Why does progesterone need to be low before modified natural FET?

[ProgesteroneQuery]

I’ve been told by my coordinator that I need to have a scan and progesterone test on cycle day 10. The progesterone needs to be ‘low’ to go ahead with planned modified natural FET this cycle.

I emailed back but they haven’t replied yet as I don’t understand why the progesterone test is needed as at my previous clinic we didn’t need this only a scan . Has anyone had this what exactly are they looking for and why does it need to be low on that day?

If it’s high too early, it can indicate you’ve ovulated so timing will be out of sync

It's to do with endometrial receptivity. There needs to be a low baseline so that the window of implantation can be predicted with confidence. If progesterone has already started to rise that can alter the window.