The Emergence of Autism Symptoms Prior to 18 Months of Age: A Systematic Literature Review

Amy Tanner & Katerina Dounavi

Although various Autism Spectrum Disorder (ASD) signs can be detected between 6 and 18 months of age, the majority of children who later on receive an ASD diagnosis do so at the age of 4 years old or older (Centre for Disease Control, 2019). The identification of very early symptoms allows for earlier intervention which is a key priority for maximizing a child’s potential and altering their developmental trajectory. Beginning behavioral intervention prior to 2 years of age has shown to be linked to achieving optimal outcomes, therefore research attempting to identify reliable early signs sparks a lot of interest among researchers and clinicians (Landa, 2018; McDonald et al., 2014).

The current systematic literature review is aimed at identifying early symptoms of ASD that emerge before 18 months of age as observed in a high-risk sample of children who later on received an ASD diagnosis (HR-ASD). Included studies used a prospective experimental design. An important contribution of the current systematic literature review is its focus on identifying where and when HR-ASD children differ from all other comparison groups (high risk no ASD or HR-no ASD, low risk ASD or LR-ASD and low risk no ASD or LR-no ASD), thereby providing a summary of early signs that are exclusive to a group that will eventually be diagnosed with ASD independently of whether there was or not a genetic risk.

The early signs of ASD have been identified through three types of studies: (a) retrospective studies, often taken from parents’ recall of their child’s behaviors during infancy; – these studies are prone to many memory errors and biases; (b) videotape reviews, consisting in home video reviews of infant and toddler behaviors from children who later on were diagnosed with ASD; – these studies have been an invaluable step in establishing early signs research (Osterling & Dawson, 1994), however they are prone to selection bias; and (c) prospective studies, often following a high-risk cohort from birth and documenting the emergence of symptoms as they unfold – these studies record data during frequent intervals in real time, allowing for a more accurate understanding of the timing and topographies of the earliest ASD symptoms.


Systematic searches were conducted in four electronic databases and were limited to peer-reviewed journal articles, written in English and published in the previous 6 years. Identical search terms were used across all four databases which included: “early signs” or “early detection” or “early symptoms” and “prospective” and “autism” or “Autism Spectrum Disorder” or “ASD”.

From 146 titles 26 studies were deemed eligible for inclusion in this review and were scored for quality. Studies should meet the following inclusion criteria: (1) participants were 18 months or younger at baseline; (2) a post-screening diagnostic tool, coding system or clinical best estimate was administered to determine the presence or absence of an ASD diagnosis; (3) the symptoms were behavioral in nature and measurable during direct observation (including video recording); (4) symptom(s) emerged before 18 months of age in the ASD group and were differentiated from comparison groups before 18 months; (5) studies were prospective in nature; and (6) no co-occurrence of additional diagnoses was reported.


The 26 included studies have been categorized into three domains on the basis of the symptoms identified in each study:

Social communication (12 studies)

Social communication includes among others shared affect and emotions, initiating and responding to joint attention, eye gaze, and facial expressions directed to others, with these skills emerging before vocal verbal behavior. Infants in the HR-ASD group obtained significantly lower expressive and receptive scores, performed below average in eye tracking task and social referencing, showed limited engagement of attention and orientation to name, greater deficits in initiating joint attention, slower growth in coordinated communication, and produced less frequent and less complex vocalizations, confirming the difficulty of HR-ASD Children in adapting in a social context.

Motor behaviors (7 studies)

Deficits in gross motor skills found to be the earliest indicative symptoms of autism in the HR-ASD group. HR-ASD infants showed motor delays in both fine and gross motor behavior in stationary tasks (e.g., “pull to sit”), object manipulation tasks (e.g., grasping) and in visual-motor integration tasks (e.g., goal directed reaching). The head tilt reflex showed to differentiate more clearly than other skills between developmental delays and ASD. In addition, delays in fine and gross motor skills were predictive of difficulties in expressive and receptive language.

Stereotypic behaviors (repetitive behaviors and restricted interests) constitute the second autism diagnostic criterion listed after social-communication. The HR-ASD group engaged in significantly more frequent motor stereotypy, indicating that motor stereotypy had greater predictive potential of an ASD diagnosis than object stereotypy.

Imitation has been considered a core deficit in children with autism and similar findings were found among children pertaining in the HR-ASD group, who engaged in less imitation overall and followed a delayed rather than an atypical trajectory.

Parental reports (8 studies)

Parents of the HR-ASD group reported a significantly greater number of sensory, motor concerns and play concerns, sleep, communication and social concerns, repetitive behaviors and challenging behavior concerns by 18 months. Notably, sensory concerns were the only domain that consistently differentiated the HR-ASD group. The biggest differentiation between the HR-ASD group and other groups, in parental concerns, were deficits in overall imitation, including vocal, motor, facial and object imitation. HR-ASD children were also reported to be less active, more adaptable and more likely to approach socially unfamiliar targets between 6 and 12 months of age and they exhibited lower surgency (approaching, vocal reactivity, smiling, laughter and high intensity pleasure) and greater negative affect, including sadness, fear, and emotional reactivity. Gastrointestinal symptoms (as a biomarker rather than a behavioral marker) were also reported more frequently in children with ASD.

