly associated with a reduction in the risk of premenopausal breast cancer (P for trend = 0.003). As compared with parous women who did not lactate, the relative risk of breast cancer among women who first lactated at less than 20 years of age and breast-fed their infants for a total of six months was 0.54 (95 percent confidence interval, 0.36 to 0.82).

Conclusions There is a reduction in the risk of breast cancer among premenopausal women who have lactated. No reduction in the risk of breast cancer occurred among postmenopausal women with a history of lactation.

Source Information

From the University of Wisconsin Comprehensive Cancer Center, Madison, (P.A.N., B.E.S.); the Department of Epidemiology, UCLA School of Public Health, Los Angeles (M.P.L.); the Department of Obstetrics and Gynecology, Pritzker School of Medicine, University of Chicago, Chicago (R.M.); the Departments of Community and Family Medicine and Medicine, Dartmouth Medical School, Hanover, N.H. (E.R.G.); the Massachusetts Cancer Registry, Massachusetts Department of Public Health, Boston (R.W.C.); the Division of Disease Control, Maine Bureau of Health, Augusta (K.P.B.); the Departments of Epidemiology (W.C.W., B.M.) and Nutrition (W.C.W.), Harvard School of Public Health, Boston; the Channing Laboratory, Harvard Medical School, Boston (W.C.W.); and the Department of Medicine, Brigham and Women's Hospital, Boston (W.C.W.).

Address reprint requests to Dr. Newcomb at the University of Wisconsin Comprehensive Cancer Center, 1300 University Ave., No. 4780, Madison, WI 53706.

Full Text of this Article

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N Engl J Med 1994; 330:1682-1684, Jun 9, 1994. Correspondence

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