In the context of doppler assessments, what does the S/D ratio measure?

The S/D ratio, which stands for systolic to diastolic ratio, is primarily used in Doppler assessments, particularly with umbilical artery waveforms. This ratio measures the relative velocities of blood flow during the systolic and diastolic phases of the cardiac cycle within the umbilical artery. An elevated S/D ratio indicates increased impedance to flow, which may occur due to various factors, such as placental insufficiency or fetal distress. Monitoring this ratio is crucial in assessing the fetal well-being and can help detect potential complications in pregnancies, such as intrauterine growth restriction (IUGR).

Other options do not directly relate to the primary purpose of the S/D ratio. For instance, heart rate variability involves different measurements and is not captured through systolic and diastolic flow ratios. Similarly, while fetal oxygenation is critical and can be indirectly assessed through other means, it is not the direct measurement of the S/D ratio. Lastly, fetal movement frequency refers to assessing fetal activity over time, which is unrelated to the Doppler measurements of blood flow dynamics, making it distinct from what the S/D ratio evaluates.