Normal Umbilical Artery Doppler Values at 28 Weeks Gestation
At 28 weeks gestation, the normal umbilical artery pulsatility index (PI) is approximately 1.1 (ranging from 0.8-1.4 for the 5th-95th percentiles), and the resistance index (RI) is approximately 0.6 (ranging from 0.5-0.7 for the 5th-95th percentiles). 1
Expected Normal Values
Based on longitudinal research data from low-risk pregnancies:
Pulsatility Index (PI) at 28 weeks:
- Median (50th percentile): ~1.1
- 5th percentile: ~0.8
- 95th percentile: ~1.4
- These values show a continuous decline throughout pregnancy 1, 2
Resistance Index (RI) at 28 weeks:
- Median (50th percentile): ~0.6
- 5th percentile: ~0.5
- 95th percentile: ~0.7
- The RI demonstrates a steady decrease from early pregnancy through term 1
Clinical Interpretation Framework
Normal findings: PI and RI values below the 95th percentile indicate adequate placental perfusion and low risk for fetal growth restriction 3.
Abnormal findings requiring action:
- PI or RI >95th percentile = Increased placental resistance, warrants serial monitoring 3
- Absent end-diastolic velocity (AEDV) = Severe compromise, requires 2-3x weekly assessment 3
- Reversed end-diastolic velocity (REDV) = Critical compromise, requires hospitalization and delivery consideration at 30-32 weeks 3
Important Clinical Caveats
The measurement location matters minimally but should be consistent—waveforms obtained near the placental insertion show slightly higher end-diastolic flow than those near the fetal insertion, though this variation rarely affects clinical decisions 3.
Once fetal growth restriction is diagnosed, serial umbilical artery Doppler assessment must be performed to detect deterioration 3. Initial surveillance should occur every 1-2 weeks, with frequency adjusted based on severity.
Values continue to decline physiologically throughout pregnancy without plateau near term when measured longitudinally, contrary to some older cross-sectional studies 2. This continuous decline reflects progressive decrease in placental vascular resistance as the placental circulation matures.