Husband could have possible affair child

My husband and I were split for a little while to work on ourselves and we are now in a place where we are ready to give our marriage another shot and work things out. However he had unprotected sex with a woman while we were split and she is now 12 days late on her period and has take 3 test which all were negative and is going to get a blood test on thursday. I need advice. My husband and I have no children together yet and i really want children. It would crush my world if she is pregnant. I don't know if I should stay with him through it or if I should get a divorce and let them figure things out. Help!!