why does proctor persist even he hears that elizabeth is pregnant

Persist in what exactly?

persist when he comes to know that elizabeth is pregnant

I'm still not completely sure what you are asking. In any case, Proctor persists in trying to tell the truth because he realizes that his honour is all that he has left. It took a beating when he and Abigail enjoyed some extra activities. Now John wants to come clean. He also wants to prove to Elizabeth that he can still be an honourable man.

means why does proctor accept even he hears that elizabeth is pregnant