Hi Polly,
I think your dream could mean many things. It may have been a message from your Mom, or perhaps not. I spent a couple years examining my dreams with a Jungian analyst, and would frequently dream of family members. I learned not to interpret these dreams literally. Very often, upon reflection, the person in the dream was symbolic of an aspect of myself or a situation I was experiencing in my life at that point. This doesn't mean you can't communicate with your Mom. A lot of people who post to this board have good advice about how that can be done. So I don't think your faith in that needs to be shaken. Maybe it's just a case of not getting too attached to a certain meaning of this dream.
Then again, those ultrasounds can be wrong! I know someone who was expecting a girl, and a boy popped out