My 2nd the placenta extended to the cervix but didn't cover it. A repeat scan at 32weeks showed it was about 4-5cm away. I'd have to look back at reports to get the exact figure.
My 3rd the placenta covered the entire internal os and apparently I had large placental lakes. I was expecting it to not move enough for a vaginal delivery but it did. I didn't have any bleeding with either. They did class it as a grade 3 placenta previa by memory in the report at 19weeks where as with number 2 it just said low lying placenta and repeat at 32weeks.
This pregnancy I expected it to be low too but it was 5cm away at 19weeks. Only pregnancy (apart from my m/c) I have had bleeding but yet the placenta is further away then any of them.