The best time to check the corpus luteum is one week after ovulation because generally follicles in the follicular phase do not secrete progesterone. The granulosa cells of mature follicles before ovulation luteinize under the effect of LH ovulation peak and start to secrete a small amount of progesterone, after ovulation the corpus luteum secretion of progesterone gradually increases to the peak when the corpus luteum matures 7-8 days after ovulation, and then gradually decreases to the follicular phase level at the onset of menstruation. Therefore, sometimes a luteal level is measured after ovulation, which represents the good or bad follicular corpus luteum formation and is directly related to pregnancy. If the corpus luteum is low, there may be problems with embryo implantation or the possibility of miscarriage, so it is usually best to measure it one week after ovulation.