Cardiprin is baby aspirin 100mg - it prevents the platelets from sticking together.
Heparin is an injectable anti-coagulant that prevents clots forming through a different process.

Both prevent clots from forming. Aspirin seems to play a role in placental development / implantation for those women with certain clotting conditions.

I have been on the cardiprin since 12 weeks (was meant to be sooner but there were other issues) for placental implantation. I was on clexane (similar to heparin but also has a component of immune system repression) for Factor V Leiden but stopped due to an allergy.

I have no idea about the role / timing etc for FET but I hope this helps a little. Good luck.