She finds out that the kid is actually from completely different parents.
Wife: “Honey, I have something very serious to tell you.”
Husband: “What’s up?”
Wife: “According to DNA test results, this is not our kid … “
Husband: Wow, you don’t remember, do you? When we were leaving the hospital that morning, you noticed that our baby had just pooped and you said: “Oh my, please go change the baby before we leave; I’ll wait for you here.”