[peggy]

The 21 day progesterone test is a blood test designed to measure your progesterone levels at day 21 of your menstrual cycle or, more precisely, 7 days post-ovulation. It checks that they have risen sufficiently to indicate a) that you have ovulated and b) that your uterine lining is developing sufficiently to sustain a pregnancy.

It's important to realise that if you don't ovulate until day 16, having the test on day 21 would be too early. It should really be taken 7 days post ovulation, which for convenience is called day 21, which is for an average 28 day cycle.

If your period does not arrive 6-8 days after the test, you probably had your dates wrong and will need to try again

http://kayoz.typepad.com/blog/21-day-progesterone-test-progesterone-levels.html