You can't have a true period while pregnant.
You can't have a true period while pregnant because a true period is when the lining of the uterus sheds to dispose of an unfertilized egg. Because your pregnant there isn't an unfertilized egg so there's nothing to shed.
You can spot or bleed during pregnancy though. This can be implantation bleeding, old blood that is still coming out, or slight bleeding due to placental position/abnormalities. If you do spot during pregnancy you should see your doctor right away just to make sure everything is okay. about 50% of spotting is harmless but you may need an Rh factor shot.