Ultimately only you can decide. How long have you been together and how did it come up.
Reality is that people do cheat, sometimes they do it again, but it really isn’t true that someone who cheats once will go on to do so again, although I do understand that cheating on someone during pregnancy is on another level.
Interestingly I know two people who this happened to. The one was a guy at work who when I started there I quite fancied, until someone told me that he’d cheated while his wife was pregnant, his wife did in fact give birth a few weeks later so it had been pretty recent.
So I was fairly surprised when he got together with a friend of mine shortly afterwards, especially given her first husband had done exactly the same to her.
However, they went on to get married, and were married for 25 years until he sadly died two years ago. I have no idea what the details of his affair were, but I do know that he didn’t cheat again, and that they had a happy marriage.
The other one was my DP’s BIL, who cheated on his SIL while she was pregnant. There is no question his behaviour was horrible during the time, he went around flaunting the other woman e.g. at the church they were part of, until the pastor announced casually that he, and his wife, (the actual wife not the OW,) needed to be congratulated on their pregnancy. He never came back to the church after that little bit of stirring. 
But she did go on to have the baby, and he married the OW, and they’re still married about 15 years later, have more children, and she and the OW even have a decent relationship.
I would want to know how all this came about, and I would want to give it some thought. But I wouldn’t necessarily bin him because of it.