What is normal umbilical artery flow in a pregnant individual with a history of preeclampsia or placental abruption?

Normal Umbilical Artery Flow

Normal umbilical artery flow demonstrates continuous forward flow throughout the cardiac cycle, with low impedance allowing persistent end-diastolic velocity from as early as 14 weeks of gestation. 1

Physiologic Characteristics of Normal Flow

Normal umbilical artery Doppler reflects optimal fetoplacental perfusion with these key features:

• Continuous forward flow during both systole and diastole is the hallmark of normal umbilical circulation, established by 14 weeks of gestation when low impedance develops in the fetoplacental unit 1
• The presence of robust end-diastolic velocity indicates adequate placental vascular development and unobstructed blood flow through placental tertiary villi 1
• Normal flow can be quantified using systolic-to-diastolic (S/D) ratio or pulsatility index (PI), with values below the 95th percentile for gestational age considered normal 1

Technical Considerations for Assessment

When evaluating umbilical artery flow, proper technique ensures accurate interpretation:

• Doppler waveforms should be obtained at the abdominal cord insertion for optimal reproducibility, though any segment along the umbilical cord is acceptable 1
• Measurements must be taken in the absence of fetal breathing movements and when the waveform appears uniform 1
• Waveforms obtained near the placental end show higher end-diastolic velocity than those at the fetal insertion, though this variation typically does not affect clinical decision-making 1

Abnormal Flow Patterns and Their Significance

Understanding what constitutes abnormal flow helps define normal parameters:

• Decreased end-diastolic flow (PI, RI, or S/D ratio >95th percentile) represents the earliest stage of placental compromise 1
• Absent end-diastolic flow (AEDV) indicates severe placental insufficiency with obliteration of approximately 70% of placental tertiary villi arteries 1
• Reversed end-diastolic flow (REDF) represents the most advanced stage of placental failure, associated with 50% perinatal mortality in historical series and requiring immediate intervention 2, 3, 4

Special Populations: High-Risk Pregnancies

In patients with history of preeclampsia or placental abruption, normal umbilical artery flow patterns remain the same, but surveillance intensity differs:

• Serial umbilical artery Doppler assessment every 1-2 weeks initially is recommended once fetal growth restriction is diagnosed, even with normal flow 1
• If flow remains normal after initial assessment, surveillance intervals can be extended to every 2-4 weeks 1
• The progression from normal to abnormal flow, if it occurs, typically manifests within the first 2 weeks after diagnosis of growth restriction 1, 2

Critical Clinical Pitfalls

• Do not assume normal flow excludes placental insufficiency in late-onset growth restriction - 15-20% of late-onset growth-restricted fetuses with normal umbilical artery Doppler show cerebral vasodilation on middle cerebral artery assessment 1
• Avoid single measurements for clinical decisions - serial assessments are essential as deterioration can occur rapidly, particularly in the first 2 weeks after abnormal findings emerge 1, 2
• Do not delay escalation of surveillance when absent end-diastolic flow develops - progression to reversed flow can occur within days and requires Doppler assessment 2-3 times per week 2, 5