Fetal growth restriction (FGR) is defined as failure of the fetus to achieve its genetically determined growth potential due to an underlying pathological process [1]. FGR affects approximately 10% of all pregnancies and is a major determinant of perinatal and childhood mortality and morbidity, as well as chronic disease in adulthood [2–4]. A challenge in studying FGR is the lack of a gold standard definition and clear diagnostic criteria. Small for gestational age (SGA) is often used interchangeably with FGR but fails to differentiate between the constitutionally small but healthy fetus and the pathologically growth-restricted fetus. SGA is typically defined as a baby <10th centile, but 40% of these babies are physiologically small and healthy, therefore fetal size alone cannot be used to differentiate SGA from FGR. Assessment of functional parameters has been proposed to improve diagnostic accuracy but may still miss the larger baby (>10th centile) that is also in fact growth restricted. The importance of accurately diagnosing FGR is that it identifies the potential risk of fetal demise or perinatal complications, which may be averted via appropriate monitoring and optimized delivery.