Ultrasound Biometry and Fetal Growth Restriction

With the ascendancy of biophysical assessment with Doppler velocimetry, and the establishment of routine scan dating in early pregnancy, there needs to be a radical re-think of the role of ultrasound biometry in the de®nition and assessment of fetal growth restriction (FGR). Doppler ̄ow velocimetry of the umbilical artery has proven its value in de®ning the FGR fetus. It is more useful than cardiotocography (CTG) or biophysical pro®le scoring. However, the sensitivity of any test depends on the prevalence of the condition being looked for. While Doppler is of value in fetuses which are small-forgestational age (SGA), it is less useful in predicting growth restriction or adverse outcome in the general population. The question is therefore, how to detect those pregnancies for which further fetal assessment is indicated. In pregnancies which are `high risk', serial ultrasound biometry will allow assessment of growth, and both AC and EFW below the tenth percentile have a useful role. Slowing of growth (dropping below an action line like the tenth percentile) would indicate the need for further investigations. Relative slowing within `normal' limits, e.g. a drop from the 70 to the 20 centile in consecutive measurements, should also be a matter of concern, but the limits of deviation from normal growth have not been de®ned. There is a limit to the usefulness of frequently performed third trimester scans: if the increment of a biometric parameter is approximately 5% per week, and the random error of a scan measurement is 10% (see below), then more than fortnightly scans are not likely to give valuable information about true growth.