If mistress got pregnant by married man? She is 20yrs old and single!What criminal charges or cases can she file?

None, as Adultery is NOT a criminal, or even legal, matter, although the mistress maybe able to file rape charges, but this is also unlikely.

She can't file any charges if she was a willing participant. The only thing she can file for are custody and child support.

We did not ask her where she was, perhaps if she is in some nations, but if she is in the US or Canada she can file for child support when child is born, she can ask for day care costs and medical insurance for the chld.

Adultery is still against the law in some US states and some countries. This is why its important to include a general locale when posting questions on law, as laws vary by area. But, in places where the laws still exist, they are rarely prosecuted.

The likely result of trying to pursue criminal charges is the prosecutor looking at it as buyer's remorse. Getting criminal charges prosecuted is not likely at all. And Adultery is the only possible charge based on the information provided.

Who are you in this scenario? The screen name is Selflesswife. So are you the wife he cheated on or the mistress, or who?

Adultery is still against the law in some US states and some countries.
Especially in ND and LA;)