The poem begins with the speaker announcing that she is "a riddle in nine syllables." The nature of this "riddle" isn't immediately clear. In calling herself, a "riddle" in the first place, however, readers might get the sense that the speaker feels herself to be a bit of an enigma, a puzzle that needs to be solved. There's also something playful and amusing about calling herself a "riddle."
While readers don't know exactly what the "answer" to this riddle is yet, there's a major clue in this opening line: this riddle has "nine syllables," as does this first line (and, indeed, every line that follows). A quick glance at the entire poem also shows that it has nine lines total. The number nine, then, is clearly important here.
The sounds of these lines, with their liquid /l/ and nasally /n/ consonance and clipped assonance, add intensity to the poem as well:
I'm a riddle in nine syllables,
The jumble of sounds makes this first line a bit of a mouthful, again suggesting the speaker's discomfort as she struggles with a body that feels strangely new.
And in the next line, the speaker gives two more metaphors that act as clues about her state. She's an "elephant" and a "ponderous" (or big, bulky, and awkward) "house." What both of these images have in common is their massive size.
Readers might already guess at the solution to the puzzle here: what combines the number nine with feeling huge, as big as a house? Pregnancy!
The speaker is describing her changing body, which has become mysterious, riddle-like, to the speaker herself. The house metaphor suggests not only the uncomfortable clumsiness of trying to move around in a body that is suddenly so unfamiliar, but also the speaker's disorienting realization that her body is no longer hers alone: it "house[s]" another.