Hi Jules,
Your story sounds very similar to mine. I have had several very early miscarriages. My husband and I had all of the tests for this including karyotyping and clotting disorders- all came back normal (which I was warned about). Although my progesterone blood levels have been normal, my FS wonders whether my levels locally are too low after implantation; hence the early losses. As a result she has started me on hormone support to help keep my progesterone levels up. Fingers crossed this helps. The next step would be IUI but I'm hoping we have some luck prior to that.
So this is something worth addressing with your doctor.
Good luck!
Skipping girl