Editor's note: The following is an excerpt from "The Falling Age of Puberty in U.S. Girls: What We Know, What We Need to Know," written by Sandra Steingraber, Ph.D., and published by the Breast Cancer Fund. In the full report (downloadable here), Steingraber reviews several causes of and contributors to early puberty, including environmental factors. Are chemicals making girls grow up too fast? Photo: iStockphoto Over the course of just a few decades, the childhoods of U.S. girls have been significantly shortened. Girls get their first periods, on average, a few months earlier than did girls 40 years ago. But …