Which hormone is measured to assess ovulation on day 21 of the cycle?

To assess ovulation on day 21 of the menstrual cycle, progesterone is the hormone measured. After ovulation, the corpus luteum forms from the ruptured follicle and produces progesterone, which is crucial for preparing the endometrium for potential implantation of a fertilized egg. On day 21, which corresponds to the luteal phase of the menstrual cycle in a typical 28-day cycle, progesterone levels should be elevated if ovulation has occurred.

Measuring progesterone at this time helps in determining whether ovulation has taken place, as the hormone is usually low during the follicular phase and peaks after ovulation. If progesterone levels are within the expected range, it indicates that ovulation occurred and the corpus luteum is functioning properly. If levels are low, it might indicate that ovulation did not happen or that there are issues with the corpus luteum.