Ethan was the last child born and conceived on the island. That was in 1977. The Incident with Jughead happened in 1977. The reason for the fertility problem was because once Jacob had the last of the candidates he needed to finish his game, he no longer wanted any additions to the island population. He couldn't stop conception off the island (Rousseau and Claire), but he could make it so that no more conceptions on the island could end in live births. He had found his candidates and was done bringing anymore people to the island. Whatever the Losties did at the Swan during the Jughead incident in 1977 also caused Jacob to know that he no longer needed candidates.