Breast cancer risk and hormone receptor status in older women by parity, age of first birth, and breastfeeding: a case-control study.
BACKGROUND
Early age at first birth and multiparity reduce the risk of estrogen receptor-progesterone receptor (ERPR)-positive breast cancer, whereas breastfeeding reduces the risk of both ERPR-positive and ERPR-negative cancers.


METHODS
We used multivariable logistic regression analysis to investigate whether age at first birth (<25 or > or =25 years) and breastfeeding (ever/never) modify the long-term effect of parity on risk of ERPR-positive and ERPR-negative cancer using 1,457 incident breast cancer cases and 1,455 controls ages > or =55 years who participated in the Women's Contraceptive and Reproductive Experiences Study.


RESULTS
Women who gave birth before age 25 years had a 36% reduced risk of breast cancer compared with nulligravida that was not observed for women who started their families at an older age (P(heterogeneity) = 0.0007). This protective effect was restricted to ERPR-positive breast cancer (P(heterogeneity) = 0.004). Late age at first birth increased the risk of ERPR-negative cancers. Additional births reduced the risk of ERPR-positive cancers among women with an early first birth (P(trend) = 0.0001) and among women who breastfed (P(trend) = 0.004) but not among older mothers or those who never breastfed. In women with a late first birth who never breastfed, multiparity was associated with increased risk of breast cancer.


CONCLUSIONS
These findings suggest that the effect of parity on a woman's long-term risk of breast cancer is modified by age at first full-term pregnancy and possibly by breastfeeding.
Lord
Sarah J
SJ
National Health and Medical Research Council Clinical Trials Centre, University of Sydney, Sydney, New South Wales, Australia.
Bernstein
Leslie
L
Johnson
Karen A
KA
Malone
Kathleen E
KE
McDonald
Jill A
JA
Marchbanks
Polly A
PA
Simon
Michael S
MS
Strom
Brian L
BL
Press
Michael F
MF
Folger
Suzanne G
SG
Burkman
Ronald T
RT
Deapen
Dennis
D
Spirtas
Robert
R
Ursin
Giske
G
D000328:Adult; D000367:Age Factors; D014408:Biomarkers, Tumor / Q000097:blood; D001942:Breast Feeding / Q000453:epidemiology*; D001943:Breast Neoplasms / Q000097:blood / Q000453:epidemiology*; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D008875:Middle Aged; D009017:Morbidity; D010298:Parity; D011247:Pregnancy; D011379:Prognosis; D011960:Receptors, Estrogen / Q000097:blood*; D011980:Receptors, Progesterone / Q000097:blood*; D012189:Retrospective Studies; D012307:Risk Factors; D013997:Time Factors; D014481:United States / Q000453:epidemiology; D055815:Young Adult
D014408:Biomarkers, Tumor; D011960:Receptors, Estrogen; D011980:Receptors, Progesterone