How do I know if my fetus has an abnormally small head based on only one ultrasound?

Based on other scans. All dimensions of the baby are measured. The gestational age of your baby is determined. His/her measements should be consistent with the range of normal for other babies the same gestational age. And all of the measurements should be consistent with eachother. Ex. You should not have a leg the measures 28wk and a head that measures like a 22wk baby.