Personally, no matter how much it hurts or kills me, I would still rather know. Because I cannot live through a lie. What is the use of a 'good' and 'happy' life when it is all act?
You say that "I feel I have thrusted myself into marriage because of my wife's pregnancy"...i think you have to really ask yourself the REAL reason why you want to stay married to her. Don't stay on out of obligation. It may seem horrible, but if you stay on only out of obligation or love of your child, everyone will suffer in future, consciously or subconsciously, because there will always be a quiet resentment. If...and ONLY IF you think you really love the other woman, you have to be honest with yourself and everyone else. Not all women may agree on this, but to me if you don't love me for the right reasons, even if you are my husband, you are still not doing justice to me. I would rather you be honest and pursue what you really want.
That being said, if you deeply feel that the love of your life is still your wife...her pregnancy definitely makes it difficult to tell the truth. I can't say for sure which is better, to tell now or later. But if you do not wish to aggravate her, and want to tell her only after she gives birth (although this is tricky also cos many women get post-natal blues), you should stop seeing that woman right now. Doesn't mean jus cos you haven't told your wife you can continue seeing that woman, even if it is just for meals or movies. Plus at that point in time, although your wife will be devastated, you can at least tell her that "I had stopped seeing her COMPLETELY since one month ago". I guess it will also be a test to see if you can really restrain yourself from now till your wife gives birth.
Neither is it a time to feel regretful towards the other woman. You have no obligations towards her. She did not swear under the institution of marriage with you. The person you are responsible towards is the one at home carrying your flesh and blood.
If you really love your wife, cut off all connections with that woman (after your wife knows it will continue to hurt her if you meet that woman even if there is nothing going on), AND SPEND THE REST OF YOUR LIFE proving to your wife that you are a worthy husband. You made a promise to her.