A pediatrician weighs in.

One in 91 American children has an autism spectrum disorder (ASD), a disturbing new study has found. In 2007, that number was 1 in 150 kids. Why is this number increasing?

Pediatrician Dr. Cara Natterson, author of the upcoming book Dangerous or Safe (which hits bookstores Thursday), says: "This study, though tragic in its results, continues to support the theory that vaccine preservative did not cause the spike in autism seen in the last two decades. There has been no thimerosal in vaccines (except flu vaccines) for more than eight years, and the incidence of autism seems to be steadily climbing. Now it is also possible that the definition of autism is changing, and the number of kids who actually have autism isn't really going up, but it's clearly not going down either. If mercury preservative were to blame, then the cohort of kids born since 2001 should have significantly lower rates of autism, and they don't appear to."

Why do you think cases of autism are increasing? Do you think environmental influences are to blame? Comment below.

Dr. Cara Natterson, a graduate of Harvard University and the Johns Hopkins School of Medicine, and author of "Your Toddler: Head To Toe," is a pediatrician and mother of 2. She is working on her forthcoming book, "Dangerous or Safe?"