Phebee - I have a blood clot (subchorionic haemorrhage) behind the placenta that has been there from the beginning. I think as long as you are not bleeding then they are happy to just wait and see what happens. Most are reabsorbed by the body by 20 weeks. I don't know why mine hasn't gone but I haven't had any bleeding since around week 18 and baby is the size they are supposed to be. Yours will likely be gone over the next few weeks.