Can I ask why is everyone being such nobs about this?
Necro person, whoever you are, go away. Post one more insult on this thread and I'll have the administrator remove your comments and your account. It's one thing voicing an opinion and another calling someone a bint.
Velvetee, I sometimes think men are programmed to philander. Especially when wives/partners are pregnant. The whole dynamic of your relationship changes when a baby is on the way.
The good news is, he hasn't slept with her, and it sounds like he could have if he'd really wanted to! The bad news is that he obviously gets a buzz out of the attention she's paying him.
Can I recommend a trick I found in a book about the psychology of lying. Basically, the trick is to behave as though you already know about his indescretions, and play the empathy card, i.e. "Look, I know I'm pregnant and haven't given you much attention lately, blah blah, and if I was in your shoes I'd be tempted to do the same thing, etc etc. But we can get over this if we can just talk honestly about what you did ...".
I know it's a bit of a sneaky trick, but often if the partner thinks he/she is not going to get caught up in a massive row about it, and if you leave enough appropriate silences in the conversation, you are likely to get a good idea of the real truth.
I have tried this technique on a partner and it really works. Even if he starts tumbling out confessions, don't go mad, just keep listening till you have all the info you need.
At least when you know what's really going on you can make an INFORMED decision. If you rush out into the night now, you might regret it later. Lots of relationships recover from affairs, and pregnancy is a flash point for a lot of men. It doesn't mean he even necessarily likes this woman, so try and keep that in your thoughts.
Hope you get on OK! Good luck,, xx