Most parents notice that something is not right with their children when the children are two or three years old. In some cases, parents pick up signs even earlier, when their children are in infancy. They may notice that their babies don't look at them or seem to recognize familiar faces. Perhaps their babies don't cry when they leave the room, exhibit anxiety around strangers, make babbling sounds, imitate gestures such as clapping and pointing, or enjoy playing games like peekaboo--all signs of a typically developing infant.

There's no single personality type that represents the model of an ASD baby. Some parents of children with ASDs look back and describe their children as having been angels when they were babies, hardly making a peep and demanding very little attention. Others describe their children as screamers. Still others describe their babies' behavior as typical -- nothing out of the ordinary. According to the National Institute of Mental Health (NIMH), some possible early indicators of ASDs include the following:

Parents may also notice that their child doesn't meet the physical, mental, language, and social developmental standards that most typical children reach. Their one-year-olds may not imitate their actions when they clap or wave, or respond to their smiles, as most one-year-olds do. Their two-year-olds may not be able to understand simple two-step instructions ("Go get your cup, and put it on the table.") or do such things as point to basic body parts (nose, ears, or eyes), identify objects, ask simple questions (or even speak at all), engage in common physical activities (jumping, running, or climbing), or draw circles and lines on paper -- as most typical two-years-olds do. Typical three- and four-year-olds drive their parents crazy with constant "Why?" and "What?" questions, eagerly answer simple "Where?" and "Who" questions, enjoy picture books and being read to, and like to play with other children, whereas most three- and four-year-olds with ASDs do not. As toddlers, children with ASDs may not show their curiosity by leaning out of their strollers to look at things that interest them or pointing things out to their parents.