I have a sub amniotic hematoma which is a bit different as its in the sac with the baby. Its also stupidly rare. However, I believe the treatment plan is similar.
Mine was found at 20 week scan (I've never had so much as a spot of blood). I am now on 4 weekly growth scans with doppler (to check blood flow between baby and placenta) and seeing the consultant at my 28 week scan (should have been my 24 week scan but there was a muddle up with the booking).
I think if you still have it at your 20 week scan they will keep a closer eye but before then they don't because they can't do proper growth scans much before 24 weeks anyway.
However, I also understand that most early ones are absorbed much before that point so it is very likely you'll carry on with a midwife led pregnancy.
As long as baby is growing they do not take any action. Obviously, if babys growth looks to slow down then they may intervene.
I am 28 weeks tomorrow and next scan is next week, but I am very lucky that I have a wiggly squirmy baby so get constant reassurance.
The wait between scans is horrible (especially the first one). The hospital here have been amazing and said I can be checked as often as I need to be for my own peace of mind, which I've managed not to take them up on so far. FYI baby was measuring exactly 50th centile at 24 weeks.