late bfps???

hey ladies have you ever heard those stories about women who say that their HCG levels were undetected by Blood for like months and they didn't find out they were pregnant until the end of the first or second trimester? I've Heard lots of Stories on here and other sites saying that they tested negative clear up till like halfway through their pregnancy. Do any of you actually believe that because it sounds so unreal and I have yet to find any legit Medical case study of that happening.