When infant motor skills are compared around the world the earliest independent walkers are seen in?

FULL STORY

On average, children take the first steps on their own at the age of 12 months. Many parents perceive this event as a decisive turning point. However, the timing is really of no consequence. Children who start walking early turn out later to be neither more intelligent nor more well-coordinated. This is the conclusion reached by a study supported by the Swiss National Science Foundation (SNSF).

Because parents pay great attention to their offspring, they often compare them with the other children in the sandpit or playground. Many of them worry that their child is lagging behind in terms of mental development if it sits up or starts to walk a bit later than other children. Now, however, in a statistical analysis of the developmental data of 222 children born healthy, researchers headed by Oskar Jenni of the Zurich Children's Hospital and Valentin Rousson of Lausanne University have come to the conclusion that most of these fears are groundless.

Considerable variance

Within the framework of the Zurich longitudinal study, the paediatricians conducted a detailed study of the development of 119 boys and 103 girls. The researchers examined the children seven times during the first two years of their life and subsequently carried out motor and intelligence tests with them every two to three years after they reached school age. The results show that children sit up for the first time at an age of between slightly less than four months and thirteen months (average 6.5 months). They begin to walk at an age of between 8.5 months and 20 months (average 12 months). In other words, there is considerable variance.

The researchers found no correlation between the age at which the children reached these motor milestones and their performance in the intelligence and motor tests between the age of seven and eighteen. In short, by the time they reach school age, children who start walking later than others are just as well-coordinated and intelligent as those who were up on their feet early.

More relaxed

Although the first steps that a child takes on its own represent a decisive turning point for most parents, the precise timing of this event is manifestly of no consequence. "That's why I advise parents to be more relaxed if their child only starts walking at 16 or 18 months," says Jenni. If a child still can't walk unaided after 20 months, then further medical investigations are indicated.

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Journal Reference:

Oskar G Jenni, Aziz Chaouch, Jon Caflisch, Valentin Rousson. Infant motor milestones: poor predictive value for outcome of healthy children. Acta Paediatrica, 2013; 102 (4): e181 DOI: 10.1111/apa.12129

