I heard the same thing in my ultrasound - and I was kind of upset at first. However, I have been told by my doctor that as the baby grows it can pull everything up. I also have a friend who is a doula and she mentioned that it would be helpful if I went for a prenatal massage. Make sure to tell them that the placenta is low... they should be able to do some things to relax the ligaments to encourage the baby and placenta to move into more favorable positions.
Good Luck!