Detection and management of breast abnormalities that develop during pregnancy and lactation is difficult for both the clinician and the radiologist. This article reviews the hormonal and physiologic effects on the breast during pregnancy and lactation. Breast masses that occur in pregnant or lactating patients, including pregnancy-associated breast cancer, are discussed and the corresponding ultrasound and mammographic findings are presented. Finally, a rationale for the imaging evaluation and management of the pregnant or lactating patient with a breast mass are presented.