Third Trimester Estrogens and Maternal Breast Cancer: Prospective Evidence.

Context: Full-term pregnancy is associated with a transient increase and life-time decrease in maternal breast cancer risk. Estrone (E1), estradiol (E2), and estriol (E3) are in high concentration during the third trimester. E1 and E2 metabolism produces carcinogenic intermediaries, and E3 metabolism does not.