I had a low lying placenta. They found it at my 20 week scan. It's generally not anything to worry about. Usually the most common effect from it is that some women have bleeding/spotting, but not all do. I never had any.
In almost every pregnancy, the placenta will move up far enough to not be a problem. It's only in the rare case that it doesn't. (Something like less than 5% of pregnancies) It's nothing to worry about. In my case, by the time I had my 24 week scan it had already moved up.
Sorry, I forgot to mention that if it DOES evolve into full or partial placenta previa, it could require bedrest and can cause moderate to heavy bleeding. I will reiterate that at this point, it is simply low-lying, meaning it isn't on the top or side where it typically is. At this point there aren't any risks and it's a matter of just waiting and seeing what it does.
A low placenta means that because of where the embryo implanted, your placenta is located further down on your uterus closer to the placenta. If it is just low, it may not cause any problems and could migrate higher as your uterus grows. However, the risks involved with a low lying placenta are that it can cover the cervix (placenta previa) either fully or partially. IF this were to become the case, and it did not correct itself by the time you go into labor, it may become necessary to have a cesarean. You can't birth a baby THROUGH the placenta, that would cause disastrous problems.
Chances are good though that as your pregnancy progress your placenta will migrate higher and away from the cervix. This is actually a fairly common occurance. I wouldn't worry at this point, your doctor will monitor the placental position and let you know if there's any need to worry or discuss possible cesarean section.
At this point it doesn't potentially pose any risks to the health of your baby.