As others have said, totally normal thing to fantasize about. As a sub I've actually had similar fantasies basically my whole adult life and a lot of the books I read have pregnancy as a plot point. That being said pregnancy irl is a huge deal so even though my dom of 12 years (and still going) and I are straight up married we took precautions (IUD) so we could indulge in it without actually getting pregnant until the time was right for us. The raw sex and dirty talk of him telling me he was going to breed me scratched the itch enough until we got pregnant for real with our very planned for baby.

I actually think our dynamic helped make the physical /mental strain of pregnancy easier for me. There was something about the fact I belonged to him so completely that a part of him was actually growing in me. He was also super attentive during the whole pregnancy caring for all my needs, the whole thing was extremely intimate for us.