The luteal phase is the second half of the ovarian cycle. It begins at ovulation and lasts approximately 14 days — the most consistent and predictable portion of the menstrual cycle. After the follicle releases its egg, the remaining structure transforms under the influence of LH into the corpus luteum ("yellow body"), a temporary endocrine gland that becomes the principal source of progesterone during the cycle's second half, along with some estradiol and inhibin A. Progesterone is the defining hormone of this phase. It converts the estrogen-primed proliferative endometrium into a secretory, glycogen-rich lining ready for implantation, thickens cervical mucus into a sperm-blocking barrier, raises basal body temperature through its action on the hypothalamic thermostat, quiets uterine contractions, and stimulates breast alveolar tissue. Many familiar premenstrual experiences — breast tenderness, bloating, fatigue, and mood changes — arise from these progesterone-related effects and the brain's response to them. The phase has only two possible endings. If fertilization and implantation do not occur, the corpus luteum has a fixed lifespan of about 14 days; it then degenerates into the corpus albicans, progesterone...