Risk of Breast Cancer in Prematurely Born Women

Extract

Epidemiologic studies of breast cancer [reviewed in (1)] have for several decades focused on the role of reproductive factors during adult life. A new line of research opened when it was suggested that perinatal events and conditions may influence a woman's breast cancer risk throughout her life (2,3). Five separate epidemiologic studies (4–8) have tested this hypothesis. Besides a weak association between increasing birth weight and increased risk for breast cancer [seen in four studies (4–7)], most pronounced in women with premenopausal breast cancer, two studies (4,7) also demonstrated an inverse association between preeclampsia during pregnancy and breast cancer in the offspring out of three that examined this hypothesis. This finding supports indirectly that early hormonal exposures affect risk of breast cancer, since preeclampsia is characterized by decreased levels of pregnancy hormones (9,10).

Two independent observations led to the present investigation. One study (7) indicated that female babies born prematurely (before the 33^rd gestational week) had an increased risk for breast cancer. Girls born before the 33^rd gestational week have markedly increased levels postnatally of gonadotropins (11) that stimulate the ovaries to produce excessive amounts of estradiol during several months after birth (12,13). Since women born before the 33^rd gestational week during the first half of the 20^th century constitute a very small fraction, 10 of 1068 case patients, of breast cancer patients in the study mentioned above, this will not affect the results in the studies analyzing the association between birth weight and breast cancer risk. However, women born extremely preterm are an ideal group in which to test the hypothesis of an association between early exposures to elevated estrogen levels and breast cancer.