SummaryBackgroundMenarche and menopause mark the onset and cessation, respectively, of ovarian activity associated with reproduction, and affect breast cancer risk. Our aim was to assess the strengths of their effects and determine whether they depend on characteristics of the tumours or the affected women.MethodsIndividual data from 117 epidemiological studies, including 118 964 women with invasive breast cancer and 306 091 without the disease, none of whom had used menopausal hormone therapy, were included in the analyses. We calculated adjusted relative risks (RRs) associated with menarche and menopause for breast cancer overall, and by tumour histology and by oestrogen receptor expression.FindingsBreast cancer risk increased by a factor of 1·050 (95% CI 1·044–1·057; p<0·0001) for every year younger at menarche, and independently by a smaller amount (1·029, 1·025–1·032; p<0·0001), for every year older at menopause. Premenopausal women had a greater risk of breast cancer than postmenopausal women of an identical age (RR at age 45–54 years 1·43, 1·33–1·52, p<0·001). All three of these associations were attenuated by increasing adiposity among postmenopausal women, but did not vary materially by women's year of birth, ethnic origin, childbearing history, smoking, alcohol consumption, or hormonal contraceptive use. All three associations were stronger for lobular than for ductal tumours (p<0·006 for each comparison). The effect of menopause in women of an identical age and trends by age at menopause were stronger for oestrogen receptor-positive disease than for oestrogen receptor-negative disease (p<0·01 for both comparisons).InterpretationThe effects of menarche and menopause on breast cancer risk might not be acting merely by lengthening women's total number of reproductive years. Endogenous ovarian hormones are more relevant for oestrogen receptor-positive disease than for oestrogen receptor-negative disease and for lobular than for ductal tumours.FundingCancer Research UK.

Context Women using combined estrogen and progestin hormone replacement therapy (CHRT) have an increased risk of breast cancer; however, data on use for long durations and on risk associated with patterns of use are lacking.Objective To evaluate relationships between durations and patterns of CHRT use and risk of breast cancer by histological type and hormone receptor status.Design Population-based case-control study.Setting Three counties in western Washington State.Participants Nine hundred seventy-five women 65-79 years of age diagnosed with invasive breast cancer from April 1, 1997, through May 31, 1999 lobular cases, 656 ductal cases, 114 cases with other histological type, and 9 cases with an unspecified histological type; estrogen receptor (ER)/progesterone receptor (PR) status: 646 ER+/PR+ cases, 147 ER+/PR− cases, and 101 ER−/PR− cases [6 ER−/PR+ cases and 75 cases with unknown ER/PR status were not included in the analyses herein]) and 1007 population controls. Main Outcome MeasuresRisks of invasive lobular, ductal, ER+/PR+, ER+/PR−, and ER−/PR− breast carcinomas. ResultsWomen using unopposed estrogen replacement therapy (ERT) (exclusive ERT use), even for 25 years or longer, had no appreciable increase in risk of breast cancer, although the associated odds ratios were not inconsistent with a possible small effect. Ever users of CHRT (includes CHRT users who also had used ERT) had a 1.7-fold (95% confidence interval [CI], 1.3-2.2) increased risk of breast cancer, including a 2.7-fold (95% CI, 1.7-4.3) increased risk of invasive lobular carcinoma, a 1.5-fold (95% CI, 1.1-2.0) increased risk of invasive ductal carcinoma, and a 2.0-fold (95% CI, 1.5-2.7) increased risk of ER+/PR+ breast cancers. The increase in risk was greatest in those using CHRT for longer durations (users for 5-14.9 years and Ն15 years had 1.5-fold [95% CI, 1.0-2.3] and 1.6-fold [95% CI, 1.0-2.6] increases in risk of invasive ductal carcinoma, respectively, and 3.7-fold [95% CI, 2.0-6.6] and 2.6-fold [95% CI, 1.3-5.3] increases in risk of invasive lobular carcinoma, respectively. Associations of similar magnitudes were seen among users of both sequential and continuous CHRT. Risks of ER+/PR− and ER−/ PR− tumors were not increased by use of any form of hormone replacement therapy; however, small numbers of these tumors limited power to detect possible associations.Conclusion These data suggest that use of CHRT is associated with an increased risk of breast cancer, particularly invasive lobular tumors, whether the progestin component was taken in a sequential or in a continuous manner.

Aspects of reproductive history are among the most well-established breast cancer risk factors. However, relatively little is known about how they influence risk of different molecular subtypes of breast cancer, particularly among younger women. Using data from a population-based case–control study of women 20–44 years of age, we assessed the relationships between various reproductive factors and risk of estrogen receptor positive (ER+), triple-negative, and HER2-overexpressing breast cancers. Detailed reproductive histories were obtained through structured interviewer administered in-person questionnaires. Reproductive histories among control women (n = 941) were compared to those of ER+ cases (n = 781), triple-negative cases (n = 180), and HER2-overexpressing cases (n = 60) using polytomous logistic regression. Age at menarche, parity, and number of full-term pregnancies were similarly associated with risk of all three breast cancer subtypes. In contrast, age at first live birth, the interval between age at menarche and age at first birth, and breastfeeding were inversely associated with risk of triple-negative breast cancer (P values for trend 0.002, 0.006 and 0.018, respectively), but were not associated with risk of ER+ or HER2-overexpressing cancers. A strong inverse association between breastfeeding and risk of triple-negative breast cancer has now been consistently observed across numerous studies, and at present it is the most well-established protective factor for this aggressive and lethal form of breast cancer. Further studies clarifying the biological mechanisms underlying this relationship and confirming our results with respect to age at first birth and the interval between age at menarche and age at first birth are needed.