Not a medical expert here, but having our fourth. I just went to a childbirth refresher course and we briefly touched on this.
Basically, it could work itself out and you may have no problems, or they will have to do a c-section on you if it doesn't. The placenta is supposed to be at the top of your sac. If it is lower, then it can end up below the baby. This can cause all kinds of problems for mother and baby and the baby cannot be delivered vaginally if that is the case because the placenta is the baby's lifeline and must deliver after the baby, not before. So then the doctor would do a c-section.
Since you are 20 weeks, there is still plenty of time for the placenta to move higher as the baby grows. Depending on what the doctor is seeing, they will likely want to do extra ultrasounds to watch the movement of the placenta and decide what needs to be done.
Again, not from a medical expert, but this is the info the nurse who was teaching the course gave us.
at 2:40 PM on Jun. 19, 2008