I think we’re like pots and lids–there are many of each, and many of them fit together fine. You happened to marry a good lid for you–but you’ve discovered another lid that seems like it would fit your pot just as well. It’s NOT a sign that a relationship with your coworker was meant to be. It just means you happen to work with a lid that fits your pot.
Someone else used the term recommit and I think that’s important. Do it. Don’t let yourself speculate on a life or future with your coworker. Don’t let your thoughts linger on her great attributes. Don’t start making comparisons between her and your wife. Nip it in the bud. If you have to distance yourself from her a bit emotionally, do it. I realize that might feel like you’re losing a friend, and feels doubly unfair to her. But this is your marriage we’re talking about here, something infinitely more important.
A baby is a huge change and there may be stressful times ahead. You need all your emotional energy focused on your family, not on the fantastic person in the next office who “might have been.” You may want to throw in the towel a million times in the first eight weeks of new parenthood alone, and the last thing you need is an extra temptation to follow through with that impulse.
One of the greatest gifts you can give that new offspring is a stable homelife and a living example of a happy, trusting, committed marriage. Too few kids get that, and you’re in a position to give it if you don’t blow it.
Good luck.