American Psychiatric Association. (2013). Diagnostic and statistical manual of mental disorders (DSM-5) (5th ed.). Washington DC: American Psychiatric Association.
Book Google Scholar
Arabameri, E., & Sotoodeh, M. S. (2015). Early developmental delay in children with autism: A study from a developing country. Infant Behavior and Development, 39, 118–123. https://doi.org/10.1016/j.infbeh.2015.02.017.
Article PubMed Google Scholar
Asperger, H. (1944). Autistischen Psychopathen im Kindesalter. Archiv fur Psychiatrie und Nervenkrankheiten, 117, 76–136.
Article Google Scholar
Berument, S. K., Rutter, M., Lord, C., Pickles, A., & Bailey, A. (1999). Autism screening questionnaire: Diagnostic validity. British Journal of Psychiatry, 175, 444–451.
Article Google Scholar
Bhat, A. N., Galloway, J. C., & Landa, R. J. (2012). Relation between early motor delay and later communication delay in infants at risk for autism. Infant Behavior and Development, 35(4), 838–846. https://doi.org/10.1016/j.infbeh.2012.07.019.
Article PubMed Google Scholar
Bishop, S. L., Farmer, C., Bal, V., Robinson, E. B., Willsey, A. J., Werling, D. M., et al. (2017). Identification of developmental and behavioral markers associated with genetic abnormalities in autism spectrum disorder. American Journal of Psychiatry. https://doi.org/10.1176/appi.ajp.2017.16101115.
Article PubMed Google Scholar
Bishop, D. V., & Norbury, C. F. (2002). Exploring the borderlands of autistic disorder and specific language impairment: A study using standardised diagnostic instruments. Journal of Child Psychology and Psychiatry, 43(7), 917–929.
Article PubMed Google Scholar
Bishop, S. L., Thurm, A., Farmer, C., & Lord, C. (2016). Autism spectrum disorder, intellectual disability, and delayed walking. Pediatrics, 137(3), e20152959. https://doi.org/10.1542/peds.2015-2959.
Article PubMed Google Scholar
Bolton, P. F., Golding, J., Emond, A., & Steer, C. D. (2012). Autism spectrum disorder and autistic traits in the Avon Longitudinal Study of Parents and Children: Precursors and early signs. Journal of the American Academy of Child and Adolescent Psychiatry, 51(3), 249–260.e225. https://doi.org/10.1016/j.jaac.2011.12.009.
Article PubMed Google Scholar
Boyle, C. A., Boulet, S., Schieve, L. A., Cohen, R. A., Blumberg, S. J., Yeargin-Allsopp, M., et al. (2011). Trends in the prevalence of developmental disabilities in US children, 1997–2008. Pediatrics, 127(6), 1034–1042. https://doi.org/10.1542/peds.2010-2989.
Article PubMed Google Scholar
Buja, A., Volfovsky, N., Krieger, A. M., Lord, C., Lash, A. E., Wigler, M., et al. (2018). Damaging de novo mutations diminish motor skills in children on the autism spectrum. Proceedings of the National Academy of Sciences of the United States of America, 115(8), E1859–e1866. https://doi.org/10.1073/pnas.1715427115.
Article PubMed PubMed Central Google Scholar
Cohen, J. (1988). Statistical power analysis for the behavioral sciences (2nd ed.). New York: Routledge.
Google Scholar
Constantino, J. N., & Charman, T. (2016). Diagnosis of autism spectrum disorder: Reconciling the syndrome, its diverse origins, and variation in expression. The Lancet Neurology, 15(3), 279–291. https://doi.org/10.1016/s1474-4422(15)00151-9.
Article PubMed Google Scholar
Constantino, J. N., Davis, S. A., Todd, R. D., Schindler, M. K., Gross, M. M., Brophy, S. L., et al. (2003). Validation of a brief quantitative measure of autistic traits: Comparison of the social responsiveness scale with the autism diagnostic interview-revised. Journal of Autism and Developmental Disorders, 33(4), 427–433.
Article PubMed Google Scholar
Constantino, J., & Gruber, C. (2005). Social responsive scale (SRS) manual. Los Angeles: Western Psychological Services.
Google Scholar
Constantino, J. N., & Todd, R. D. (2003). Autistic traits in the general population: A twin study. Archives of General Psychiatry, 60(5), 524–530. https://doi.org/10.1001/archpsyc.60.5.524.
Article PubMed Google Scholar
Constantino, J. N., & Todd, R. D. (2005). Intergenerational transmission of subthreshold autistic traits in the general population. Biological Psychiatry, 57(6), 655–660. https://doi.org/10.1016/j.biopsych.2004.12.014.
Article PubMed Google Scholar
Corsello, C., Hus, V., Pickles, A., Risi, S., Cook, E. H., Jr., Leventhal, B. L., et al. (2007). Between a ROC and a hard place: Decision making and making decisions about using the SCQ. Journal of Child Psychology and Psychiatry, 48(9), 932–940. https://doi.org/10.1111/j.1469-7610.2007.01762.x.
Article PubMed Google Scholar
Dworzynski, K., Ronald, A., Bolton, P., & Happe, F. (2012). How different are girls and boys above and below the diagnostic threshold for autism spectrum disorders? Journal of the American Academy of Child and Adolescent Psychiatry, 51(8), 788–797. https://doi.org/10.1016/j.jaac.2012.05.018.
Article PubMed Google Scholar
Fournier, K. A., Hass, C. J., Naik, S. K., Lodha, N., & Cauraugh, J. H. (2010). Motor coordination in autism spectrum disorders: A synthesis and meta-analysis. Journal of Autism and Developmental Disorders, 40(10), 1227–1240. https://doi.org/10.1007/s10803-010-0981-3.
Article PubMed Google Scholar
Frazier, T. W., Youngstrom, E. A., Hardan, A. Y., Georgiades, S., Constantino, J. N., & Eng, C. (2015). Quantitative autism symptom patterns recapitulate differential mechanisms of genetic transmission in single and multiple incidence families. Molecular Autism, 6(1), 58. https://doi.org/10.1186/s13229-015-0050-z.
Article PubMed PubMed Central Google Scholar
Gillberg, C. (2010). The ESSENCE in child psychiatry: Early symptomatic syndromes eliciting neurodevelopmental clinical examinations. Research in Developmental Disabilities, 31(6), 1543–1551. https://doi.org/10.1016/j.ridd.2010.06.002.
Article PubMed Google Scholar
Green, D., Charman, T., Pickles, A., Chandler, S., Loucas, T., Simonoff, E., et al. (2009). Impairment in movement skills of children with autistic spectrum disorders. Developmental Medicine and Child Neurology, 51(4), 311–316.
Article PubMed Google Scholar
Halvorsen, M., & Helverschou, S. E. (2017). Måleegenskaper ved den norske versjonen av autism diagnostic interview-revised (ADI-R). PsykTestBarn, 1, 5.
Google Scholar
Harris, S. R. (2017). Early motor delays as diagnostic clues in autism spectrum disorder. European Journal of Pediatrics, 176(9), 1259–1262. https://doi.org/10.1007/s00431-017-2951-7.
Article PubMed Google Scholar
Hatakenaka, Y., Kotani, H., Yasumitsu-Lovell, K., Suzuki, K., Fernell, E., & Gillberg, C. (2016). Infant motor delay and early symptomatic syndromes eliciting neurodevelopmental clinical examinations in Japan. Pediatric Neurology, 54, 55–63. https://doi.org/10.1016/j.pediatrneurol.2015.09.008.
Article PubMed Google Scholar
Havdahl, K. A., Hus Bal, V., Huerta, M., Pickles, A., Oyen, A. S., Stoltenberg, C., et al. (2016). Multidimensional influences on autism symptom measures: Implications for use in etiological research. Journal of the American Academy of Child and Adolescent Psychiatry, 55(12), 1054–1063.e1053. https://doi.org/10.1016/j.jaac.2016.09.490.

