Can a finalized divorce be voided if the wife knowingly did not tell the judge about being pregnant?

Most courts will not nullify a divorce decree that has been finalized.  Voidability usually hinges on questions of the court not having proper jurisdiction over one of the parties or some statutory violation such as fraud.  In Browning v. Prostok (Texas 2005), a case involving a challenge to a divorce decree where one party was not a resident of the ruling state, the court ruled that "a judgment is void only when it is apparent that the court rendering judgment 'had no jurisdiction of the parties or property, no jurisdiction of the subject matter, no jurisdiction to enter the particular judgment, or no capacity to act.'" Again, this was a case of lack of jurisdiction.

It is unlikely that in this case that a court will reverse the divorce decree based on your ex-wife's hidden pregnancy; however, you might attempt to make an argument that your ex-wife perpetrated a fraud upon the court (extrinsic fraud), although it is unlikely that this will stand. Usually courts determine that it is not good social policy to keep two people in an untenable marriage because of a pregnancy. However, in this case, the court may re-visit any divorce agreements to determine what modifications might be needed for the child at issue.