Can you solve this riddle?

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It's fairly easy; when I heard it from my friend earlier I knew the answer immediately, but lets see how you'll do. :) A man and his pregnant wife are in a car, driving in the desert. Suddenly, the car breaks down. So, the husband leaves to find someone to help them. When he returns an hour later, having failed, he sees his wife dead, and a stranger in the car. Who is the stranger?