I think the mental, physical and emotional adjustments of pregnancy are reflected in our dreams. I guess it's pretty common to feel vulnerable, unprepared for parenthood, a little bit trapped, excited, and worried about the effect that pregnancy, childbirth and parenting will have on your relationship, your body, and yourself. I know that during my pregnancy with Emily, I used to frequently dream that I was stuck in a car that was going SOOOOOOO slowly, and I couldn't speed it up or slow it down, and it was heading for a crash. I was trapped, and couldn't change the course of the car. Never dreamt like that before or after, but I think it was related to a feeling of "here we go again, wow this is going to be long, and I hate the last month of pregnancy, and the intense demands of the first few weeks are going to be tough, along with caring for two other little ones...." And I was excited about my pregnancy, and looking forward to having another little person to care for, but it's always a mixed thing, I think, and some of what we feel comes out in our dreams.
Anyways, hope the scary ones end soon! All the best!