Article PubMed PubMed Central Google Scholar
Hilton, C., Wente, L., LaVesser, P., Ito, M., Reed, C., & Herzberg, G. (2007). Relationship between motor skill impairment and severity in children with Asperger syndrome. Research in Autism Spectrum Disorders, 1(4), 339–349.
Article Google Scholar
Hilton, C. L., Zhang, Y., Whilte, M. R., Klohr, C. L., & Constantino, J. (2012). Motor impairment in sibling pairs concordant and discordant for autism spectrum disorders. Autism, 16(4), 430–441. https://doi.org/10.1177/1362361311423018.
Article PubMed Google Scholar
Hus, V., & Lord, C. (2013). Effects of child characteristics on the autism diagnostic interview-revised: Implications for use of scores as a measure of ASD severity. Journal of Autism and Developmental Disorders, 43(2), 371–381. https://doi.org/10.1007/s10803-012-1576-y.
Article PubMed PubMed Central Google Scholar
Hus, V., Taylor, A., & Lord, C. (2011). Telescoping of caregiver report on the autism diagnostic interview-revised. Journal of Child Psychology and Psychiatry, 52(7), 753–760. https://doi.org/10.1111/j.1469-7610.2011.02398.x.
Article PubMed Google Scholar
Jenni, O. G., Chaouch, A., Caflisch, J., & Rousson, V. (2013). Infant motor milestones: Poor predictive value for outcome of healthy children. Acta Paediatrica, 102(4), e181–184. https://doi.org/10.1111/apa.12129.
Article PubMed Google Scholar
Kanner, L. (1943). Autistic disturbances of affective contact. Nervous Child, 2, 217–250.
Google Scholar
Karasik, L. B., Tamis-Lemonda, C. S., & Adolph, K. E. (2014). Crawling and walking infants elicit different verbal responses from mothers. Developmental Science, 17(3), 388–395. https://doi.org/10.1111/desc.12129.
Article PubMed Google Scholar
Kim, Y. S., Leventhal, B. L., Koh, Y. J., Fombonne, E., Laska, E., Lim, E. C., et al. (2011). Prevalence of autism spectrum disorders in a total population sample. American Journal of Psychiatry, 168(9), 904–912. https://doi.org/10.1176/appi.ajp.2011.10101532.
Article PubMed Google Scholar
Kirkovski, M., Enticott, P. G., & Fitzgerald, P. B. (2013). A review of the role of female gender in autism spectrum disorders. Journal of Autism and Developmental Disorders, 43(11), 2584–2603. https://doi.org/10.1007/s10803-013-1811-1.
Article PubMed Google Scholar
Lai, M. C., Lombardo, M. V., Auyeung, B., Chakrabarti, B., & Baron-Cohen, S. (2015). Sex/gender differences and autism: Setting the scene for future research. Journal of the American Academy of Child and Adolescent Psychiatry, 54(1), 11–24. https://doi.org/10.1016/j.jaac.2014.10.003.
Article PubMed PubMed Central Google Scholar
Landa, R., & Garrett-Mayer, E. (2006). Development in infants with autism spectrum disorders: A prospective study. Journal of Child Psychology and Psychiatry, 47(6), 629–638. https://doi.org/10.1111/j.1469-7610.2006.01531.x.
Article PubMed Google Scholar
Lane, A., Harpster, K., & Heathcock, J. (2012). Motor characteristics of young children referred for possible autism spectrum disorder. Pediatric Physical Therapy, 24(1), 21–29. https://doi.org/10.1097/PEP.0b013e31823e071a.
Article PubMed PubMed Central Google Scholar
Lemcke, S., Juul, S., Parner, E. T., Lauritsen, M. B., & Thorsen, P. (2013). Early signs of autism in toddlers: A follow-up study in the Danish National Birth Cohort. Journal of Autism and Developmental Disorders, 43(10), 2366–2375. https://doi.org/10.1007/s10803-013-1785-z.
Article PubMed Google Scholar
Leonard, H. C., Bedford, R., Charman, T., Elsabbagh, M., Johnson, M. H., & Hill, E. L. (2014). Motor development in children at risk of autism: A follow-up study of infant siblings. Autism, 18(3), 281–291. https://doi.org/10.1177/1362361312470037.
Article PubMed Google Scholar
Levy, S. E., Giarelli, E., Lee, L. C., Schieve, L. A., Kirby, R. S., Cunniff, C., et al. (2010). Autism spectrum disorder and co-occurring developmental, psychiatric, and medical conditions among children in multiple populations of the United States. Journal of Developmental and Behavioral Pediatrics, 31(4), 267–275. https://doi.org/10.1097/DBP.0b013e3181d5d03b.
Article PubMed Google Scholar
Lichtenstein, P., Carlstrom, E., Rastam, M., Gillberg, C., & Anckarsater, H. (2010). The genetics of autism spectrum disorders and related neuropsychiatric disorders in childhood. American Journal of Psychiatry, 167(11), 1357–1363. https://doi.org/10.1176/appi.ajp.2010.10020223.
Article PubMed Google Scholar
Lord, C., Elsabbagh, M., Baird, G., & Veenstra-Vanderweele, J. (2018). Autism spectrum disorder. Lancet, 392(10146), 508–520. https://doi.org/10.1016/s0140-6736(18)31129-2.
Article PubMed PubMed Central Google Scholar
