I've heard about that too, but my experience has oddly been the opposite. I've always had vivid dreams and could usually recall several in great detail each morning. Since being preggo, dreams that I remember are less frequent and usually just sketchy at best. Almost like snapshots or something.
Check out www.dreammoods.com to look up the meanings of your dreams. You might be suprised at what you learn about your subconscience... I always enjoyed that before!