A 27-year-old woman in the third trimester of her third pregnancy discovers a lump in her left breast. On physical examination, a 2-cm, discrete, freely movable mass beneath the nipple is palpable. After the birth of a term infant, the mass appears to decrease in size. The infant is breastfed without difficulty. What is the most likely diagnosis?

Fibroadenomas are common and may enlarge during pregnancy or late in each menstrual cycle. Most intraductal papillomas are smaller than 1 cm and are not influenced by hormonal changes. Lobular carcinoma in situ is typically an ill-defined lesion without a mass effect. Medullary carcinomas tend to be large; they account for only about 1% of all breast carcinomas. Phyllodes tumors are uncommon and tend to be larger than 4 cm.