Breast cancer is the most common non-skin cancer in women in the United States. The incidence rate of breast cancer among migrant women from Asian countries, where the breast cancer incidence is low, could achieve the rate of the United States within two generations. Maternal exposures to various environmental factors during pregnancy have been hypothesized to be associated with offsprings’ breast cancer risk. These exposures may change various carcinogenesis-related hormone levels and alter the epigenome among offsprings, which increase their breast cancer risk in later life. The effect of maternal pregnancy exposures on offsprings’ breast cancer via epigenetic modifications could be carried out through multiple generations of offspring. In this chapter, we aim to summarize findings from both experimental and epidemiological studies investigating associations between maternal pregnancy exposures and offspring’s breast cancer risk.