Luteinizing hormone is progesterone, which is mainly secreted by the corpus luteum of the ovary. At the time of menstruation, the level of estrogen and progesterone in the body is the lowest. Then, influenced by the ovarian secretion of estrogen, the endometrium begins to proliferate, and at the time of ovulation, the follicles are discharged to form the corpus luteum, which begins to secrete progesterone. The level of progesterone can be checked by blood test and the level of progesterone, which is the lowest level during menstruation, doubles every other day before and after ovulation, and remains at a high level after ovulation. If the follicle is not fertilized, the corpus luteum atrophies, the body’s estrogen and progesterone levels drop at the same time, and menstruation occurs. If conception occurs, the corpus luteum will continue to grow and secrete progesterone, which will gradually increase during early pregnancy and can be monitored by checking blood progesterone values. Low progesterone values in early pregnancy need to be treated with progesterone supplementation.