Estrogen - Out of all the that contribute to development, has to be the main or most important one. Follicle stimulating and release help make estrogen. The contribute to producing estrogen during , and at this time is when the begin to grow. Estrogen increases the fat build up around the connective tissue in the breasts which makes them expand more. It also matures the lactiferous ducts, tissues, lobules and glands. However, these aspects of the breasts develop even more during . During the first half of a woman's , estrogen increases, and this swells the mammary glands in the breasts for a potential upcoming pregnancy if fertilization were to occur. If the was not fertilized, estrogen decreases back to its regular amount, which also decreases the expansion of the mammary glands. If the egg was fertilized, then the amount of increased estrogen during the first half of the menstrual cycle stays the same and elevated throughout the pregnancy. This way the breasts continue to grow for , and milk ducts can develop. But, towards the end of the pregnancy, estrogen levels start to decrease in order to allow to produce colostrum and milk within the milk ducts. Once one hormones job is complete, the next comes along to continue the breasts physiology. At times there are issues with producing milk due to a continuance of high estrogen after . This prevents and prolactin from being stimulated and released to produce milk. This is called estrogen dominance. It can be treated with exercise, eating and drinking less fats and alcohol, and eating more vitamin B6 foods.

Progesterone - Similar to estrogen, progesterone also promotes breast development, but it is more specific when it comes to development during pregnancy. Progesterone, like estrogen, is released around puberty during , and it helps grow the milk buds of the breast. It increases and decreases during a menstrual cycle in the opposite way of estrogen. During the first half of menstruation, progesterone is at a less amount, and during the second half it is at a higher amount. If the woman’s egg was fertilized, progesterone stays high throughout the pregnancy to develop milk ducts and increase the amount of milk buds, and prepare it for breastfeeding. Instead of just growing the breasts more, it works alongside estrogen to regulate this growth and maintain it. Progesterone makes sure the breast and tissues are growing and staying at its pregnancy size. At the start of pregnancy, progesterone contributes to the swelling, soreness, tenderness, and color changes of the breasts. It also increases blood supply to the breasts during the first trimester. During pregnancy, progesterone is produced by the corpus luteum for the first 10 weeks, and then the takes over to produce it. When the placenta is delivered (with this being the main source of progesterone release), progesterone levels drop dramatically, along with estrogen. This drop is progesterone allowing for the increase in prolactin.

Prolactin - only begins after the baby is born, and it is due to two hormones: prolactin,and . When estrogen and progesterone are increased, prolactin is decreased. These hormones can not function together at its optimal height during the same time. At the start of pregnancy, the hormone cascade that contributes to the subsequent flood of prolactin is actively inhibited by other neuronal transmitters. During the first stage of labor, prolactin peaks and allows for the production of milk, in the anticipation of breastfeeding. Of the last two hormones, prolactin is first produced during puberty as well helping to facilitate menstruation, and pregnancy. During puberty and pregnancy, prolactin works alongside estrogen for production. It spikes during the third trimester of pregnancy and continues to be prevalent during the immediate postpartum (and later) period to continue milk production. Since the breasts no longer need to develop any more after the baby is born, now the role is to produce milk and nurse. This is also why progesterone and estrogen are no longer needed.

Oxytocin - Oxytocin comes into play during the actual act of breastfeeding. When the baby latches and begins to feed, the feedback from the sensation on the breast and the , causes the body to produce oxytocin. Oxytocin also stimulates and facilitates the release of milk during feeding by triggering a series of reactions that thus causes the milk to be physically “ejected” from the sysnus of the nipple. When it comes to breast development, oxytocin helps trigger the release of the other growth-stimulating hormones associated with puberty. A body of research now suggests that if oxytocin levels are too high before or during puberty, the raised levels have been observed, to negatively affect the development of secondary sexual characteristics in the growing body.

Adrenaline - Based on the collective body of knowledge currently available, the adrenaline hormone negatively affects the production of milk. During pregnancy, adrenaline assists in the mothers cognition and stamina. The side effect of the stress hormone is that it can find its way into the milk during production. Substantially, the research (in regards to the baby) is neutral stating that the half life of adrenaline (that makes its way into the milk) is too short to pose any negative effect on the baby. But, adrenaline also comes in bursts during the second stage of labor to give the mother energy to push her baby out. At this point she is extremely tired, fatigued, and catecholamines will get her through it. Not only this, but at this point it travels through the breast milk via the mothers blood stream, giving the baby energy as well when they are born. Energy to open their eyes, cry, naturally remove fluid from the lungs, etc.

Cortisol - along with adrenaline (the other stress hormone), is produced in the mothers body and can affect the production and quality of the breast milk. Subsequently, it affects the composition in what we would believe to be negative. There is even some research that shows that cortisol and other stress-related hormones can alter the babies mood and temperament. Although this is an interesting point and warrants future study. The current findings are mixed and are not conclusive enough to substantiate a connection between the data. Research shows that breastfed babies have an average 40% more cortisol in their system compared to formula fed babies. Experiments on animals show that animals that had more cortisol in the system from nursing developed more cortisol receptors, which is thought to handle stress better.