The ovaries produce several types of hormones at different times in the menstrual cycle, which trigger different functions in the body. Estrogen enables healthy general functioning of the reproductive system, causes changes to physical characteristics during puberty, and makes the ovary grow and release egg follicles. Progesterone works to stabilize the menstrual cycle and prepare the body for pregnancy by causing the lining of the uterus to thicken to be able to support a possible fertilized egg. If there is no implantation of a fertilized egg, the progesterone level drops, which causes the lining to be shed in menstruation. Menopause is a natural process, where the ovaries gradually produce less estrogen and progesterone, and stop releasing eggs, causing the menstrual cycle to stop and signals the end of fertility. It is a gradual process that can take years before full menopause is reached. Menopause is permanent, unpreventable, and irreversible.