Getting married after committing zina

I had a question, I commited zina with a woman who is my wife. She got pregnant at the time however we did not repent at that time. The imam that married us was aware that she was pregnant but he did not make us do tobah. We got married and then we had repented for what we had done. We both were not aware at the time that you must repent or the marriage may be invalid. It has been four years since and I wish to know is our marriage valid or invalid.