High estriol levels protect against breast cancer in pregnancy

High levels of estriol may protect pregnant women in their third trimester from breast cancer, whereas elevated levels of estrone and estradiol may increase cancer risk, study data showed.

“[Previous researchers] suggested that estriol could protect against breast cancer, proposing that the ratio of estriol to estrone reflects ‘the balance between estrogens of widely varying carcinogenicity, growth-inducing activity and estrogen-impeding activity in the pathogenesis of disease,” Barbara A. Cohn, PhD, of the Public Health Institute, Berkeley, California, and colleagues wrote. “[Other research] suggested a complementary hypothesis that the abundant estriol in the third trimester of pregnancy would help clear the breast and other organs of accumulated carcinogens via increased blood flow.”

The researchers performed a prospective, case-cohort study of 620 women nested in 38 years of follow-up on 15,528 pregnant women who were enrolled in the Child Health and Development Studies. Cohn and colleagues measured serum concentrations of estriol, estrone and estradiol, evaluating the hormones’ associations with risk for breast cancer. All women were pregnant between June 1959 and September 1966. The main outcome was breast cancer through 1997.

Doubled levels of serum estrone and estradiol were associated with an increased risk for breast cancer, the researchers reported (HR = 1.7; 95% CI, 1.2-2.4). However, there was an association between protection against cancer and doubling of estriol (HR = 0.7; 95% CI, 0.5-1) or of the ratio of estriol to estrone and estradiol (HR = 0.6; 95% CI, 0.4-0.8).

These associations were more pronounced for breast cancer diagnosed between 2 and 15 years after delivery than for disease diagnosed after 16 to 38 years (P = .0002) and for women aged older than 27 years at delivery compared with those aged 27 years or younger (P = .01), as well as for a first-time delivery compared with multiple deliveries (P = .02).

“A re-evaluation of the role of estriol in human pregnancy suggests that its importance may have been underestimated for many years,” the researchers wrote. “The present study suggests that relatively high late-pregnancy estriol may play a role in protecting parous women from breast cancer, even as pregnancy exposure to estrone and estradiol enhances risk.” – by Andy Polhamus

Disclosures: The authors report no relevant financial disclosures.