Renal agenesis is difficult to diagnose as it depends on the demonstration of empty renal fossae. Both false positive and false negative diagnoses have been reported and this is of particular concern if termination of pregnancy is exclusively dependent on ultrasound diagnosis. An assessment of the severity of oligohydramnios is useful both in terms of underlying aetiology and for prognosis. In the past, oligohydramnios has been assessed subjectively, but increasingly one of two methods for objective assessment are used. The first involves measuring the single deepest vertical pool devoid of cord or limbs and measurements of less than 1 to 3 cm have been variously used to define oligohydramnios. The second, and preferable method, measures the amniotic fluid index (AFI which is the sum of the deepest vertical pool in each of the four quadrants of the uterus. There is a gestation related reference range for AFI which can be used to diagnose and classify oligohydramnios. The use of colour Doppler to identify loops of cord with either of these techniques may improve the accuracy of information obtained