Lord, C., Rutter, M., DiLavore, P. S., & Risi, S. (1999). Autism diagnostic observation schedule (ADOS). Los Angeles: Western Psychological Services.
Google Scholar
Lord, C., Rutter, M., & Le Couteur, A. (1994). Autism diagnostic interview-revised: A revised version of a diagnostic interview for caregivers of individuals with possible pervasive developmental disorders. Journal of Autism and Developmental Disorders, 24(5), 659–685.
Article PubMed Google Scholar
Lundstrom, S., Chang, Z., Kerekes, N., Gumpert, C. H., Rastam, M., Gillberg, C., et al. (2011). Autistic-like traits and their association with mental health problems in two nationwide twin cohorts of children and adults. Psychological Medicine, 41(11), 2423–2433. https://doi.org/10.1017/s0033291711000377.
Article PubMed Google Scholar
MacDonald, M., Lord, C., & Ulrich, D. A. (2013). The relationship of motor skills and social communicative skills in school-aged children with autism spectrum disorder. Adapted Physical Activity Quarterly, 30(3), 271–282.
Article PubMed Google Scholar
MacDonald, M., Lord, C., & Ulrich, D. A. (2014). Motor skills and calibrated autism severity in young children with autism spectrum disorder. Adapted Physical Activity Quarterly, 31(2), 95–105. https://doi.org/10.1123/apaq.2013-0068.
Article PubMed Google Scholar
Mandy, W., Pellicano, L., St Pourcain, B., Skuse, D., & Heron, J. (2018). The development of autistic social traits across childhood and adolescence in males and females. J Child Psychol Psychiatry. https://doi.org/10.1111/jcpp.12913.
Article PubMed Google Scholar
Matson, J. L., Mahan, S., Kozlowski, A. M., & Shoemaker, M. (2010). Developmental milestones in toddlers with autistic disorder, pervasive developmental disorder-not otherwise specified and atypical development. Developmental Neurorehabilitation, 13(4), 239–247. https://doi.org/10.3109/17518423.2010.481299.
Article PubMed Google Scholar
Moreno-De-Luca, A., Myers, S. M., Challman, T. D., Moreno-De-Luca, D., Evans, D. W., & Ledbetter, D. H. (2013). Developmental brain dysfunction: Revival and expansion of old concepts based on new genetic evidence. The Lancet Neurology, 12(4), 406–414. https://doi.org/10.1016/s1474-4422(13)70011-5.
Article PubMed PubMed Central Google Scholar
Oien, R. A., Schjolberg, S., Volkmar, F. R., Shic, F., Cicchetti, D. V., Nordahl-Hansen, A., et al. (2018). Clinical features of children with autism who passed 18-month screening. Pediatrics. https://doi.org/10.1542/peds.2017-3596.
Article PubMed Google Scholar
Onis, M. (2006a). Assessment of sex differences and heterogeneity in motor milestone attainment among populations in the WHO Multicentre Growth Reference Study. Acta Paediatrica, 450, 66–75.
Google Scholar
Onis, M. (2006b). WHO motor development study: Windows of achievement for six gross motor development milestones. Acta Paediatrica, 450, 86–95.
Google Scholar
Ozonoff, S., Young, G. S., Goldring, S., Greiss-Hess, L., Herrera, A. M., Steele, J., et al. (2008). Gross motor development, movement abnormalities, and early identification of autism. Journal of Autism and Developmental Disorders, 38(4), 644–656. https://doi.org/10.1007/s10803-007-0430-0.
Article PubMed Google Scholar
Posserud, M. B., Lundervold, A. J., & Gillberg, C. (2006). Autistic features in a total population of 7-9-year-old children assessed by the ASSQ (Autism Spectrum Screening Questionnaire). Journal of Child Psychology and Psychiatry, 47(2), 167–175. https://doi.org/10.1111/j.1469-7610.2005.01462.x.
Article PubMed Google Scholar
Rodwell, L., Lee, K. J., Romaniuk, H., & Carlin, J. B. (2014). Comparison of methods for imputing limited-range variables: A simulation study. BMC Medical Research Methodology, 14(1), 57.
Article PubMed PubMed Central Google Scholar
Rutter, M., Bailey, A., & Lord, C. (2003a). The social communication questionnaire. Los Angeles: Western Psychological Services.
Google Scholar
Rutter, M., Le Couteur, A., & Lord, C. (2003b). Autism diagnostic interview-revised. Los Angeles: Western Psychological Services.
Google Scholar
Ryland, H. K., Hysing, M., Posserud, M. B., Gillberg, C., & Lundervold, A. J. (2012). Autism spectrum symptoms in children with neurological disorders. Child and Adolescent Psychiatry and Mental Health, 6(1), 34. https://doi.org/10.1186/1753-2000-6-34.
Article PubMed PubMed Central Google Scholar
Skuse, D. H., Mandy, W., Steer, C., Miller, L. L., Goodman, R., Lawrence, K., et al. (2009). Social communication competence and functional adaptation in a general population of children: Preliminary evidence for sex-by-verbal IQ differential risk. Journal of the American Academy of Child and Adolescent Psychiatry, 48(2), 128–137. https://doi.org/10.1097/CHI.0b013e31819176b8.

