even i had the same issue. but dont worry am on my 8th month. Its just a matter of taking little more care. dont take much weights, avoid sitting in floors. as doctor said there is chance to move placenta upward. If it didn move also dont need to worry much. Only our anxiety raises more complications. If its too risky then the gynaec will suggest to have process of encercelage that is putting internal stitch in cervix so that to avoid chances of abortion.