I take it from your post that you are not overly thrilled with the news. I've been there and didn't take it well. My W never forgot that and held it against me for 6 years before becoming a WAW (one of several factors, but this was definitely a watershed moment). At the time it was important that I be able to discuss my feelings with my wife -- wrong unless your feelings are 100% positive, she's going to be way stressed!
Bottom line is this is a super critical juncture as I'm sure you appreciate. You must be 100% positive and supportive. If you have doubt, angst, regrets I would urge finding someone to discuss that with besides your wife and be the best partner you can be for her right now.
Good luck and congratulations!
Married 18, Together 20, Now Divorced M: 48, W: 50, D: 18, S: 16, D: 12 Bomb Dropped (EA, D): 7/13/11 Start Reconcile: 8/15/11 Bomb Dropped (EA, D): 5/1/2014 (Divorced) In a New Relationship: 3/2015