Hi,
For us, it was being tested for reasons for implantation failure and finding out that I had antinuclear antibodies (ANA) which was stopping our little embies from sticking. As we were only a week out from a FET when we found out, I started aspirin straight away and then heparin injections from the day of transfer. We got our BFP from that FET after two failed cycles/fresh transfers and 3 FET's.

Good luck.
FG