The imaging changes in the breast associated with pregnancy and the perinatal period may not only deviate from the baseline appearance for each patient, but may also mimic disease or confound evaluation of disease. The hormonal changes can influence a range of disorders, from benign or inflammatory changes to malignant tumors. Moreover, outcomes differ from those of similar pathologies in nonpregnant patients, owing to either physiological changes or delays in diagnosis and treatment. Ultrasonography is the preferred imaging modality for evaluation during pregnancy and lactation, as its sensitivity for carcinoma is nearly 100% (Sabate JM, Clotet M, Torrubia S, et al, 2007; Radiographics 27 suppl 1:S101-S124). Therefore, an understanding of the specific pregnancy-associated pathologies and their radiologic appearances is indispensable to the interpreting radiologist.