Fetal alcohol spectrum disorder (FASD) can affect a child's central nervous system in many ways. Signs of central nervous system effects include:

• A small head size at birth. This is defined as having a head circumference at or below the 10th percentile. This means that the measurement around the largest part of the head, starting and ending at the forehead, is less than that of 90% of other newborns of the same gender.
• Structural brain abnormalities, such as small brain size. These abnormalities can sometimes be detected with imaging tests, such as an MRI.
• Signs of nervous system abnormalities, such as mental retardation, problems with fine motor skills, or poor eye-hand coordination.

At least one of these signs must be present for alcohol to be considered as a possible cause.