What does it mean when you dream that you have a baby?
No one can say
Oh, if only we still had dr. Sigmund freud with us! but since his time, with ideas that dreams are "wish fulfillment" fantasies, the best guess is that dreams are what we experience as our brains go about consolidating and organizing our recent experiences.
You may have thought of having a baby recently or know or heard of someone expecting or having had a baby, and so on. A good dream!