I enjoyed Children of Men very much, so I read the book as well. In it, the infertility of the world is likewise unexplained - because no one in the story knows why it has happened. It's a fundamental part of the disorientation and hopelessness that possesses the created universe that babies have stopped coming and no one knows why, despite mandatory fertility testing for everyone. Both book and film are great examples of science fiction, though of the HG Wells, social thought experiment type, not the Asimov, hard tech type.