I think that with IVF, they're bound by duty of care to ensure they check up on you regularly, in case of ectopics or multiples, or all manner of things which carry a higher risk with IVF, but with a natural pg, the onus is not on them to rule out an IVF-related risk maybe? But I do understand feeling abandoned after all the monitering you've undergone previously.

Why don't you wait for your BT and then after your results are ready, give your FS a call and get him to explain your P4 levels to you, and why he thinks you should stop the supps. I know we are not encouraged to call our FSs with questions, but that doesn't mean you shouldn't. In fact it is your right to know why your particular treatment (or withdrawal of treatment in this case) was decided on. And it's his job to explain it to you.

BTW the nurse was not right to firstly undermine your FS's instructions, and secondly to plant the seed of doubt in your head without offering you a possible plan of action to resolve it, IMO. If it had happened to me, I would also be mentioning to my FS that so-and-so told me I should be on P4 for 12 weeks, just so he's aware you're getting differing advice that is stressing you out.