There's no way this can happen.

Given the circumstances, you're surprised that you're not panicking. Perhaps it's the fact you're the older one out of the two of you, perhaps it's you're in shock, but whatever it is, you absolutely cannot have a child with this girl. It will absolutely ruin your life, as well as probably ruin hers. You've never wanted a child - otherwise, you'd already have one - and she's only 18, not to mention the absolute shitshow that will occur once your wife, her father, and the entire town find out. However you realize how sensitive the situation is, and take care not to give away your true feelings, otherwise things could go south very quickly.

"Well, we should have a think about this," you say, trying not to sound too committed one way or another, "And then discuss our options."

"You're right," she says as if reading your mind. "I'm way too young to be a mother."

"OK," you say, relieved she feels the same way you do. "Let's look at what's available in the city and go from there."