The Second Riddle:

              In the second riddle the witches explain to Macbeth that no child born of a woman can harm him. Macbeth interprets this to mean that no one can harm or kill him; because, as far as he knows, all humans are born from a woman (as opposed to a man or creature). This makes him think that he is invincible. What the riddle actually meant was that no child that is born "naturally" can harm him. The catch in the riddle was that Macduff was not born naturally, but was born by cea-section. Macduff was able to beat Macbeth for that reason. This quote is really confusing, you get so many different ideas of what it could really mean. The fact that Macduff was born cea-section was a surprising twist that made the riddle make complete sense. If Macbeth didn't take the riddle so literally, I wonder if he would have understood from the beginning.