First of all, tell your wife. I have (recently) been on the wife's side in a similar situation and if the other woman does tell her first I can almost guarantee that there is no way your wife would be willing to work things out so, if you truly want to make your marriage work, your wife needs to come first. Will she stay with you? Who knows BUT she deserves to know. Then you can tackle the pregnancy issue. First and foremost though, you need to take away this woman's power over you.