Birth weight has been positively associated with breast cancer risk in adult life and is positively associated with the principal pregnancy estrogen estriol. Birth weight is lower among Chinese women than among Caucasian women, but paradoxically, pregnancy estriol levels are higher among the former than the latter. We studied a cohort of 317 Caucasian pregnant women in Boston, MA, and 339 Chinese pregnant women in Shanghai, China. We investigated whether maternal height, which is inversely associated with pregnancy estriol levels, interacts with this hormone in relation to birth weight, thus accommodating the apparently contradictory ecologic and analytic evidence concerning the role of pregnancy estrogens on breast cancer risk in the offspring. In both Boston and Shanghai, there was a positive association of pregnancy estriol with birth weight among taller women, whereas among shorter women the association was essentially null. The relevant interaction terms were highly significant in Boston (p approximately 0.006), whereas in Shanghai, where pregnant women were generally shorter, the interaction term was suggestive (p approximately 0.14). We conclude that maternal height should be considered an important risk factor for breast cancer in the offspring since it is a crucial determinant of birth weight, both directly and through positive interaction with the principal pregnancy estrogen estriol.
(c) 2005 Wiley-Liss, Inc.