What devastating mistake occurs when Mrs. McNair is in the hospital in the novel, Dream Children?

Mrs. McNair's terrible experience is in fact made worse the morning after losing her child, an exhausted nurse mistakenly brings in someone else's baby, cheerfully telling Mrs. McNair that it is hers, and lays it in her arms. When the error is explained to her, the poor woman will not let go of the baby, and her agonizing ordeal takes on a new form of suffering.