I wouldn't worry to much about it. Progesterone levels will fluctuate somewhat and as long as they are in the normal range, which yours are, there is no concern.
HCG levels are the ones they monitor in terms of rising regularly. Sounds to me like everything is looking good!
Congrats and all the best to you!!
what did your doctor say?
mine didn't even measure my progesterone after my IVF, just kept me on suppositories until 10 weeks.
I wouldn't panic, but it might be a decent idea to at least talk to your Dr about progesterone supplements. My progesterone levels at almost 8 weeks was 16.5 and they put me on supplements and baby's doing well. I hear mixed things about progesterone levels and minimums, I read 20 was average at 4 weeks but beyond that, just a range for each trimester. You are with in the range of normal. I would be more concern with the fact that it went down, but if the ultrasound is good ask for the supplements. Good luck and lots of sticky baby dust.