A Small for Gestational Age (SGA) foetus is one that is below the tenth percentile for age of gestation, as measured by weight or abdominal circumference (using ultrasound). A severe SGA foetus is below the 3rd percentile.

Foetal growth restriction implies a pathological reduction in growth. Some, but not all of these babies are small for gestational age, whilst 50–70 % of small for gestational age babies have normal growth, and are just constitutionally small.