When Macduff reveals the details of his birth, and Macbeth realizes he is doomed, is that the climax of "Macbeth"?
Yes, it is! For the specators as well as Macbeth realize absolutely what they have presumed beforehand: from the moment of Duncan's murder, there has been 'no way out' for Macbeth, there is no other recourse of action since his fate has irrevocably been set.
Implicit is that despite destiny Macbeth is still responsible for his choices and his downfall. Had he resisted his wife's urgings, had he obeyed his deeper sense of moral duty, had he not carried out the murder of Duncan, his doom would have been waived....
The climax in "Macbeth" is a bit incongruent, however, with its usual context. Normally, the climax of a story - its heightened point of interest and suspense - corresponds in time to the crisis point: that is to say the moment of decision in which the choice a character makes determines the outcome of the story. An irrevocable act is committed which cannot be "undone." In "Macbeth," this is obviously the murder of Duncan.
However, after the actual killing of the king, there is still much "unfinished business" in "Macbeth." The spectator still wants to see how the personal choices of Macbeth link up with the greater forces of the occult, and this is what maintains interest until his final conflict with Macduffand his approaching army. When Mabeth hears of the circumstances of Macduff's birth (a Caesarian section - 'not of woman born'), he knows that all prophesy has been fulfilled and that he is already "history."