The breast becomes fully differentiated during pregnancy and lactation. The terminal lobular units are most affected during this physiologic state. Several histologic changes occur in the terminal ducts and lobules with enlargement and formation of new lobular units early in pregnancy. By the end of the second trimester secretory changes are well developed and the cuboidal luminal epithelial cells show evidence of vacuolization. Accumulation of fat droplets occurs within the cytoplasm of the epithelium of the lobules during the third trimester. The lobules undergo striking hyperplasia to occupy most of the breast parenchyma during this time, with a decrease in the interlobular connective tissue. At term the expanded lobular units show marked distention of their lumina by colostrum. Ref: Cunningham F.G., Leveno K.J., Bloom S.L., Hauth J.C., Rouse D.J., Spong C.Y. (2010). Chapter 5. Maternal Physiology. In F.G. Cunningham, K.J. Leveno, S.L. Bloom, J.C. Hauth, D.J. Rouse, C.Y. Spong (Eds), Williams Obstetrics, 23e.