Timing of serum progesterone testing

[confused__0]

  Is there an optimal time for a serum progesterone test?  My cycles have been 26 days, with ovulation on day 17/18.  I'm 34, have had one pregnancy and lost it, not due to LPD, but a blighted ovum with extremely high HCG (not molar) that required DNC @ 11 weeks because I hadn't spontaneously AB'd yet. I faxed Dr. copies of BBT chart and asked about a progesterone screening.  Nurse called back and said to do test on day 21.  
  1. What are normal values for day 21?  
  2. Are they based are based on ovulating day 14?
  3. If so, would ovulating day 17/18 cause test on day 21 to be abnormally high or low?  
  I haven't been able to talk to my Dr. about these questions; nurse says to do it on day 21 then we'll see where to go next.  I found something on Medline from 1989 says optimal time is day 25-26, but I don't know if this is still valid/current.  Also, Dr. counts day 1 as any spotting (usually a day or two prior to red flow); I've read you should count day 1 as first day of red flow.  However you count it, it's still too short between ovulation and spotting.  I'm really confused about when I should have the test and whether it will be valid.  Please help.  Thank you.

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Dear "confused" person:
Serum progesterone rises with ovulation, peaks 5-9 days following ovulation, and declines as the menstrual period approaches. Pregnancy rescues the corpus luteum and the progesterone level does not fall.
Progesterone is best tested 6,7, or 8 days following ovulation. Testing on day 21 presumes ovulation occurred on day 14. Ovulation on day 17/18 would mean that a day 21 progesterone level measures less than the = or >10 ng/ml desired peak value because one has observed the rise rather than the top of the curve.
Day 1 of the menstrual cycle is the first day of "real flow". Spotting belongs the the previous cycle.
Keywords: luteal phase progesterone
This information is provided for education purpose and is not a medical consultation. If you have specific questions, please speak with your physician.