Progesterone, estrogen, placental lactogen, pr olactin, cortisol and insulin action contribute on growth and development of alveolar and ductal system of the breast during pregnancy, and the alv eolar secretory epithelium synthesizes several milk components. α -lactalbumin stimulates lactose syn thase to increase milk lactose. Progesterone inhibits and prolactin stimulates α -lactalbumin producti on. With delivery, there is an abrupt and profound decrease in the levels of progesterone (and estrogen). Progesterone withdrawal allows prolactin to act unopposed in its stimulation of α - lactalbumin production. Prolactin is very important for lact ation, and women with extensive pituitary necrosis (S heehan syndrome) do not lactate.
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