Your answer is very informative. I still don't understand why P4 is not measurable in the blood and will research more. But my number was on the low side.
Big congratulations to your preg. I read your reply below and really appreciate your understanding and comforting words. I had three losses so far and it was a even harder journey for you. I am so glad for your pregnancy. All the best to you - an uneventful pregnancy and a wonderful baby in your hands.