Nothing, because she isn't. People read too deeply into things. Starting with what we know, she can't have children, she's barren. The dragons are her babies.
For the better explanation, there isn't time for this. Let's say she is pregnant, do you think she has 9 months to relax and wait around for little Ned to be born? The Night King is already south of the wall when Jon and Dany copulate. It won't take him 9 months to reach Winterfell. He's going to arrive within a month or two if I had to estimate, not accounting for D&D's sense of time in this show.
She also can't ride Drogon with a bulging belly. It'll be a big no from me on this theory.