Hey mom to be need not worry. This is quite possible as the placental position keep changing in the womb. Best thing is we can come to know about this through the scan and it helps the doctor to discuss this with you every time you visit them. Till the last date of delivery the position of the placenta will be clear to the doctor and also she will guide you timely as per your condition. Best in not to be worried about it now as it can cause anxiety and stress which may make you feel a little uncomfortable