Breast Cancer Risk After Recent Childbirth: A Pooled Analysis of 15 Prospective Studies.
Level 3 - non-randomized controlled study
Pooled analysis of 15 prospective cohort studies (observational follow-up data).
PubMed 30534999 · doi:10.7326/M18-1323
What was done
Researchers conducted a pooled analysis of individual-level data from 15 prospective cohort studies within the international Premenopausal Breast Cancer Collaborative Group. The study included women younger than 55 years followed over 9.6 million person-years. Cox proportional hazards regression was used to evaluate hazard ratios (HRs) and 95% confidence intervals for incident breast cancer in relation to time since childbirth, adjusting for variables such as breastfeeding, family history, and tumor receptor status.
What was found
Among 18,826 incident breast cancer cases, parous women had a higher breast cancer risk compared with nulliparous women that peaked approximately 5 years postpartum (HR 1.80, 95% CI 1.63 to 1.99). This risk gradually declined, crossing over from elevated to reduced risk at about 24 years post-delivery and reaching an HR of 0.77 (95% CI 0.67 to 0.88) at 34 years. The crossover pattern was driven by estrogen receptor (ER)-positive breast cancer, with no crossover observed for ER-negative disease. Risk increases were more pronounced in women with a family history of breast cancer, older age at first birth, or higher parity; breastfeeding did not modify the overall risk pattern.
Why it matters
While parity is known to protect against breast cancer long-term, this study shows that childbirth is associated with an elevated risk that persists for more than two decades before that protection appears. This informs clinical risk assessment for premenopausal women.
Limits
Diagnoses made during pregnancy could not be uniformly distinguished from early postpartum diagnoses. Data regarding HER2 oncogene overexpression were limited, and the analysis was restricted to women under 55 years of age.
Cited by
- context Giving birth before age 40 reduces the long-term risk of developing breast cancer.