Final answer:
The peptide hormone called prolactin enhances breast development and initiates lactation. Produced by the anterior pituitary gland, its levels increase notably during pregnancy and lactation to stimulate breast milk supply and the transfer of nutrients to the milk.
Step-by-step explanation:
The peptide hormone responsible for enhancing breast development and initiating lactation, with levels that are normally elevated in pregnant and nursing women, is prolactin. This hormone is produced by the anterior pituitary gland and is crucial not only for the establishment and maintenance of the supply of breast milk but also for the mobilization of maternal micronutrients into the breast milk.
Near the fifth week of pregnancy, prolactin levels begin to rise significantly, reaching about 10-20 times the pre-pregnancy concentration. However the hormone's effect on milk synthesis is inhibited during pregnancy by other hormones such as estrogen and progesterone. Once the placenta is expelled after childbirth this inhibition is lifted and milk production commences. In the postpartum period prolactin levels fluctuate, with increases typically observed during each breastfeeding session to promote milk production for the subsequent feeding.