my husband cheated on me with his baby mama

Anna
I had been visiting my mother for the past 5 days and the whole time I was gone I had this really weird feeling about him and his baby mama. He told me he was taking the kids to see her at work, which shook me even more. I called him twice that day and no answer. He calls me later that night and says his phone was on silent and that he was working in the yard....right. 
The next day when I get home,( I can just feel that he has cheated on me before any actual evidence) The sheets and all the blankets are torn off the bed. Red flag #1 I was so enraged I couldn't keep It inside anymore. I asked him and he wouldn't deny it or admit it. But i finally got the truth out of him.
I feel so awful. No words can describe. He had unprotected sex with her, she told him she was on her period. But he didn't see any blood. What do I do if she gets pregnant? I think she did this just to be malicious towards me because he left her for me. We've been trying to have a baby and now this. He said he didn't want any of this and it was a mistake. He wants to go to counseling  but I don't know what to do!Do I walk away? How can I stay if there's a chance she could be pregnant?
Any