What Does Positive Anti-m Indicate?

Hi, I had blood tests done as i'm wanting to have a baby. I have never been pregnant before. I got my blood tests back and I tested positive for the anti-m antibody and the doctor said he didn't really know about it but would research it and he said that it means i have been pregnant before as you can only test positive for anti-m if you have been pregnant, is this true? Also can you tell me what it means and the impacts of it? Thank you