Progesterone is prescribed to women with a uterus who are taking estradiol during menopause. This is because estrogen promotes the continuous growth of cells in the uterus. When you’re menstruating, those cells are shed every month through your period. Once menstruation stops (through menopause, for example), estrogen alone can make the the cells build up, thickening your uterus and leading to abnormal growths, which can then lead to uterine cancers. Progestin helps prevent this by keeping the uterine lining thin.