First of all, it is entirely possible that there is no pregnancy and no baby. How many times did you sleep with this woman? If the answer is "once," then the odds are that she is lying. A woman who would sleep with her friend's husband is fully and totally capable of telling a big, fat lie.

So, I would say this:
1) Say nothing about this to your wife.
2) Stay completely away from this other woman, and just wait to see what she does next.

This woman, basically, just wants to break up your marriage. I don't know why, and that is not very important to know. Do you want to stay married to your wife? If you do, then ignore this woman completely. She is playing a head game with you - one of the oldest head games.

Even if this woman is pregnant, it's entirely possible that you are not the father. Assume nothing.