The luteal phase is the day after ovulation to the day before the onset of menstruation. After ovulation, the corpus luteum of the ovary forms, secretes progesterone, maintains the thickness of the endometrium, inhibits uterine contraction, and facilitates the fertilization of the egg. If the follicle is not fertilized, the endometrium will disintegrate and fall off, bleeding for menstruation. During the menstrual period, the body estrogen and progesterone levels are low, with the growth of follicles, secretion of estrogen endometrial hyperplasia, after ovulation, the corpus luteum is formed and begins to secrete progesterone, through the blood can detect the progesterone value gradually increases, if the follicle is not fertilized, the corpus luteum will shrink, progesterone drops sharply, and menstruation comes. During the luteal phase, the basal body temperature will increase, but it is within the normal range. The basal body temperature can determine whether there is ovulation and whether the luteal function is sound.