This line moves us back into some of the more obvious metaphors for pregnancy. It's easy to picture a pregnant woman as a loaf of bread rising, what with the slow swelling of her belly as a child grows inside.
And even though we love freshly baked bread just as much as the next person, there's something not very appetizing about thinking about a baby along the same lines as bread fresh out of the oven.
In fact, it sounds a little sad and cynical the way that the speaker thinks of pregnant women. No pregnant woman that we know wants to think of herself as a loaf of bread. Or a melon on skinny legs, either, for that matter.