Article PubMed Google Scholar
Staples, K. L., MacDonald, M., & Zimmer, C. (2012). Assessment of motor behavior among children and adolescents with autism spectrum disorder. International Review of Research in Developmental Disabilities, 42, 179–214.
Article Google Scholar
Storvold, G. V., Aarethun, K., & Bratberg, G. H. (2013). Age for onset of walking and prewalking strategies. Early Human Development, 89(9), 655–659. https://doi.org/10.1016/j.earlhumdev.2013.04.010.
Article PubMed Google Scholar
Suren, P., Bakken, I. J., Aase, H., Chin, R., Gunnes, N., Lie, K. K., et al. (2012). Autism spectrum disorder, ADHD, epilepsy, and cerebral palsy in Norwegian children. Pediatrics, 130(1), e152–158. https://doi.org/10.1542/peds.2011-3217.
Article PubMed PubMed Central Google Scholar
Van Buuren, S. (2018). Flexible imputation of missing data. New York: Chapman and Hall.
Book Google Scholar
Walle, E. A., & Campos, J. J. (2014). Infant language development is related to the acquisition of walking. Developmental Psychology, 50(2), 336–348. https://doi.org/10.1037/a0033238.
Article PubMed Google Scholar
Wechsler, D. (1999). Wechsler abbreviated scale of intelligence. San Antonio: The Psychological Corporation.
Google Scholar
Wechsler, D. (2003). Wechsler intelligence scale for children (WISC-IV) (4th ed.). San Antonio: The Psychological Corporation.
Google Scholar
Wechsler, D. (2008). Wechsler adult intelligence scale (WAIS-IV) (4th ed.). San Antonio: The Psychological Corporation.
Google Scholar
Wechsler, D. (2012). Wechsler preschool and primary scale of intelligence (4th ed.). San Antonio: The Psychological Corporation.
Google Scholar
West, K. L. (2018). Infant motor development in autism spectrum disorder: A synthesis and meta-analysis. Child Development. https://doi.org/10.1111/cdev.13086.
Article PubMed Google Scholar
West, K. L., Leezenbaum, N. B., Northrup, J. B., & Iverson, J. M. (2017). The relation between walking and language in infant siblings of children with autism spectrum disorder. Child Development. https://doi.org/10.1111/cdev.12980.
Article PubMed PubMed Central Google Scholar
World Health Organization. (1992). The ICD-10 classification of mental and behavioural disorders: Clinical descriptions and diagnostic guidelines. Geneva: World Health Organization.
Google Scholar
Zwaigenbaum, L., Bauman, M. L., Stone, W. L., Yirmiya, N., Estes, A., Hansen, R. L., et al. (2015). Early identification of autism spectrum disorder: Recommendations for practice and research. Pediatrics, 136(Suppl 1), S10–40. https://doi.org/10.1542/peds.2014-3667C.
Article PubMed Google Scholar

