Progesterone is a steroid hormone, which is secreted by the corpus luteum of the ovary and by the placenta and that helps to prepare the uterus for implantation of the fertilized ovum and to maintain pregnancy. Changing progesterone levels can contribute to abnormal menstrual periods and menopausal symptoms. It's function is to regulate the condition of the inner lining (endometrium) of the uterus. It is this hormone mainly required for implantation.