S/D ratio
Pronunciation: es-dee RAY-shee-oh
Systolic to diastolic ratio, Doppler measurement reflecting vascular resistance in fetal and placental circulation.
Full Definition
The systolic to diastolic (S/D) ratio is a Doppler ultrasound parameter that measures the relationship between peak systolic and end-diastolic velocities in blood vessels, most commonly applied to umbilical artery assessment. The S/D ratio reflects downstream vascular resistance and placental function. Normal values decrease with advancing gestational age as placental vascular development progresses. Elevated S/D ratios indicate increased placental resistance and may suggest placental insufficiency, while absent or reversed end-diastolic flow represents severe compromise. The S/D ratio is used alongside other Doppler indices such as pulsatility index (PI) and resistance index (RI) to comprehensively assess fetal-placental circulation and guide management decisions in high-risk pregnancies.
Usage
Usage note: Use forward slash, not hyphen; specify vessel being measured.
In Context
- "Umbilical artery S/D ratio was elevated at 4.2, indicating increased placental resistance." — Doppler study report
- "The S/D ratio showed progressive increase over serial examinations, concerning for worsening placental function." — Longitudinal monitoring