Do you mean placenta over the scar or over the cervix (placenta previa)? I am not aware of any research about the likelihood of problems with the placenta being over the scar but not the cervix. When the placenta overlays the cervix, a planned cesarean is often necessary, because as the cervix dilates, the placenta will hemorrhage. Sometimes, though, depending on how much of the cervix is covered, it is possible to make it through to a vaginal birth. You wouldn't want to try this outside of a hospital that could perform an urgent c/sec at any time.

-- Henci

P.S. Important note: you cannot tell until toward the end of pregnancy if placenta previa is going to pose a problem. Many a previa disappears as the pregnancy advances. As the uterus grows larger, the placenta is often carried upward sufficiently far that it moves away from the cervix.

A woman with a cesarean-scarred uterus has a higher risk of having placenta accreta (the placenta invades the uterine muscle sometimes growing through it into other organs) in a subsequent pregnancy compared with women who have no uterine scar, and that risk rises as she accumulates more scars. A woman with a cesarean scar is also more likely to have placenta previa in combination with accreta than women who have placenta previa and unscarred uteruses. One researcher looking at cesarean-scar ectopic pregnancy theorized that if the embryo tries to grow inside the cesarean scar, you get an ectopic pregnancy. If only the placenta grows inside the scar but the embryo develops inside the uterus, you get placenta accreta.

-- Henci