A pregnant woman has high blood levels of pregnancy hormones that stimulate the breast tissue to develop and prepare for milk production (Mohrbacher 2010). Starting at 3 – 4 weeks of pregnancy, a woman’s breasts undergo growth and differentiation. This causes ductal branching and lobular formation occurring from the action of a range of hormones including estrogen, progesterone, prolactin, growth hormone, epidermal growth factor, fibroblast growth factor, insulin-like growth factor, human placental lactogen, and parathyroid hormone related protein. The completion of this stage results in differentiation of mammary lobules and alveoli, or milk making cells, which will be needed for milk production (Hale and Hartman 2007).
Breast Development in Pregnancy
“The hormones affecting lactation play different roles during pregnancy, birth and breastfeeding”
(Mohrbacher p 393)
The growth of breast tissue and the beginning of colostrum production is called ‘secretory differentiation’ or ‘lactogenesis I,’ and it begins mid-way through the pregnancy and ends on around day 2 after the birth. This stage is involved in producing the colostrum, the “first milk,” in the alveoli cells and milk ducts (Mohrbacher 2010).
While progesterone is one of the active hormones in the development of the mammary gland during pregnancy, another action of progesterone is to inhibit significant milk production during pregnancy. While the breast begins producing colostrum in pregnancy, progesterone’s presence during pregnancy directly inhibits the onset of copious milk secretion (Walker 2017, Mohrbacher 2010, Hale and Hartman 2007, Riordan 2005).
The expulsion of the placenta at birth triggers the hormonal chain of events that transitions a woman from the pregnancy hormonal state to the breastfeeding hormonal state. This hormonal cycle is what causes milk production to rapidly increase, and is called Lactogenesis II.
Lactogenesis II is the initiation of copious milk secretion and is triggered by the delivery of the placenta at birth, and the resulting withdrawal of progesterone from circulation. “Since it is the delivery of the placenta that initiates the fall in progesterone, lactogenesis II is therefore delayed until progesterone clears from the circulation, some 30 – 40 hours postpartum.” (Hale and Hartman p 98)
“Studies have shown that lactogenesis II is delayed in women who have retained placental products after birth. The capacity of the remaining placental fragments to secrete progesterone prolongs the inhibition of milk secretion in these women” (Hale and Hartman p 91).
Lactogenesis III is also known as Galactopoiesis, or the maintenance stage of milk production.
From the delivery of the placenta, estrogen, progesterone and placental lactogen blood levels fall quickly and stay low during breastfeeding, while prolactin levels stay high due to its secretion from the pituitary gland. At this stage of lactation, the hormones important to milk production are prolactin, oxytocin, cortisol, thyroid-stimulating hormone, and insulin. (Walker 2017, Mohrbacher 2010, Hale and Hartman 2007, Riordan 2005).
After the hormonal chain of events of the first days after the birth, hormones have only a minor role in maintaining milk production. Lactogenesis III is under autocrine, rather than hormonal control, which means that effective milk removal is the primary method for influencing the rate of milk supply. At this stage in lactation, milk removal is what is meant to keep prolactin levels high and is the primary driver of milk production (Mohrbacher 2010).
The pituitary gland’s secretion of prolactin for ongoing maintenance of milk production depends on a positive feedback loop. The infant suckling at the breast, and the regular emptying of the breast, are essential to the promotion of prolactin secretion. Levels of prolactin in the blood are highest in early lactation and will decline as lactation progresses. There is evidence that the function of prolactin may change during the course of lactation (Hale and Hartman 2007).
Understanding lactation physiology is important to see how the body is meant to maintain milk supply. The author of the book, Breastfeeding and Human Lactation explains the importance of prolactin receptors in long term milk supply, stating “the controlling factor in breastmilk output is the number of prolactin receptors rather than the amount of prolactin in serum” (Riordan p 77). The early weeks after birth may be a critical period for activation of enough prolactin receptors in the breast for ample long-term milk production (Mohrbacher 2010).
Oxytocin is released by the posterior pituitary gland in response to the baby sucking at the breast. The oxytocin hormone is released in pulsatile waves, causing surges of high levels that act on the muscles fibers surrounding alveolar cells in the breast, causing them to contract and release the milk. This is known as the “milk ejection reflex” (Walker 2017).