When Macbeth fights Young Siward he is very confident of the outcome? Why is this? Is he right to be so confident?

Macbeth is still believing in the second apparition’s prophesy that, "none of woman born Shall harm Macbeth." Macbeth is cnfident when he makes short work of young Siward, "Thou wast born of woman." Macbeth's overconfidence will prove to be his undoing.