Ultrasound
- Conventional and Doppler ultrasonography are used to assess fetal wellbeing through the following parameters: 3,4,5,6
- Fetal anatomy: measurements include biparietal diameter, head circumference, abdominal circumference, femur length, and estimation of fetal weight. Abdominal circumference is the most sensitive measurement for assessing fetal wellbeing.
- Biophysical profile: includes fetal heart rate, breathing, movements, tone, amniotic fluid and duplex Doppler waveform of the umbilical artery, including a calculation of the systolic to diastolic (S/D) ratio.
- Percentile charts are used to assess the estimated fetal weight for age. Normal values are arbitrarily chosen as between the 10th and 90th percentiles.
- During early pregnancy, the placental vascular resistance is normally high, and in normal pregnancies vascular resistance falls so that by 20 weeks of gestation, there is continuous diastolic flow toward the maternal system through the umbilical arteries. 4,5
- Changes in the Doppler characteristics of the umbilical artery may reflect IUGR and placental dysfunction. Changes in the end-diastolic velocity of the umbilical artery range from reduced, to absent or reversed and may indicate progressive abnormal increases in the placental circulatory resistance. The normal S/D ratio is less than 3.5, and varies with gestational age. 4,5
- Doppler studies of the umbilical artery alone are not diagnostic of intrauterine growth restriction as the sensitivity is low. A combination of conventional and Doppler ultrasound has a sensitivity, specificity, positive predictive value and negative predictive value of 31%, 99%, 77%, and 93% respectively. 7
- Increasingly, middle cerebral artery, ductus venosus, and umbilical vein doppler studies are being used to aid assessment. 8
- In diagnosed IUGR, the degree of umbilical artery abnormality directly correlates with adverse fetal outcomes and adverse events during labour and the immediate post-partum period. It can be used to guide management and has been shown to significantly reduce the risk of perinatal mortality by 38%, but without a significant effect on neonatal morbidity. 4,5,7,9
- If IUGR is diagnosed and there are no indications for immediate delivery, fetal monitoring is appropriate. This usually consists of serial cardiotocography and ultrasound (biophysical profile, S/D ratios, and assessment of fetal growth). One randomised control trial has shown that compared to serial cardiotocography, serial Doppler ultrasound of the umbilical arteries significantly reduces the need for emergency cesarean deliveries for fetal distress in-utero. 1,3,10
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