If you have ever been pregnant or know someone who has been, one of the obvious transformations during pregnancy (besides the belly of course!) is the breasts. During pregnancy the body produces hormones such as estrogen, progesterone, prolactin, human chorionic somatomammotrophin, thyroid hormone and corticosteroids. The secretion of these hormones by the placental and corpus luteum causes the terminal duct epithelium in the breasts to grow and more secretory acini to be developed . Inner secretory cells in the breast tissue also become bigger and the alveoli eventually contain breast milk .
Figure 5: a) The lobules (Lo) have enlarged while the septa (S) remain the same size while dividing the breast into lobes. b) acini (A) are dilated and the epithelial cells (E) show cuboidal structure as well as low columnar. The dilation of the acini and the ducts is due to the accumulation of colostrum .Colostrum is the first secretion of the breasts after pregnancy. It consists of a laxative and maternal antibodies.
When a women begins lactating the breast again changes.
Figure 6: The lactating breast. The important feature to take from this is the myoepithelial cells (M) which are the secretory cells that contract to actually expel the milk. The lipid droplets (L) also seem to take up a large amount of space and are discharged in apocrine secretion , which could account for why breast milk is so high in fatty content. IgA, produced by plasma cells in the connective tissue surrounding alveoli , is also released into the milk via transcytosis by the small vesicles (V). Protein is developed by the rough endoplasmic recticulum (rER) and small protein granules can be seen by the black dots in the micrograph .
Once the baby is born and begins suckling the breast, two neurohormonal reflexes occur:
1) prolactin is released by the anterior pituitary which stimulates milk production 
2) oxytocin is released by the posterior pituitary which causes myoepithelial cells to contract. These cells are located around the secretory acini and ducts, so it causes milk to be pushed down the ducts and eventually out the nipple 
If suckling stops then these hormones are not released and the lactating breast regresses .
DID YOU KNOW??
For effective output of the milk, it is suggested that the baby should have a large portion of the areola in the mouth .