placenta can also be explained as the part which feed ur baby, when this part changes its position and come close to internal OS(can also be termed as the mouth of uterus) it is termed as placenta -posterior and low-lying placenta. minimum req gap is 2.5-3 cm between OS and placenta.
this is serious problem take simple precaution n u will be out of danger. try to have bed rest, dnt lift heavy load, dnt bend n avoid traveling and above all if this is the report of 5th month then u have one more reason to shed ur worries reason being, main development phase of ur baby is still to come if u take simple precaution u will get good report in ur next scan.