Why Day 21?
This day is chosen on the assumption that the women are having a 26-30 day cycle, and luteal phase is normal and lasts about 10-14 days after ovulation. Thus, assuming ovulation is between Days 12-16 , Day 21 is the middle of the luteal phase when the production of progesterone from the corpus luteum gland peaks in an ovulatory cycle (and if there was then pregnancy, the HCG would prevent the corpus luteum from then dying and it would produce even more progesterone). If ovulation has occurred later than Days 12 – 16, then the progesterone check would be better done a few days later so as to strike the middle of the luteal phase. Conversely, if ovulation has occurred earlier than Day 12, an earlier progesterone test at about Day 18 is required.
Progesterone levels should be interpreted with respect to the number of days before the onset of the next period. Luteal function ( and hence ovulation) cannot be assumed if the period starts < 5 days later or > 9 days after the blood test.