MFM Clinical Presentation

Single Umbilical Artery

Diagnosis • Pathophysiology • Clinical Management

ACOG / SMFM Aligned Evidence-Based Surveillance Protocol

Atlanta Perinatal Associates | DoctorsWhoCode.blog

The Normal Umbilical Cord

Three-Vessel Cord

🔴1 Umbilical Vein — oxygenated blood to fetus
🔵2 Umbilical Arteries — deoxygenated blood to placenta
✅Present in ~94–99.5% of pregnancies

Single Umbilical Artery (SUA)

Two-Vessel Cord

🔴1 Umbilical Vein — present
🔵1 Umbilical Artery — one is absent
📊Prevalence: 0.5% – 6% of pregnancies

Three-Vessel Cord vs. Single Umbilical Artery

Feature	Normal 3-Vessel Cord	SUA (2-Vessel Cord)
Umbilical Arteries	2	1
Umbilical Vein	1	1
Prevalence	~94 – 99.5%	0.5 – 6%
Artery Diameter	Standard	Compensatory dilation
Umbilical Artery PI	Standard reference range	~20% lower than 3VC
Mean Birth Weight	~3,183 g	~3,013 g

Compensatory Hemodynamic Adaptation

Mechanism of Absence

Primary agenesis or secondary atrophy of one umbilical artery

Compensatory Response

Remaining artery undergoes dilation to maintain placental perfusion

Doppler Implication

Pulsatility Index (PI) is ~20% lower than 3-vessel cord controls

Standard reference ranges may overestimate resistance in SUA — use adjusted nomograms when available

Confirming Isolated SUA

A comprehensive evaluation is required before labeling SUA as "isolated"

1 Identify SUA on routine obstetric ultrasound — confirm 2-vessel cord in free loop
2 Level II Anatomy Survey — exclude co-existing structural malformations (mandatory)
3 Fetal Echocardiography — consider selectively; not universally required if no additional markers
4 Chromosomal Microarray Analysis (CMA) — discuss with patient; risk lower in truly isolated cases
5 Confirm isolation — only after complete anatomical and genetic evaluation

SUA: Associated Structural Anomalies

Organ System	Odds Ratio	Risk Level
Cardiovascular	OR 9.98 – 24.02	High
Esophageal Atresia / Stenosis	OR 25.33	High
Genitourinary	OR 2.45 – 15.66	Moderate–High
Trisomy 18 / Trisomy 21	Elevated	Discuss CMA
Isolated SUA (no anomalies)	Significantly lower	Favorable

Odds ratios from pooled cohort studies; risk applies to non-isolated SUA

Cardiovascular anomalies carry the highest association with SUA

Isolated SUA — Prognosis is Generally Favorable

✅

When Truly Isolated

No structural anomalies on Level II survey • No chromosomal markers • No additional ultrasound findings

🫀CHD prevalence is low in the absence of other markers — selective echo approach is appropriate
🧬Chromosomal risk is significantly reduced in confirmed isolated cases
📋Prognosis supports expectant management with enhanced surveillance

Perinatal Outcomes: iSUA vs. Normal Cord

Clinical Outcome	Isolated SUA (iSUA)	Normal 3-Vessel Cord
Mean Gestational Age at Delivery	~38 weeks	~39 weeks
Mean Birth Weight	~3,013 g	~3,183 g
Preterm Delivery Risk	Significantly Increased	Baseline
FGR / SGA Risk	OR ~2.0	Baseline
NICU Admission Rate	Increased	Baseline
5-min Apgar / Cord pH	Reduced	Standard

iSUA fetuses are twice as likely to experience low birth weight and preterm delivery

Fetal Growth Restriction Risk

FGR Odds Ratio

~2.0×

vs. 3-vessel cord baseline

Birth Weight Difference

−170 g

Mean reduction vs. 3VC

SGA Frequency

↑

Below 10th percentile — more frequent in iSUA cohorts

📉Lower 5-minute Apgar scores and reduced umbilical cord blood pH at delivery
⚠️iSUA is an independent risk factor for preterm delivery even without other complications
💧Oligohydramnios occurs more frequently — warrants closer antenatal testing

Recommended Surveillance Protocol

18–22 wks

Level II Anatomy Survey — confirm isolation, exclude anomalies

24–28 wks

Consider fetal echo if additional markers present

28–32 wks

Initiate serial growth ultrasounds every 4 weeks

32–36 wks

Umbilical artery Doppler with adjusted PI reference ranges

36–40 wks

Monitor AFI; assess for preterm labor; individualize delivery timing

📏Serial growth scans every 4 weeks starting at 28–32 weeks
🌊Umbilical artery Doppler — use iSUA-adjusted reference ranges
💧Assess amniotic fluid index (AFI) at each visit

Doppler Interpretation Requires Adjustment

Why PI is Lower in SUA

The single remaining artery dilates compensatorily, increasing diastolic flow and reducing vascular resistance

Clinical Risk

Standard 3VC reference ranges may falsely suggest low resistance, masking true placental insufficiency

Recommendation

Use iSUA-specific nomograms for PI and RI when available; interpret in clinical context

PI values ~20% lower in iSUA vs. 3VC; S/D ratio and RI similarly reduced

Areas of Clinical Debate

Clinical Question	Current Evidence	Recommendation
Universal fetal echocardiography?	CHD prevalence low without additional markers	Selective approach
Right vs. left artery absence?	No significant prognostic difference in isolated cases	No clinical distinction
CMA in isolated SUA?	Risk significantly lower when truly isolated	Discuss; individualize
Optimal delivery timing?	No consensus in literature	Individualize by clinical context
Doppler reference ranges?	Standard ranges may overestimate resistance	Use iSUA-adjusted nomograms

SUA: Clinical Pearls

1 Confirm isolation with Level II anatomy survey before counseling — do not label as isolated prematurely
2 Adjust Doppler reference ranges — compensatory dilation lowers PI by ~20%; standard ranges may be misleading
3 Serial growth monitoring every 4 weeks from 28–32 weeks — iSUA is an independent FGR risk factor
4 Reassure when truly isolated — prognosis is generally favorable; avoid excessive alarm
5 Individualize echo and CMA decisions based on additional ultrasound findings and patient values

ACOG / SMFM Aligned Evidence-Based Management Surveillance Protocol

Atlanta Perinatal Associates | DoctorsWhoCode.blog