Blood tests and brain scans are inaccurate autism diagnostic tools. Instead, doctors use interviews and observations. Does the person exhibit speech delays, contact avoidance, repetitive movements, or social impairment? The more signs the doctor can pinpoint, the more autism is likely. Since an autism diagnosis relies on observations, doctors must wait until babies age. Very small children can't describe how they experience the world, and their brain cells are still growing and changing. Only by age 2 or 3 does the full picture of autism appear.