A linked-registry study of gestational factors and subsequent breast cancer risk in the mother

Women who were younger at their first live birth have a reduced breast cancer risk. Other pregnancy characteristics, including complications, also may affect risk but because they are rare, require large datasets to study.

The association of pregnancy history and breast cancer risk was assessed in a population-based study including 22,646 cases diagnosed in Washington State 1974 to 2009, and 224,721 controls, frequency matched on parity, age, calendar year of delivery, and race/ethnicity. Information on prediagnosis pregnancies derived from linked birth certificate and hospital discharge databases. Adjusted odd ratios (ORs) and 95% confidence intervals (CI) were calculated.

Multiple gestation pregnancies were associated with decreased breast cancer risk (OR, 0.65; 95% CI, 0.57-0.74) as was prepregnancy obesity (OR, 0.76; 95% CI, 0.65-0.90). Infant birth weight was positively associated (6% per 1,000 g; 95% CI, 3%-9%). The ORs for first trimester bleeding (OR, 3.35; 95% CI, 1.48-7.55) and placental abnormality/insufficiency (OR, 2.24; 95% CI, 1.08-4.67) were increased in women diagnosed at age 50+ years and 15+ years after the index pregnancy. Results were similar in analyses restricted to first pregnancies, those closest to diagnosis, and when excluding in situ disease.

These data suggest that multiple gestation pregnancies are protective, whereas delivering larger infants increases risk for later development of maternal breast cancer. Placental abnormalities that result in bleeding in pregnancy also may reverse the long-term protection in postmenopausal women associated with parity.

Certain pregnancy characteristics seem to be associated with later maternal breast cancer risk.