Normal Umbilical Artery Doppler Values at 28 Weeks Gestation
At 28 weeks gestation, normal umbilical artery Doppler values are: Pulsatility Index (PI) approximately 1.1, Resistance Index (RI) approximately 0.6, and Systolic/Diastolic (S/D) ratio approximately 3.1.
Reference Values from High-Quality Studies
The most robust reference ranges come from the INTERGROWTH-21st Project, which used rigorous international methodology across multiple populations 1. According to their longitudinal data, umbilical artery indices decrease progressively with advancing gestational age, with the mean PI at 28 weeks calculated using their equation: 1.02944 + 77.7456*(gestational age)^-2 - 0.000004455*gestational age^3.
Additional well-established reference ranges from multiple studies show consistent patterns:
- Pulsatility Index (PI): Ranges from approximately 1.0-1.1 at 28 weeks 2, 3, 4
- Resistance Index (RI): Approximately 0.6 at 28 weeks 2
- S/D Ratio: Approximately 3.1 (with 95% confidence interval of 4.6-1.6) at 28 weeks 5
Clinical Context and Interpretation
Normal Physiologic Pattern
All three Doppler indices demonstrate a continuous decline throughout pregnancy without plateau or increase near term 6. This reflects progressive decrease in placental vascular resistance as pregnancy advances. The umbilical artery shows increasing end-diastolic flow velocity as gestation progresses, indicating improving placental perfusion 2.
When to Consider Values Abnormal
According to SMFM guidelines, abnormal umbilical artery Doppler is defined as flow ratios (S/D, PI, or RI) greater than the 95th percentile 7. At 28 weeks, this would translate to:
- PI > approximately 1.3-1.4
- RI > approximately 0.7
- S/D ratio > approximately 4.6
Critical Thresholds for Management
The guidelines emphasize specific Doppler patterns that trigger management changes 7:
- Decreased end-diastolic velocity (elevated indices >95th percentile but flow still present): Weekly Doppler surveillance recommended; delivery at 37 weeks if fetal growth restriction present
- Absent end-diastolic velocity (AEDV): Doppler 2-3 times per week; delivery at 33-34 weeks if FGR present
- Reversed end-diastolic velocity (REDV): Hospitalization, daily monitoring, delivery at 30-32 weeks if FGR present
Important Clinical Caveats
Measurement technique matters significantly. Doppler should be obtained from a free-floating loop of umbilical cord to avoid compression artifacts 2, 1. The intraobserver variability for PI is approximately 10.5%, for RI is 6.8%, and for S/D ratio is 13.0% 6, so borderline values should be interpreted cautiously and repeated.
Population-specific variations exist. While the INTERGROWTH-21st standards are designed to be international 1, some studies show ethnic and geographic variations 8. However, the overall pattern of declining indices with gestational age remains consistent across populations.
Serial measurements are more valuable than single values 6. Longitudinal assessment allows detection of deterioration patterns, which is particularly important in fetal growth restriction surveillance 7, 9.