I was 7 months pregnant with my second child when I found out that my WS had a drunken ONS at a works Christmas party many moons ago. I also had a 2 1/2 year old. He gave me Chlymidia which was passed onto my unborn daughter.
When he found out that he had an infection and that he had to tell me, obviously he was contrite and promised on bended knees that he would NEVER EVER EVER betray me again. I believed him. I was alone. No family in this country and financially dependent upon him. I had no one to talk to about it (and the only 'friend' that I did tell, blanked me forever after...)
Within six months of my daughter's birth he embarked on a ten year affair with another (divorced) woman. During that time he had another two affairs. To date he has had eleven affairs, one of which lasted for twelve years subsequent to the others and has produced (so far as I know) at least one other child.
Had I had known what was to be the future, I would have left him there. Whatever the outcome of being alone...
I found out about all the others last year - I had numerous 'DDays.'
My advice: Someone who cheats on a pregnant wife is someone who has no boundaries and is severely impaired with regards to real intimacy with their wife. He has some kind of hole to fill that you won't be able to. I'm sorry to have to speak like this.
IF he goes into intensive IC and you both have MC and everything is out and above board and you feel that you can trust this man again, then give it a chance but remember that you are dealing with what could be a pattern of behaviour for the rest of your married life.
Please look after yourself and your other child. I know just how hard this is.
Hugs to you.