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Progesterone hormone. Polarised light micrograph of crystals of female sex hormone progesterone. Progesterone is the most potent of the naturally- occurring progestogens, a class of steroid hormones. It is derived from pregnenolone, which is itself made by the action of an enzyme on cholesterol. The primary function of progesterone is to prepare the inner lining of the womb for pregnancy. It is secreted by the corpus luteum of an ovary following the release of an egg. If this egg is fertilised, the ovaries continue to produce progesterone for four months, after which it is secreted by the placenta. Magnification: 200x (KEYSTONE/SCIENCE PHOTO LIBRARY/PASIEKA)