Dreams during pregnancy

When I was pregnant with my first daughter I had strange and very vivid dreams, I'm pregnant with baby girl number 2 and I'm having not very night dreams.. Most of the time I dream that my 3 year old is running away from me and I can't catch her, it's always on a road where cars go and no matter how fast I try I just can't get to her. It's so scary. What does that mean? Does anyone know? I used to have a dream book but lost it when we moved!