Autism cannot be definitively diagnosed in very young babies through a single test, but signs can appear as early as 6 months old, and screening tools become reliable starting around 16 months. The process involves watching for specific behavioral patterns, completing standardized screenings at pediatric checkups, and, when concerns arise, pursuing a formal evaluation by a specialist. Here’s what that process looks like at each stage.
Signs to Watch for Between 6 and 12 Months
Most parents first notice something feels different during everyday interactions, like feeding, playing, or trying to get their baby’s attention. Research from the Kennedy Krieger Institute identifies several specific behaviors between 6 and 12 months that have been linked to a later autism diagnosis:
- Limited eye contact and rarely smiling when a caregiver approaches
- Not responding to their name with increasing consistency as they approach 12 months
- Infrequent babbling or not trying to imitate sounds, smiles, or laughter during back-and-forth play
- Rarely seeking your attention or not reaching up when you move to pick them up
- Not using gestures to communicate by 10 months
- Unusual body movements, such as repeatedly stiffening the arms, hands, or legs, rotating hands at the wrists, or holding uncommon postures
- Motor delays, including delayed rolling over, pushing up, or crawling
No single sign on this list means a baby is autistic. These are patterns, and they matter most when several appear together or persist over time. A baby who occasionally ignores their name is behaving normally. A baby who consistently doesn’t respond to their name, rarely makes eye contact, and isn’t babbling by 10 months presents a picture worth investigating.
A study tracking infants from 9 to 12 months found that babies later diagnosed with autism already showed fewer social and early speech skills than their peers at 9 months. By 12 months, the gap had widened across nearly every measure of prelinguistic communication. Three patterns stood out: communicating with eye gaze, facial expressions, and sounds was consistently low from 9 to 12 months; creative or symbolic use of objects (like pretending with toys) fell behind by 12 months; and the use of gestures and overall frequency of communication showed a growing gap compared to typically developing infants.
Sensory Differences in Infancy
Sensory behaviors are sometimes the earliest and most noticeable clues, though they can be easy to dismiss as quirks. Autistic infants may show three broad patterns of sensory difference: overreacting to sensory input, underreacting to it, or actively seeking it out.
Overreactions might look like a baby who becomes extremely distressed during bath time (withdrawing from water on their skin), covers their ears when someone sings, or gets upset when their head is tilted back. Underreactions can be harder to spot. A baby who doesn’t turn toward a loud sound, seems unbothered by a bump or fall, or appears unusually passive may be underresponsive to sensory information. Sensory seeking shows up as prolonged staring at spinning objects, repetitive touching of textures, or an unusual fascination with lights or visual patterns.
These sensory differences aren’t exclusive to autism. But combined with the social and communication signs above, they form a fuller picture that helps clinicians assess what’s going on.
Formal Screening at 18 and 24 Months
The American Academy of Pediatrics recommends that all children be screened specifically for autism at 18 and 24 months, in addition to regular developmental surveillance at every well-child visit. This is universal screening, meaning it applies to every child, not just those with obvious concerns.
The most widely used tool at these ages is the Modified Checklist for Autism in Toddlers, or M-CHAT. It’s a parent questionnaire with 23 yes-or-no questions that takes about 10 minutes to complete. You’ll answer questions about your child’s behavior: whether they respond to their name, point to things, make eye contact, and so on. A total score of 3 or higher, or a score of 2 or higher on six specific critical items, flags the need for a follow-up interview to determine whether a full evaluation is warranted.
The M-CHAT is designed for children between 16 and 30 months. It’s a screening tool, not a diagnostic one. A high score doesn’t mean your child is autistic, and a low score doesn’t guarantee they aren’t. What it does is sort children into groups that need closer evaluation and those who don’t, with reasonable accuracy.
What a Diagnostic Evaluation Looks Like
If screening raises concerns, the next step is a comprehensive evaluation by a specialist, typically a developmental pediatrician, child psychologist, or child neurologist. This evaluation pulls together information from multiple sources: direct observation of your child, parent interviews about developmental history, and standardized assessment tools.
One of the gold-standard tools is the Autism Diagnostic Observation Schedule (ADOS-2), which includes a Toddler Module designed for children under 30 months. During this assessment, a clinician engages your child in a series of play-based activities and social interactions, carefully scoring how your child responds to social cues, uses eye contact, communicates, and plays with toys. The session typically feels more like structured playtime than a medical exam.
For very young children, this assessment has limitations. In babies with intellectual disabilities or significant language delays, the tool can sometimes over-identify autism when something else is actually going on. That’s one reason clinicians combine the ADOS with their own clinical judgment, parent reports, and developmental testing rather than relying on a single score.
The formal diagnostic criteria require two core features: difficulties with social communication (like limited eye contact, trouble with back-and-forth interaction, or reduced sharing of emotions) and restricted or repetitive behaviors (like unusual sensory interests, repetitive movements, or rigid routines). Both must be present, and they must be significant enough to affect daily functioning.
Regression: When Skills Disappear
About one-third of young children with autism experience a loss of skills they previously had, a pattern called regression. This typically happens around 20 months of age, though it can occur anywhere from the second year of life through the preschool years.
Language loss is the most commonly reported form, and it tends to happen in children who had only a very limited vocabulary to begin with. One study found that 94% of children with autism who lost speech had only single-word language at the time of regression. But regression isn’t limited to words. Parents also report loss of eye contact, loss of interest in social interaction, and loss of imitative games like peekaboo. A smaller number of children lose motor skills or basic self-care abilities like self-feeding.
If your child has been saying a few words and using them meaningfully, then stops, or if they were socially engaged and gradually become withdrawn, that’s a significant change worth raising with your pediatrician promptly. Regression doesn’t always indicate autism, but it always warrants evaluation.
Getting Help Before a Diagnosis
One of the most important things to know is that you do not need a formal autism diagnosis to start getting your child help. Under federal law (IDEA Part C), children under 3 are eligible for early intervention services if they have a developmental delay in areas like communication, social or emotional development, physical development, or adaptive skills. Each state sets its own criteria for how much delay qualifies, using different cutoff scores on standardized assessments.
Your child can also qualify if they have a diagnosed condition with a high probability of causing developmental delay, but that’s a separate pathway. The developmental delay pathway means that if your baby is behind in communication or social skills, they can receive services like speech therapy or developmental support regardless of whether autism has been formally identified. Some states even allow services to begin on an interim plan before eligibility is fully determined, with a complete assessment required within 45 days.
To start this process, you can contact your state’s early intervention program directly. You don’t need a referral from your pediatrician, though most pediatricians will help initiate one. Evaluations through early intervention are provided at no cost to families. The gap between first concerns and a formal autism diagnosis can stretch months or longer, so accessing early intervention during that waiting period gives your child support during the developmental window when it matters most.