The level of progesterone in the blood is measured by a progesterone test. The hormone progesterone is produced by a woman's ovaries. Progesterone is a crucial hormone during pregnancy. It aids in preparing your uterus to support a fertilised egg. Progesterone also aids in the preparation of your breasts for the production of milk. During a woman's menstrual cycle, her progesterone levels fluctuate. The levels begin low and gradually rise after the ovaries release an egg. Progesterone levels will continue to climb as your body prepares to sustain a growing baby if you become pregnant. Your progesterone levels will drop and your period will begin if you do not become pregnant (your egg is not fertilised).