It is totally reasonable to hit your OB with all the questions in the world about this, and that is what I would strongly recommend that you do.
Questions I would ask include (but are definitely not limited to):
- How far from the cervix does the placenta appear to be, based on the last ultrasound?
- How much distance would the doctor want to see in order to feel comfortable proceeding, as planned, with a vaginal delivery?
- Are there any other conditions that might make low-lying placenta more of a concern?
Follow-up ultrasound is strongly recommended for cases like this, where the placenta is, or might be, close to the cervix. IIRC, you want to see a certain minimum distance. IMO, it's worth taking a second look at the placental location in the third trimester, to make sure you've got that minimum clearance. If you don't have enough space, the risks of birthing unassisted get pretty ugly. The concern is that, as the lower segment of the uterus changes shape in late pregnancy and labor, the placenta will shear away from the uterine wall, causing blood loss to both you and the baby. THe seperation of the placenta from the uterus may also impede placental function, which means the baby may not be getting oxygen. If the placenta begins to shear away from the lower segment of the uterine wall, you'll experience painless, bright red bleeding, which can be trivial, or can be severe enough to kill you.
The good news for you is that your placenta is low-lying, but apparently not actually over the cervix. Odds are good that this problem will resolve by itself long before you go into labor, but it's a case where you probably really want to be sure.