Epidemiologic and experimental data have shown that a full term pregnancy reduces breast cancer risk. However, recent studies have suggested that while full term pregnancy does reduce risk for estrogen receptor and luminal breast cancers, pregnancy may actually increase risk of more aggressive basal-like breast cancers. There are complex relationships between age, race, parity, and obesity in observational human datasets making it difficult to translate these findings into public health messages. Melissa Troester, University of North Carolina Chapel Hill, addresses tumor heterogeneity, focusing specifically on the basal-like breast cancer subtype and the microenvironment changes that promote this breast cancer subtype during a vital window of susceptibility, the post partum period. (#25025)