Risk factors for pregnancy-associated breast cancer: a report from the Nigerian Breast Cancer Study.

PURPOSE: Little is known about risk factors for pregnancy-associated breast cancer (PABC), diagnosed during pregnancy or postpartum. METHODS: We enrolled 1715 premenopausal women from the Nigerian Breast Cancer Study from 1998 to 2011. Based on recency of last pregnancy from diagnosis, breast cancer cases were categorized as (1) PABC diagnosed 2 years or longer postpartum, (2) PABC diagnosed 3 to 5 years postpartum, or (3) non-PABC diagnosed more than 5 years postpartum. Controls were matched to cases on recency of last pregnancy. Multiple logistic regressions were performed comparing cases and controls within each group. RESULTS: Of the 718 cases, 152 (21.2%) had PABC 2 or more years postpartum, and 145 (20.2%) 3 to 5 years postpartum. Although not statistically significant, women with higher parity tend to have an elevated risk of PABC but reduced risk of non-PABC (p for heterogeneity = 0.097). Family history of breast cancer might be a strong predictor particularly for PABC 2 or more years postpartum (odds ratio, 3.28; 95% confidence interval, 1.05-10.3). Compared with non-PABC cases, PABC 2 or more years postpartum cases were more likely to carry BRCA1/2 mutations (P = .03). CONCLUSIONS: Parity may have different roles in the development of PABC versus other premenopausal breast cancer in Nigerian women. Prospective mothers with multiple births and a family history of breast cancer may have an elevated risk of breast cancer during their immediate postpartum period.