Hi Sally,
What I was told from my dr. was that as long as the heamatoma did not cause the placenta to come away completly from the wall that I would be fine, mine was quite large when they found it at around 14wks but over the next 4-8wks it did start to shrink and eventually reabsorb which allowed the placenta to reattach.
My Dr. basically told me to go home and relax as there was not much that could be done, if i was going to miscarry it was inevetable, thankfully i didn't and my little boy will be 3yrs in a few weeks.