- Parous women may have elevated risk for breast cancer (BCa) for 23.6 years after childbirth compared with nulliparous women, with the risk peaking at 5 years, according to a pooled analysis of 15 prospective cohort studies.
- The effect is driven by estrogen receptor-positive (ER+) BCa.
- Breastfeeding has no effect on risk.
Why this matters
- Recent childbirth should be viewed as a risk factor for BCa.
- Pooled analysis of individual-level data from 15 prospective cohorts (n=889,944) of premenopausal women in the international Premenopausal Breast Cancer Collaborative Group.
- Funding: Avon Foundation; NIH; others.
- Mean follow-up, 10.8 years; 18,826 incident BCa cases diagnosed before 55 years of age.
- Parity (vs nulliparity) was associated with increased BCa risk that lasted 23.6 years, peaking at 4.6 years after birth (HR, 1.80; 95% CI, 1.63-1.99).
- Effect was driven by ER+ BCa, which accounted for 76% of all cases.
- The combination of parity and family history was associated with an even larger increase in risk peaking at 4.9 years (HR, 3.53; 95% CI, 2.91-4.29).
- Parity gradually switched to a protective factor that reached its lowest observed point at 34.5 years after birth (HR, 0.77; 95% CI, 0.67-0.88).
- Breastfeeding had no effect on BCa risk.
- Observational design.