In Act 4, Scene 1 Macbeth returns to the witches to get more answers. He is terribly insecure at this point in the play, having recently been troubled by Banquo's ghost at the feast. Before he leaves to see the 'weird sisters', as he calls them (3.4.133) he tells his wife that he is 'bent to know, / By the worst means, the worst. (3.4.134-135). So it is with this 'bent' mind-set that Macbeth goes to the witches and demands answers. It is worth noting that when he speaks to the witches here (4.1.51-60), he hazards all of nature upon his demand to know more.
So, Macbeth comes to the witches with 'bent' intentions and is willing to risk all the world to get his answers. So it is that he cannot see 'straight' to the truths revealed by the three apparitions. Contrary to what Macbeth believes when he hears them, these apparitions foreshadow his downfall. He asks to know the worst, but does not see it when shown to him.
The first apparition is clear enough, it tells Macbeth to beware Macduff. Macbeth, naturally, is thankful for this straightforward advice. Things complicate, however, with the second apparition, the bloody child, which states enigmatically that none of woman born shall harm him. Macbeth's initial response here is to discount the warning about Macduff in the first apparition, 'what need I fear of thee?' (4.1.81), though he resolves to have him killed just to be on the safe side. But already we can see that Macbeth is looking to interpret the apparitions according to his need for reassurance.
The third apparition appears to reassure him further: another child tells him that he shall not be defeated until the woods come up the hill to his castle. Again, Macbeth cannot see this being a possibility and believes that he can now 'live the lease of nature' (4.1.99). The foreshadowing here is that Macbeth has already committed crimes against the order of nature. Macbeth's murder of Duncan was closely followed by the report that the day was unnaturally dark, a mighty hawk was killed by a little owl and Duncan's horses ate each other (2.4.1-20). Macbeth's 'vaulting ambition' has resulted in reversals of nature and so it is that the third apparition predicts natural forces rising up against him. But, again, Macbeth's anxious state to find reassurances by the worst means leads him to interpret the messages in a way that suits him. In this way, the apparitions also foreshadow the rash and foolhardy way in which Macbeth orders his troops to leave the safety of his castle and fight the advancing troops (5.5.46). At this point Macbeth echoes his earlier outburst against nature as he admits 'I 'gin to be aweary of the sun / and wish the estate o' the world were now undone' (5.5.49-50).
So Shakespeare foreshadows the destruction of Macbeth through these apparitions - straight, hard truths and natural forces that Macbeth cannot see because of his 'bent' and ambitious purposes.