They are not sure what causes them but some think it occurs when the fetus implants and causes changes to the lining. It is not uncommon. I had one but I do have three clotting factors..My hematologist said it was not related but she did stop the blood thinner until it resolved. I did stay on BA.
I think what they really look at is the size and the proximity to the fetus..and they keep an eye on it by weekly u/s's. I got mine at seven weeks and it resolved by week nine.
Good luck to you!