Did you ever have a crush before you met your wife? Because from the way you write about this woman, you spend a few hours with her every week in a group setting and only a few minutes alone - in other words you don't really know her, it sounds to me exactly like a crush. Crushes can be powerful things, but they are all within us, our dreams and desires, rather than any basis in reality.

How you feel about this woman is right now all inside your head, and unless you make a move on her it will stay that way. Perhaps she is representing something, a need or desire, that you have within you but do not recognise. It may be to do with your relationship with your wife and how that is changing (it does change, that is natural, and there are big changes underway with parenthood). Most likely it has very little to do with her and a lot to do with yourself.

Personally if you value your marriage I would take steps not to acquaint further with this other woman, I would look at your relationship with your wife and focus on that - what do you feel is lacking and what will it take to bring that relationship back to the place where it was.

If you do not value your marriage (& it's not my place to tell you that you should) then I would urge you not to make a move on this woman until you have ended your marriage. Otherwise there will be a lot of heartbreak.

Remember, strong though these feelings are they will eventually burn out, crushes do eventually. Even if you did get together with this woman that intensity would eventually burn out. It might help to acknowledge these feelings, not panic, but file them under 'crush' and go and do something else. If you can't focus on your wife then how about you make something special for your little girl? There is nothing like the birth of your first child and I urge you to enjoy this time of anticipation - it flies by so quickly.