What determines if a fetus is "measuring small?" how is this figured out?

Your Obstetrician: May suspect the fetus is measuring small for your gestational age on the basis of physical examination. An ultrasound may be ordered to assess size and growth. Fetal biometry, which includes measurements of the fetal head, abdomen and femur, is used to measure the fetus, and these numbers are compared to the expected size based on established gestational age.