To assess variation in the role of reproductive risk factors in relation to risk for breast cancer among premenopausal women we examined the relationship comparing women less than age 40 with older premenopausal women. Drawing on Data from the Nurses’ Health Studies, Dr. Warner and colleagues documented 374 incident cases of breast cancer diagnosed before age 40, and 2,533 cases diagnosed at age 40 and older among premenopausal women.1 Tumors in younger women were significantly more likely to be higher grade, larger size, and hormone receptor negative than were tumors in older premenopausal women. There was no significant difference according to age in associations between reproductive factors and risk of premenopausal breast cancer. For example, first birth at age 30 or older increased breast cancer risk in both age groups. Risk of premenopausal breast cancer decreased with each additional year of age at menarche in both age groups. As seen in postmenopausal women, among premenopausal parous women, breastfeeding was protective regardless of age at diagnosis.
The authors conclude that in the largest prospective examination of reproductive risk factors and risk of breast cancer before and after age 40, younger women were more likely to develop tumors with less favorable prognostic characteristics.