Just to say the best research on this I found was:
Ku, C., Allen Jr, J.C., Lek, S. et al. Serum progesterone distribution in normal pregnancies compared to pregnancies complicated by threatened miscarriage from 5 to 13 weeks gestation: a prospective cohort study. BMC Pregnancy Childbirth 18, 360 (2018). doi.org/10.1186/s12884-018-2002-z
Lower progesterone is linked to higher risk of miscarriage. My progesterone was on the low side, but I wasn't spotting so wouldn't have been offered progesterone by my EPU. They said in a subsequent pregnancy if I was spotting I would be able to get progesterone pessaries.
Tommy's PRISM trial shows progesterone can be useful in cases of 3 or more losses.
I tracked my BBT throughout the last 12-13 weeks. It's only now that my hCG is almost at zero that my BBT is starting to drop. It dropped slightly before my first scan at 6 weeks, which was before the embryo stopped developing, but presumably the placenta kept producing progesterone and had MMC.
I think the difficult thing is you can't tell if the embryo isn't progressing because of low progesterone, or whether you have low progesterone, because the embryo / pregnancy isn't viable... I had 3 earlier losses (5-6 weeks) between my two daughters over a decade ago.
With this latest pregnancy I tested hcg blood tests and progesterone privately with medichecks. My progesterone was 25.7 nmol/L at 18dpo and 17.2 nmol/l at 23dpo (the day after first scan at 6w3d when hcg was over 15000). The progesterone was low, but just within range. If I was pregnant again I would test these again at 7 and 8 weeks, but all my previous losses had been much earlier so thought it was going to be ok based on the hcg.
Because I had medical management I was able to request the embryo be genetically tested and asked to be referred to recurrent miscarriage clinic, plus planning to do a bunch of blood tests privately to start with when I get my next cycle (day 2-5 bloods, AMH, iron, DHEA, CRP (inflammation). I am 44, so I know any discussion will be about egg quality, so I want a better idea about FSH / AMH, DHEA, etc given my cycles are more regular than they have ever been.