Doubtful, in most states.  Once you have filed for divorce, you are living separate lives.

ah to be clear,  he isnot your ex until the divorce is final and why would you want to sue him for 20 years of aggravation he brought on himself?  The best way to manage divorce is to take it upon yourself as a positive force in this world.  Leave the hurt behind and go find happiness.

Unless you have a pre-marital contract that states that neither party is allowed to get or make someone pregnant I doubt it would be a winnable case. Even if you did, you'd still need to go the whole nine yards in a court of law to successfully sue someone.

Hopefully you haven't threatened him with legal action, as it will most likely result in embarrassment.

But if you feel that there can be some kind of legal leverage when it comes to the property settlement, which would depend on which country and state you live in, then I recommend speaking to a lawyer.

Since you are already in the process of a divorce, you are currenty in the middle of legal proceedings.  Most states have no-fault divorce clauses, which means you can divorce for any reason you feel like it basically.  In states where at-fault divorces are permitted, things like infidelity are causes of action to file for.  If that is your current situation, then there may be remedies instituted in those proceedings for situations like this (helping prove someone is an unfit parent in resolving child custody during a divorce, etc).  But you need to talk to your lawyer handling the case, and if you are unrepresented, look to your local family law court for a self-help center.  Many states now offer amazing family law resources at little to no cost to residents. 

Just to mention, in regards to "suing" him - I am assuming you mean for financial compensation - in order to file a lawsuit, you would need a cause of action - in regards to tort law, there has to be a duty that was breached, which caused you harm (physical harm) - purely emotional harm is not actionable in most, if not all states.  Please note this is not to be construed as legal advice, as you would need to consult a legal expert in the laws of your particular jurisdiction. 

Adultery is not illegal in any of the 50 states of the USA, though it is in some other countries. I believe this could be considered adultery since the divorce isn’t final, but I am not sure. And the months she has been pregnant versus the filing of the divorce would matter.

I can’t think of an actual reason to sue him for. Depending on the timeline, you could sue his girlfriend for something called “Alienation of Affection”. That is a civil law you could sue him for in a few states of the US. This law is used in cases where there is adultery and the adultery causes the divorce. You will need to ask a lawyer in your state.

Remember though; when you sue, the other party can also counter sue.