For example, kids with Asperger syndrome, sometimes called high-functioning autism, fall under autism spectrum disorder, even though Asperger syndrome tends to be more subtle. A child with Asperger's isn't necessarily antisocial, but he may have difficulty having conversations or reading social cues. However, his language is normal and he may even be precocious, says Dr. Powers. Asperger's is typically diagnosed around age 6 or older. Before that, a child doesn't usually develop the intense interest in a certain subject, like trains or math — that's the hallmark of Asperger syndrome.

Other children might be diagnosed as PDD-NOS, which stands for pervasive developmental disorder, not otherwise specified. This residual category holds a large group of kids who have maybe two out of the three classic social, language, and behavioral characteristics of autism, says Fred Volkmar, MD. There are many more of these children than those who are diagnosed as classically autistic, but they're all lumped together.

What Are the Signs of Autism?
Although autism is usually diagnosed in toddlerhood — when a lag in language or an utter disinterest in others becomes strikingly apparent — about 75 percent of parents of children with autism report feeling worried during the child's first year of life, says Fred Volkmar, MD, professor of psychology, pediatrics, and psychiatry at Yale.

One of the earliest and clearest signs of autism is the lack of joint attention — the mutual understanding that occurs when a baby communicates by catching your eye, then gazing or pointing at what she wants to show you. These gestures usually form the foundation of language, emerging by about 10 to 12 months of age, says psychologist Geraldine Dawson, PhD, director of the University of Washington Autism Center in Seattle. Another early red flag: when a baby doesn't react to hearing his name. By seven months of age, a baby should turn to look in your direction when you call him, says Dr. Dawson.