Pharmacokinetics of progesterone concentrations was around 4 hours and the elimination half-life was in the range of 30 to 40 hours. The venous blood levels observed were very low. Because progesterone is a lipophilic and hydrophobic compound, it is not absorbed efficiently by the skin. Moreover, because the body produces about 25 mg progesterone per day on average during the luteal phase, around 50% of the body would need to be used as a surface of absorption to deliver a similar quantity of progesterone. As such, the transdermal route cannot easily achieve adequate circulating progesterone levels, and this makes transdermal progesterone impractical for
Breast development the breasts by inducing the expression of the progesterone receptor (PR) in breast epithelial tissue. In contrast to the case of the PR, ER expression in the breast is stable and differs relatively little in the contexts of reproductive status, stage of the menstrual cycle, or exogenous hormonal therapy. During pregnancy, pronounced breast growth and maturation occurs in preparation of lactation and breastfeeding. Estrogen and progesterone levels increase dramatically, reaching levels by late pregnancy that are several hundred-fold higher than usual menstrual cycle levels. Estrogen and progesterone cause the secretion of high levels of prolactin from the anterior pituitary, which