Generally the placenta is only an issue for labour if it overlies or is very close to the internal os, as the PP said.
If you have an an anterior placenta it can make it hard to feel and hear the baby initially. This is because the Doppler or foetoscope can't selectively hear the baby's heartbeat, they just pick up whatever is closest. So, the placenta is often easiest to hear at first. This generally changes as the baby gets bigger though so by the end of the second trimester it's usually pretty easy to hear and feel them.
If you want to give any more info about your placenta I'm happy to be more specific if I can.