GH1 and breast carcinoma: The association with parity is weaker for ER- tumors.15 In contrast, breast cancer risk is elevated soon after pregnancy,1 when women are younger, and ER- tumors comprise a greater percentage of total breast cancers than among older women.17 Estrogens and other steroid and growth hormones rise precipitously in pregnancy and are critical in preparing the mammary gland for lactation.18 Dramatic changes to mammary tissue begin in the first weeks of pregnancy with major epithelial cell proliferation.