In the first stanza, the speaker begins the story of her abusive relationship with an unnamed man. Though the poem may not be a literally true story, the toxic dynamics between the speaker and her partner feel all too real—and the poem's dark humor thus feels especially sinister.
The speaker starts by remembering the day she turned 30, which should have been a joyful milestone in her life. And, right on cue, her partner seems to have thoughtfully made her a birthday cake, complete with "three layers of icing." How thoughtful!
Except, this is not a birthday cake. This is a weight-gain cake, celebrating the fact that the speaker is getting fatter and fatter, just as her partner wants her to. Of course, the huge, sugary cake itself is part of this plan. The candles mark, not the speaker's life in years, but her weight in stone (a unit of measure equal to 14 pounds).
Already, then, the speaker is being devalued and objectified—treated like livestock being fattened up for slaughter. Her partner celebrates, not an important moment in her life, but his own selfish glee that her body is meeting his standards.
These first three lines—which establish the poem's tercet stanza form—also put in place the poem's unusual rhyme scheme. Each stanza effectively has three of the same rhyme sound: AAA, BBB, and so on. But these are not full rhymes, but assonant rhymes: that is, they share the same vowel. In the first stanza, for instance, "cake," "made," and "weight" all use a long /ay/ sound.
This subtle linkage of sounds reflects the partner's behavior: the rhyme scheme here shapes the poem in an underhanded way, just as the partner coerces the speaker into "shaping" herself the way he wants through selfish gestures of "love."