A low lying placenta is actually defined by being less than 2.5cm away from the os generally. So without knowing exactly how far away it is, it's hard to say whether yours is more of a concern than anyone else who is diagnosed with a low lying placenta iykwim.

Perhaps if you ask him the exact distance that will give you an idea about whether it's something you should be concerned about. If it's 2.3cm, obviously that's very different to .5cm.

Lots of women (myself included) have this finding at the 20 week scan and it turns out to be a complete non issue. Knowledge is power - ask questions