An elephant, a ponderous house, (2)
These lines establish the size of the woman's appearance: absolutely humongous. We could even say elephantine. Of course, we know that pregnant women aren't actually the size of elephants and houses, but for this woman, that's what her dramatic change in size feels like.
A melon strolling on two tendrils. (3)
Now that the whole elephant and house thing has given us an idea of the huge size of this speaker's appearance, we get an image of how disproportionate and awkward she is. She feels as if she's huge, like a melon, around the middle, and as thin as two tiny curling vines everywhere else. Sounds precarious, no?
This loaf's big with its yeasty rising. (5)
Again, we get an image of largeness. It's weird that, throughout this poem, a pregnant woman is compared to something we'd like to eat, like red fruit or a fresh loaf of bread. Abundance, this poem seems to say, can be attractive. Unless you are that abundant thing.