The witches have told Macbeth that he will reign until "Birnam Wood comes to Dunsinane" and that he can be killed by 'no man of woman born".
Now that Birnam wood has come to Dunsinane (Macduff's soldiers using trees as camouflage) and Macduff reveals that he was "from his mother's womb untimely ripped" (Caesarian section) and therefore, "not of woman born", Macbeth realizes that he is definitely in the soup.
Macduff tells him to yield, but Macbeth defies the prophecy, saying: "I will not yield, To kiss the ground before young Malcolm's feet, and to be baited with the rabble's curse. Though Birnam wood be come to Dunsinane, and thou opposed, being of no woman born, het I will try the last. Before my body I throw my warlike shield: lay on, Macduff, and damned be him that first cries 'Hold, enough'."
Could that be it?