Prior studies using home movies have shown that signs of autism can be detected between 8-12 months. A study looking at head circumference found statistical differences prior to 12 months. And one study looking at movements found differences between 4-6 months.etrospective studies, largely involving review of home movies, have suggested that autism can be diagnosed as early as 6-12 months, suggesting that parental report is not an adequate screen because subtle signs are hard to detect without rigorous observation.