When did Jocasta know?

In Sophocles' Oedipus the King, the arrival of the messenger from Corinth, with his startling announcement that Oedipus is not the son of Polybos, diverts Oedipus from his search for the killer of Laios to a search for his own origins and identity. But before Oedipus realizes who he is, Jocasta recognizes the awful truth and recoils in horror.

Before the messenger’s arrival, Jocasta thought that her son – the son prophesied to kill his father – was dead. She had secretly given him with his ankles bound to a trusted servant to expose on Mt. Cithaeron. She knows that this is the same servant who witnessed the murder of Laios, and whom she recently sent for, but no one else knows.

The Corinthian messenger tells Oedipus and Jocasta that he found Oedipus as a baby on Mt. Cithaeron while he was herding sheep. Because the baby’s feet were bound and swollen, he named him Oedipus. He does not know the baby’s origin, but says that the shepherd who gave it to him would know. “What shepherd?” Oedipus asks. “The one who worked for Laios,” the messenger replies. When Oedipus asks if the man is still alive, the messenger points to the elders for an answer. Their leader identifies him as the man Jocasta just sent for.

Now she knows.