The poem is a tiny scene, and a lot depends on how you read its first line. "Knocked up" is blunt slang for being pregnant, and the title suggests a crowded, cheap apartment building, so we picture a woman in a small home getting up and going to the window. Whether the speaker is watching her or is the woman herself isn't clear, and that ambiguity is part of the poem's quiet pull.
At the window, moonlight falls on her lip. The comparison to "a light supper" is odd and lovely: a little glow, something modest and nourishing rather than a feast. It hints at hunger, perhaps the restless appetite of pregnancy, and at the thin comfort the night offers. The mood is hushed and tender, with a touch of unease. The rough slang sits against the gentle image, and that contrast suggests worry about money, space, or the future, but also a small moment of calm and beauty.