I would have to agree with Anila on the intuition thing, there was no exact reason why for me just a series of little things that all sort of added up, I also had a few "weird" experiences that really quite scared me and had me thinking that something was wrong, I was also REALLY aprehensive about going to my scan (where I found out my bub had no HB) but I had no reason at all to be, just something in my head/body that was niggling constantly "this isnt gonna be good"
I hope that makes sense!