Page 2

From: Age of First Walking and Associations with Symptom Severity in Children with Suspected or Diagnosed Autism Spectrum Disorder

	ASD (n = 376)	Non-ASD (n = 114)
n	(%)	Mean (SD)	n	(%)	Mean (SD)
Sex
Male	292	(77.7)		85	(74.6)
Early motor development
AOW (months)	376		14.7 (4.3)	114		13.8 (2.9)
“Late walking” (≥ 16 months)	117	(31.1)		28	(24.6)
Diagnoses
ASD (F84)	376	(100.0)		0	0
Former ASD	0	0		6	(5.5)
Intellectual Disability (F70–79)	49	(15.1)		9	(9.8)
ADHD (F90)	114	(36.2)		52	(55.9)
Communication disorder (F80)	12	(3.8)		23	(23.0)
Specific learning disorder (F81 + F83)	18	(5.7)		22	(23.4)
Motor disorders (F82 + F95)	35	(11.1)		27	(29.0)
Epilepsy	25	(6.6)		8	(7.0)
Cerebral Palsy	1	(.3)		1	(.9)
Other NDD (F88 + F89 + F94)	4	(1.3)		8	(8.4)
No of NDDs
0	0	0		8	(7.6)
1	179	(48.2)		53	(50.5)
2–3	182	(49.1)		40	(38.1)
≥ 4	10	(2.7)		4	(3.8)
Verbal language	305	(92.7)		100	(100.0)
Age (years) at inclusion	376		11.4 (3.8)	114		10.2 (3.6)
Age (years) at ASD diagnosis	326		9.3 (4.2)
Nonverbal IQ	254		102.3 (17.7)	85		100.9 (17.5)
Verbal IQ	258		89.1 (17.8)	86		92.9 (18.0)
Paternal age (years)	213		32.5 (6.3)	69		32.9 (6.5)
Maternal age (years)	231		30.3 (5.0)	73		30.8 (5.5)
Prematurity	50	(14.8)		15	(15.5)
Ethnicity
European (Caucasian)	281	(81.2)		98	(89.1)

AOW age for onset of independent walking, ASD autism spectrum disorder, ADHD attention-deficit/hyperactivity disorder, NDD neurodevelopmental disorder. Data are expressed as n (%) or mean (SD). The denominator for the reported proportions in this table excludes those with missing data: 151 participants for nonverbal IQ; 146 for verbal IQ; 61 for language level; 56 for prematurity; 34 for ethnicity; 208 for paternal age; 186 for maternal age; 14 for number of NDDs, and 52 ASD cases for age at diagnosis. IQ was obtained from various age-appropriate standardized tests