Findings from observational studies suggest an inverse association
between lactation and premenopausal breast cancer risk, but results
have been inconsistent, and data from large prospective cohort studies
We used information from 60,075 parous women participating in the
prospective cohort study of the Nurses' Health Study II from 1997 to
2005. Our primary outcome was incident premenopausal breast cancer.
We ascertained 608 incident cases of premenopausal breast cancer
during 357,556 person-years of follow-up. Women who had ever breastfed
had a covariate-adjusted hazard ratio (HR) of 0.75 (95% confidence
interval [CI], 0.56-1.00) for premenopausal breast cancer compared
with women who had never breastfed. No linear trend was found with
duration of total lactation (P = .95), exclusive lactation (P = .74),
or lactation amenorrhea (P = .88). The association between lactation
and premenopausal breast cancer was modified by family history of
breast cancer (P value for interaction = .03). Among women with a
first-degree relative with breast cancer, those who had ever breastfed
had a covariate-adjusted HR of 0.41 (95% CI, 0.22-0.75) for
premenopausal breast cancer compared with women who had never
breastfed, whereas no association was observed among women without a
family history of breast cancer.
In this large, prospective cohort study of parous premenopausal
women, having ever breastfed was inversely associated with incidence
of breast cancer among women with a family history of breast cancer.
Author Affiliations: Division of Maternal-Fetal Medicine (Dr
Stuebe) and Obstetrics and Gynecology Epidemiology Center (Drs Xue and
Michels), Department of Obstetrics, Gynecology, and Reproductive
Biology and Channing Laboratory, Department of Medicine (Drs Willet
and Michels), Brigham and Women's Hospital and Harvard Medical School,
and Departments of Nutrition (Dr Willet) and Epidemiology (Dr
Michels), Harvard School of Public Health, Boston, Massachusetts. Dr
Stuebe is now with the Department of Maternal-Fetal Medicine,
University of North Carolina at Chapel Hill.
Arch Intern Med. 2009;169(15):1364-1371