Abstract: ObjectiveTo study the relationship between the geographical variation in breast cancer incidence and the difference of pregnant hormone levels which form the intrauterine hormonal millen for the offspring.MethodsAn ecological study is used to compare the pregnancy hormone levels in a population with high breast cancer incidence(Boston,304 subjects)with a population in a low incidence(Shanghai,334 subjects).To compare the level of pregnancy hormones at weeks of 16 and 27 between the two places.ResultsShanghai women have higher serum levels on almost all studied pregnancy hormones at weeks 16 and 27 of gestation except the low level of pregnancy hormone at 27 weeks,and the differences of hormone levles between women have statistical significance.ConclusionThe differences of hormone levels between US Caucasian and Shanghai women are reasonable to explain the remarkable difference of breast cancer incidences between them.