I think that the motivations behind Proctor persisting in his defense when he hears about Elizabeth's pregnancy is to reflect the beginning of his pivot towards "goodness" that will be a part of the trajectory of his characterization. When Proctor hears that Elizabeth is pregnant and the eventual questioning of her pregnancy, Proctor snaps into what will amount to be a persistent defense: " If she say she is pregnant, then she must be! That woman will never lie, Mr. Danforth.” While there has been some emotional struggle in the relationship between Elizabeth and John, John recognizes her immediate sense of honor, something that he sees as being under siege in the condition in which Salem is operating. For John, his persistence in the defense of what he sees as right is almost validated in hearing that Elizabeth is pregnant. It almost gives him a greater purpose in fighting for what he knows as right. When he sees her being questioned, it galvanizes him into action. His persistence is a marked difference than the emotional ambivalence that had been seen in the first two acts. It is in this idea, understanding that Elizabeth is pregnant, in jail, and being doubted for her own certainty that he persists in what he does. Proctor persists because he recognizes that he is right in what he is doing and is more committed than ever in pursuing what he sees as truth in a Salem condition in which so much is false and inauthentic.