Progesterone is the female reproductive hormone. Its presence is considered a marker for estrus in milking cows hence its presence is often measured in milk where its concentrations may be up to 60 ug/g.Progesterone is one of the steroid hormones. It is secreted by the ovaries (corpus luteum) and by the placenta and is responsible for preparing the body for pregnancy and, if pregnancy occurs, maintaining it until birth. Progesterone secretion by the corpus luteum occurs after ovulation and continues the preparation of the endometrium for a possible pregnancy . It inhibits contraction of the uterus as well as the development of a new follicle. If pregnancy does not occur, secretion wanes toward the end of the menstrual cycle, and menstruation begins. If pregnancy does occur, the placenta begins to secrete progesterone which supplements that of the corpus luteum. In fact, by the fifth month of pregnancy, the placenta secretes sufficient progesterone by itself that the corpus luteum is no longer essential to maintain pregnancy. Progesterone, like all steroids, is a small hydrophobic molecule. Thus it diffuses freely through the plasma membrane of all cells. However, in target cells, like those of the endometrium, it becomes tightly bound to a cytoplasmic protein, the progesterone receptor. The complex of receptor and its hormone moves into the nucleus. There it binds to a progesterone response element. The progesterone response element is a specific sequence of DNA in the promotion of certain genes that are needed to turn those genes on (or off). Thus, the complex of progesterone with its receptor forms a transcription factor. Some target cells also have other progesterone receptors that are exposed at the surface of the cell embedded in the plasma membrane. These are G-protein-coupled receptors, and binding of the hormone to them produces more rapid effects than those of the nuclear receptors.