Babies that are either very small or very big have increased perinatal morbidity and mortality. Predicting which babies will fall into these groups is traditionally done with risk assessment and third trimester manual palpation, however neither of these techniques are sensitive and a considerable number of affected pregnancies are missed. This results in stillbirth for small babies or birth trauma for larger ones. Serial scanning in the third trimester can improve detection rates but this is expensive and cannot currently be provided to all NHS patients.