Hello girls, sorry for sitting in the bushes, but I am shy
I just wanted to write something to Clairesmummy because I know a bit about the LP. So here it goes:

The LP has to be at least 10 days long to support pregnancy. If it is shorter, then yes, it can cause an early miscarriage. If it is short it usually means that you have progesterone deficiency and you need progesterone to support pregnancy. I was reading your posts about 6 days LPs and I could not believe that no doctor wanted to check this yet. It is just not right.
After my first miscarriage, my doctor checked for the LP defect, because it is the most straightforward thing they can check. She did an endometrial biopsy - to check for the thickness of it about 5 days past ovulation. If it is not thick enough, I think this is one of the indicators that progesterone may be lacking as it makes endometrium thicken. (Lucky for me, everything was fine.) It is very unpleasant test, as they did it with just local anestesia, I swear D&C is less pain.
What they can do for it - I believe if you really have progesterone problems, they can give you progesterone supplements. It is best if your doctor prescribe them, since they will know what the dosage should be. I would certainly INSIST on the LP defect test. Good luck, I hope you get some answers.