The reliability of parental reports was examined in general, concluding that parental reports are more accurate when collected prospectively on current performance and using dimensional ratings, rather than collected retrospectively using categorical ratings (Ozonoff et al., 2018).


The early signs research led to the early detection assessments and screening tools in populations that do have a high genetic risk. However, research should extend in low genetic risk populations in order to identify whether ASD early markers manifest equally across HR and LR populations. Potential differences in early markers according to gender is also an area that should be further explored.

Regression of Skills as its Own “Early Sign”

Regressive onset is now considered to be the rule rather than the exception among HR-ASD children (Ozonoff & Iosif, 2019). The loss of acquired skills, such as social communicative skills, was reported in 88% of children within their first year of life and this was identified by using a prospective dimensional approach. Assessing the same first year skills across multiple visits using dimensional ratings may provide significant insight and capture the decreasing trajectory of skills in HR-ASD children; such an approach can be easily adopted by clinicians and even offered through telehealth to rural or low resourced families.

Motor skills

Deficits in fine and gross motor skills continue to be among the earliest markers of ASD. This signposts the importance of establishing strong multidisciplinary teams that will best support pre-diagnostic intervention; such teams should include pediatric occupational and physical therapists that can support the development of pivotal motor skills.

Parental Concerns

Parental reports can significantly contribute in indentifying infants at risk and should continue to be an imperative component in early detection screeners alongside clinical observation.

Excesses and Deficits

The majority of early ASD symptoms emerge as deficits rather than excesses. Early screening tools used with HR children should focus on comparing the frequency of typical behaviors (e.g., initiating joint attention, using gestures, imitating, etc.) to the expected frequency of such behaviors among typically developing children.


The first limitation is that the quality assessment revealed that only 68% of the studies were considered high quality and the rest 32% scored a moderate quality. Although the current review is in distinction from previous literature reviews, that not only does not utilize a replicable review methodology but also does not contain a quality assessment of the studies included (Zwaigenbaum et al., 2015).

The second limitation of the present SLR is that only two of the 19 studies (approximately 10%) used a normative population sample and not a high-risk sample, and this can create limitations with the generalizability of the results. That may be failing to identify ASD in low risk populations (such as the first child population) that do not have an increased genetic risk. Acknowledging that ASD cannot yet be determined by one symptom underscores the importance of using early screening tools which examine cumulative risk.

Lastly the limited search scope may also lead to reveal less relevant studies. Although we trust that the number of potentially relevant but omitted studies is negligible.

Future Directions

One significant finding of this study is the fact that regression seems to be an exclusive early symptom among HR-ASD infants and the future development of regression tools is promising for identifying greater true positive and fewer false positive cases, with an aim being to establish whether regression occurs more frequently among HR-ASD infants as compared to LR-ASD infants. Established early behavioral symptoms of ASD, such as deficits in social smiling, motor control and the use of gestures, should be interpreted as posing a cumulative risk, which means that the more symptoms the greater the risk. Cumulative risk thresholds have been established and tested across multiple early detection tools, therefore clinicians should strive to select and routinely use age-appropriate screening tools, such as the M-CHAT-R/F (Robins et al., 2014) or the AOSI (Bryson et al. 2008).


Bryson, S. E., Zwaigenbaum, L., McDermott, C., Rombough, V., & Brian, J. (2008). The Autism Observation Scale for Infants: scale development and reliability data. Journal of Autism and Developmental Disorders, 38(4), 731–738.

Landa, R. J. (2018). Efficacy of early interventions for infants and young children with, and at risk for Autism Spectrum Disorders. International Review of Psychiatry, 30(1), 25–39.

MacDonald, R., Parry-Cruwys, D., Dupere, S., & Ahearn, W. (2014). Assessing progress and outcome of early intensive behavioral intervention for toddlers with autism. Research in Developmental Disabilities, 35(12), 3632–3644.

Osterling, J. A., & Dawson, G. (1994). Early recognition of children with autism: A study of first birthday home videotapes. Journal of Autism and Developmental Disorders, 24, 247–257.

Ozonoff, S., Gangi, D., Hanzel, E. P., Hill, A., Hill, M. M., Miller, M., et al. (2018). Onset patterns in autism: Variation across informants, methods, and timing. Autism Research, 11(5), 788–797.

Ozonoff, S., & Iosif, A. M. (2019). Changing conceptualizations of regression: What prospective studies reveal about the onset of autism spectrum disorder. Neuroscience and biobehavioral reviews, 100, 296–304.

Robins, D. L., Casagrande, K., Barton, M., Chen, C. A., Dumont-Mathieu, T., & Fein, D. (2014). Validation of the Modified Checklist for Autism in Toddlers, Revised with Follow-up (M-Chat-R/F). Pediatrics, 113, 37–45.

Zwaigenbaum, L., Bauman, M., Stone, W. L., Yirmiya, N., Estes, A., Hansen, R., et al. (2015). Early identification of Autism Spectrum Disorder: Recommendations for practice and research. Pediatrics, 1(36), 10–40.

Summary by Sophia Petrogiannaki and Katerina Dounavi

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