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To identify possible early signs of autism spectrum disorder (ASD) within the Danish National Birth Cohort, we studied prospectively collected interviews from 76,441 mothers about their children's development and behaviour at 6 and 18 months. In Danish national registries, 720 children with ASD and 231 children with intellectual disability (ID) were identified. At 6 months, associations between early signs and ASD or ID were found only in few areas. At 18 months social, language, and motor skills were delayed, and suspicion of vision and hearing problems were increased for both groups. Signs distinguishing ASD from ID were unclear, and the positive predictive values regarding ASD were below 10 % for individual predictors and aggregated risk scores.

Prevalence statistics for autism spectrum disorders (ASD) vary widely across geographical boundaries. Some variation can be explained by diagnostic methods, case ascertainment and age at diagnosis. This study compared prevalence statistics for two distinct geographical regions, Denmark and Western Australia, both of which have had population-based registers and consistent classification systems operating over the past decade. Overall ASD prevalence rates were higher in Denmark (68.5 per 10,000 children) compared with Western Australia (51.0 per 10,000 children), while the diagnosis of childhood autism was more prevalent in Western Australia (39.3 per 10,000 children) compared with Denmark (21.8 per 10,000 children). These differences are probably caused by local phenomena affecting case ascertainment but influence from biological or geographical factors may exist.

Exposure to prenatal infection has been suggested to cause deficiencies in fetal neurodevelopment. In this study we included all children born in Denmark from 1980, through 2005. Diagnoses of autism spectrum disorders (ASDs) and maternal infection were obtained through nationwide registers. Data was analyzed using Cox proportional hazards regression. No association was found between any maternal infection and diagnosis of ASDs in the child when looking at the total period of pregnancy: adjusted hazard ratio = 1.14 (CI: 0.96-1.34). However, admission to hospital due to maternal viral infection in the first trimester and maternal bacterial infection in the second trimester were found to be associated with diagnosis of ASDs in the offspring, adjusted hazard ratio = 2.98 (CI: 1.29-7.15) and adjusted hazard ratio = 1.42 (CI: 1.08-1.87), respectively. Our results support prior hypotheses concerning early prenatal viral infection increasing the risk of ASDs. (Contains 4 tables.)

The purpose of this study was to assess the validity of the diagnosis of childhood autism in the Danish Psychiatric Central Register (DPCR) by reviewing medical records from 499 of 504 total children with childhood autism born 1990-1999. Based on review of abstracted behaviors recorded in case records from child psychiatric hospitals, case status determination was performed using a standardized coding scheme. In 499 children diagnosed with childhood autism in the DPCR, the diagnosis could be confirmed in 469 children (94%). Of the 30 non-confirmed cases, five were classified by the reviewers as non-autistic cases and the remaining 25 cases were either classified with another ASD diagnosis or the specific diagnosis was not possible to determine.

In a previous study, we found that infants transferred to a neonatal ward after delivery had an almost twofold increased risk of being diagnosed with infantile autism later in childhood in spite of extensive controlling of obstetric risk factors. We therefore decided to investigate other reasons for transfer to a neonatal ward, in particular hyperbilirubinaemia and neurological abnormalities. We conducted a population-based matched case–control study of 473 children with autism and 473 matched controls born from 1990 to 1999 in Denmark. Cases were children reported with a diagnosis of infantile autism in the Danish Psychiatric Central Register. Conditional logistic regression was used to calculate odds ratios (OR) and 95% confidence intervals [CI] and likelihood ratio tests were used to test for effect modification. We found an almost fourfold risk for infantile autism in infants who had hyperbilirubinaemia after birth (OR 3.7 [95% CI 1.3, 10.5]). In stratified analysis, the association appeared limited to term infants (≥37 weeks gestation). A strong association was also observed between abnormal neurological signs after birth and infantile autism, especially hypertonicity (OR 6.7 [95% CI 1.5, 29.7]). No associations were found between infantile autism and low Apgar scores, acidosis or hypoglycaemia. Our findings suggest that hyperbilirubinaemia and neurological abnormalities in the neonatal period are important factors to consider when studying causes of infantile autism. [ABSTRACT FROM AUTHOR]

Research suggests that heredity and early fetal development play a causal role in autism. This case-control study explored the association between perinatal factors, parental psychiatric history, socioeconomic status, and risk of autism. The study was nested within a cohort of all children born in Denmark after 1972 and at risk of being diagnosed with autism until December 1999. Prospectively recorded data were obtained from nationwide registries in Denmark. Cases totaled 698 children with a diagnosis of autism; each case was individually matched by gender, birth year, and age to 25 controls. Analyses by conditional logistic regression produced risk ratios and 95% confidence intervals. Adjusted analyses showed that the risk of autism was associated with breech presentation (risk ratio (RR) = 1.63, 95% confidence interval (CI): 1.18, 2.26), low Apgar score at 5 minutes (RR = 1.89, 95% CI: 1.10, 3.27), gestational age at birth <35 weeks (RR = 2.45, 95% CI: 1.55, 3.86), and parental psychiatric history (schizophrenia-like psychosis: RR = 3.44, 95% CI: 1.48, 7.95; affective disorder: RR = 2.91, 95% CI: 1.65, 5.14). Analyses showed no statistically significant association between risk of autism and weight for gestational age, parity, number of antenatal visits, parental age, or socioeconomic status. Results suggest that prenatal environmental factors and parental psychopathology are associated with the risk of autism. These factors seem to act independently. [ABSTRACT FROM AUTHOR]