I had a friend who didn't know she was pregnant until well over the 6 month mark. She was a big girl to start with and having problems with irregular periods already so she wasn't worried about no period for months. There was also a nasty bug circulating in the area during the months where she would have been getting morning sickness, so she thought it was just a prolonged case of the bug. And she did actually lose a little weight during that time. It wasn't until she felt the baby moving in a big way that she realised what was going on.