Birth size and breast cancer risk among young California-born women.

Objective: Evaluate the risk of breast cancer associated with birth size among young California-born women.

Methods: Invasive breast cancer cases diagnosed 1988-2004 among women born in California during the 1960s were identified from the California Cancer Registry. Breast cancer cases (n = 3,712) were linked to their California birth records. Controls (n = 8,615) were randomly selected from California birth records for women, frequency matched to cases by birth year. Odds ratios (ORs) and 95% confidence intervals (95% CI) were estimated from unconditional logistic regression.

Results: The adjusted OR for breast cancer associated with the highest category of birth weight (≥4,000 g) was 1.12 (95% CI 0.89-1.41), p-trend = 0.02. The adjusted OR for the highest category of birth length (>20 inches) was 1.13 (95% CI 1.02-1.25), p-trend = 0.02. These relationships appeared to be confined to cases with estrogen receptor positive (ER+) tumors (p-trend ≤0.01) or progesterone receptor positive (PR+) tumors (p-trend ≤0.02). No significant associations were found among cases with ER or PR negative tumors.

Conclusions: Our results confirm previous findings of elevated breast cancer risk associated with increases in birth size. These risks may be confined to ER+ and PR+ tumors, highlighting the potential mechanistic role of sex steroid hormonal pathways.