Umbilical artery velocity waveforms: poor association with small-for-gestational-age babies

A total of 205 high-risk pregnancies was studied using continuous wave Doppler ultrasound examination of the umbilical artery to investigate the ability of the test to detect small-for-gestational-age (SGA) infants. The waveforms were analysed by calculating a ratio of the peak systolic to end diastolic frequency (A/B ratio). An A/B ratio greater than 95th centile from our derived normal values was classified as abnormal. Three outcome variables were examined: birthweight for gestational age, the standard deviation birthweight score and the ponderal index. Although of the 56 pregnancies with an abnormal Doppler result 34 (61%) were associated with a SGA infant, only 41% of all the SGA infants had an abnormal Doppler result. Alternative measures of growth, the ponderal index and the SD birthweight score, showed that on average the babies in the Doppler abnormal group were smaller than those in the Doppler normal group, but the overlap between the normal and abnormal groups was large. Therefore although Doppler ultrasound appears to identify groups of smaller babies, it does not identify individual pregnancies where the baby will be small at birth.