What exactly is natural progesterone and how does it differ from synthetic progesterone?

Natural pregesterone, a cholesterol derivative, comes from the Mexican yam. It matches exactly the chemical formulation of the body's own progesterone. Manufacturers alter the chemical makeup of progesterone in order to create progestins. Progestins in their altered molecular structure may cause many side effects, such as birth defects or abortion, fluid retention, epilepsy, migraine, asthma, cardiac or kidney dysfunction and depression.