From the Guidelines
Absent or reversed diastolic flow in fetal umbilical artery Doppler is indicated by an S/D ratio that is immeasurable or not applicable.
Key Points
- The S/D ratio is used to quantify the Doppler waveform in the umbilical artery, along with the pulsatility index (PI) and resistance index (RI) 1.
- An abnormal umbilical artery Doppler is defined as a PI, RI, or S/D ratio greater than the 95th percentile for gestational age or an absent or reversed end-diastolic velocity (AEDV or REDV) 1.
- When end-diastolic flow is absent, the S/D ratio is immeasurable, and PI may be used instead 1.
- The presence of abnormal umbilical artery Doppler indices, such as an elevated S/D ratio, has been found to predict adverse perinatal outcomes 1.
- Delivery timing is influenced by the severity of fetal growth restriction and the presence of abnormal umbilical artery Doppler indices, with recommendations for delivery at 37 weeks of gestation for pregnancies with FGR and decreased diastolic flow, 33-34 weeks for pregnancies with FGR and AEDV, and 30-32 weeks for pregnancies with FGR and REDV 1.
From the Research
S/D Ratio and Diastolic Flow in Fetal Umbilical Artery Doppler
- The S/D ratio is used to assess the blood flow in the umbilical artery, with higher ratios indicating increased resistance to blood flow 2, 3.
- An absent or reversed diastolic flow in the umbilical artery is associated with serious fetal compromise and increased risk of adverse outcomes, including neonatal death and cerebral palsy 4, 3, 5.
- A S/D ratio above the 95th percentile is associated with increased risk of adverse outcomes, while absent or reversed end-diastolic flow is a more specific indicator of fetal compromise 3.
- The presence of absent or reversed end-diastolic flow in the umbilical artery is an independent predictor of adverse outcomes, including neonatal death and cerebral palsy, in preterm growth-restricted fetuses 3.
- Intermittent absent end-diastolic flow is associated with a similar rate of composite neonatal morbidity as persistently elevated Doppler waveforms, and there is no difference in composite neonatal morbidity between the three groups when corrected for gestational age at delivery and antenatal steroid administration 6.
Clinical Significance of S/D Ratio
- The S/D ratio is a useful tool for assessing fetal well-being and predicting adverse outcomes, but its clinical significance must be interpreted in the context of other maternal and fetal factors 2, 3.
- The presence of absent or reversed end-diastolic flow in the umbilical artery is a critical indicator of fetal compromise and requires immediate attention and potential delivery 4, 5.
- The management of intermittent absent end-diastolic flow is less clear, but it may be associated with a similar risk of adverse outcomes as persistently elevated Doppler waveforms 6.