Your pituitary gland, located beneath the front of your brain, starts puberty. Your pituitary gland sends a signal to 2 other glands, your ovaries, telling them to make a special chemical, or hormone, called estrogen. Estrogen is the main female hormone that triggers most of the changes you'll see in puberty and feel. Boys have a different hormone, testosterone, that causes most of their changes.
Attribution: Straight Talk Foundation