A major instance of ambiguity comes in two of the second set of prophecies the witches offer Macbeth in Act IV, scene 1. When he visits the witches as his reign appears to be unravelling, they tell him the following:
Be bloody, bold and resolute; laugh to scorn
The power of man, for none of women born
Shall harm Macbeth
The Third Apparition says:
Be lion-mettled, proud, and take no care . . .
Macbeth shall never vanquished be until Great
Birnam Wood to high Dunsinane Hill shall come against him.
The Second Apparition tells Macbeth that nobody born of a woman will hurt Macbeth, but, as will later learn, the words "of women born" are deliberately ambiguous, having a double meaning. Macbeth takes them to mean that no human can harm him, because he assumes all people are born of women. However, by "born of women" the Apparition means born vaginally, not through a Caesarean section: one born by Caesarean section, Macduff, will defeat Macbeth.
The Third Apparition states that Birnam Wood would have to move to a nearby hill for Macbeth to be defeated. Macbeth interprets this to mean that since a forest can't transplant itself, he will be victorious. However, the meaning is again ambiguous. The forest can't transplant itself, but Macduff's troops can disguise themselves with branches from Birnam Wood as they climb Dunsinane, as happens. In this meaning, the "woods" do move.
Antithesis comes early in the play, In Act I, scene v, as Lady Macbeth tries to "unsex" herself in order to harden her heart to the murder of Duncan. For instance, she asks that the maternal milk from her breasts be turned to "gall," the antithesis or opposite of nourishing and sweet tasting mother's milk.