The speaker imagines pregnancy as a kind of hunger that takes in everything: a trout, a sandwich, whatever is at hand. The poem treats the body as a vessel, and the "stolen words" seem to be language the speaker has gathered from elsewhere, from reading, listening, or borrowing. Calling them "unborn water" makes them feel fluid and not yet shaped, something that hasn't become a voice yet.
That water presses against "the drum / of my big belly," so the belly becomes an instrument, tight and ready to vibrate. Out of that pressure comes the ending: the speaker begins to hear "a new dark voice." Judging by the images, the poem seems to link physical gestation with the making of a poem, or of a new poetic self. The mood is quiet and a little eerie. There's wonder here, but also unease, since the voice is dark and unfamiliar, and it seems to be growing out of the speaker without being fully theirs.