I took low dose aspirin daily during my second and third IUI. The first IUI resulted in a miscarriage so after some research I suggested aspirin to my consultant and he said I could take it if I wished to and it wouldn't do me any harm!
My third IUI was a success and I asked if I could continue to use progesterone until I was 12 weeks pregnant (rather than just for the two weeks after the IUI itself) and again was told the same thing.
IME NHS fertility treatment (amazing as it was) followed protocols which couldn't be altered. It was one size fits all. I really had to be my own advocate to try anything different.
My DS is now five months old. I will never know if the aspirin or progesterone contributed to his safe arrival but at the time I felt like I was doing everything I could to maximise the success of my BFP. When you've only seen two in four years of TTC and one has ended in MC that's very important.