Why do you think that he's cheating? Has he changed behavior? Done things others have mentioned below? Implied he's seeing someone?
Reason I ask is that after our son was born, my hormones were so out of whack that when our son was somewhere between 16 and 20 weeks old, I was CONVINCED my husband was cheating on me with a friend of ours.
I confronted him about it and he said he never has, never wanted to and never would. He answered all of my questions but I still thought maybe he did something. I had no proof other than a gut feeling.
Well, it was purely hormonal and months later I finally saw that. There was nothing about his behavior or habits that had changed, it really was just all in my head. I've apologized to him and we've moved past it. He was never defensive - rather he was very concerned why I felt that way and if there was something HE wasn't providing to ME that I ended up going down that path.
He was sad that I would think that about him and our marriage. In the end, it's made me trust him even more, it's strengthened our marriage and I've never had a thought like that since.
In reality, he doesn't have the time to have an affair -- we work in the same building, we commute in to work together and on occasion, my son and I will go watch him play baseball or volleyball on the nights he plays. There simply isn't time in his day or life for another woman because if he isn't at work, or at his organized sports, he's home with us.
Bottom line, hormones are a funky thing that can really play with your mind and emotions - even though you're that much further from pregnancy.
Just a thought.