I don't belive some of the stories either...but you never know. I talked to an ultrasound tech who had to inform a 50yr old woman she was pregnant, like 6+ months and she didn't know. She thought she had a tumor.
Honestly if one of my friends didn't go in for blood work I'm not sure she would know right now. She has had no symptoms and she has lost weight which she was originally trying to do. She doesn't look pregnant at all and she's 6 months.