The number of twins born in the United States has doubled over the last three decades, according to the Centers for Disease Control and Prevention. The huge increase is largely the result of new fertility treatments. "We seem to be making improvements, refinements to fertility-enhancing therapies, so that could then result in a lowering of the increase of the pace in twin and other multiple births,” said the author of the CDC study. More than 137,000 twins were born in the U.S. in 2009, accounting for one in every 30 babies.
