A mother looks back nineteen years to the moment her son began, an awkward, passionate act that she describes in plain, almost blunt terms. She carried him, bore him, and then spent years raising him, and the poem admits that this wasn't all tenderness. She says she beat him and hurt him even as she fed and loved him, which is an unusually honest thing to confess.
The ending is where the feeling lands. Her son now "stands beyond" her, out in the world, with his own sharp mind and quick heart. The final image is the surprise: it is as if she had given birth to a tree. A tree is rooted, independent, and not something she made in the way she made a baby. The mood is a mix of wonder, distance, and quiet pride, with a hint of guilt. She seems amazed that someone so beautiful and strange came from her, and a little awed that he is no longer hers to shape.