What is the next step for an 18-week pregnant female with a single umbilical artery (SUA) and no detected congenital anomalies?

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Management of Isolated Single Umbilical Artery at 18 Weeks

For a pregnant patient at 18 weeks with an isolated single umbilical artery and no other congenital anomalies detected, the answer is C: Routine Antenatal Care with enhanced surveillance—specifically, a comprehensive cardiac assessment should be performed immediately (if not already adequately visualized on the anomaly scan), followed by third-trimester growth ultrasound and consideration of weekly antenatal surveillance beginning at 36 weeks. 1, 2

Immediate Cardiac Assessment

  • A comprehensive assessment of cardiac anatomy should be performed immediately, as cardiovascular abnormalities are the most commonly associated structural defects with SUA (occurring in 23.8% of non-isolated cases). 1, 2, 3
  • If the required cardiac views were adequately visualized and normal on the anomaly scan, fetal echocardiography is not routinely warranted. 1, 2
  • The cardiovascular system shows particularly strong associations with SUA, with odds ratios ranging from 5.9 to 20.3 for various cardiac defects. 3, 4

Genetic Counseling is NOT Indicated

  • No additional evaluation for aneuploidy is recommended for isolated SUA, regardless of whether previous aneuploidy screening was negative, low-risk, or declined. 1, 2
  • Isolated SUA (without other structural abnormalities) shows no increased risk of aneuploidy. 1, 2, 3
  • This is a critical distinction: when SUA occurs with other structural abnormalities, aneuploidy risk ranges from 4-50%, but isolated SUA carries no increased chromosomal risk. 1, 3

Enhanced Surveillance Protocol (Not Just "Routine" Care)

Third-Trimester Growth Monitoring

  • A third-trimester ultrasound examination to evaluate fetal growth is recommended, as isolated SUA has been associated with increased risk of fetal growth restriction in some studies. 1, 2
  • Growth assessments should be performed at intervals of 3-4 weeks for more reliable measurements given inherent biometric error. 5

Antenatal Fetal Surveillance

  • Consider weekly antenatal fetal surveillance beginning at 36 0/7 weeks of gestation for fetuses with isolated SUA. 1, 2
  • This recommendation is based on population-based data showing SUA is associated with increased odds of stillbirth (OR 4.80; 95% CI 2.67-8.62), as well as increased risks of oligohydramnios, placental abruption, cord prolapse, and perinatal mortality. 5, 3, 4

Why Other Options Are Incorrect

Option A (Genetic Counseling) - Incorrect

  • Genetic counseling and amniocentesis are not indicated for isolated SUA, as there is no increased aneuploidy risk. 1, 2
  • The Society for Maternal-Fetal Medicine explicitly states: "We do not recommend diagnostic testing for aneuploidy solely for the evaluation of an isolated soft marker following a negative serum or cfDNA screening result." 1

Option B (ECHO) - Partially Correct but Not the Primary Answer

  • While cardiac assessment is important, a comprehensive anatomic survey with adequate cardiac views is sufficient. 1, 2
  • Fetal echocardiography is only indicated if the standard cardiac views were inadequate or abnormal findings were detected. 1

Option D (Termination) - Absolutely Incorrect

  • Termination is never indicated for isolated SUA without other structural or chromosomal abnormalities. 6
  • Studies show that when no other malformations are present, only routine physical examination is needed at birth, with excellent prognosis. 6

Postnatal Considerations

  • Pediatric providers should be notified of the prenatal finding of isolated SUA at delivery, as postnatal examination has revealed structural anomalies in up to 7% of infants with prenatal diagnosis of isolated SUA. 2, 5
  • However, if the prenatal evaluation was thorough and no abnormalities were detected, the prognosis is excellent. 6

Common Pitfalls to Avoid

  • Do not order amniocentesis based solely on isolated SUA—this represents unnecessary invasive testing with procedural risk when there is no increased aneuploidy risk. 1
  • Do not provide false reassurance by calling this "routine" care without explaining the need for enhanced third-trimester surveillance and potential weekly testing at 36 weeks. 1, 2
  • Do not forget to verify adequate cardiac imaging, as this is the most critical associated abnormality to exclude. 1, 2

References

1
Guideline

Guideline Directed Topic Overview

Dr.Oracle Medical Advisory Board & Editors, 2025

2
Guideline

Management of Isolated Single Umbilical Artery

Praxis Medical Insights: Practical Summaries of Clinical Guidelines, 2025

3
Guideline

Single Umbilical Artery: Associated Findings and Management

Praxis Medical Insights: Practical Summaries of Clinical Guidelines, 2025

4
Research

Single umbilical artery and risk of congenital malformation: population-based study in Norway.

Ultrasound in obstetrics & gynecology : the official journal of the International Society of Ultrasound in Obstetrics and Gynecology, 2020

5
Guideline

Management of Two-Vessel Umbilical Cord in Pregnancy

Praxis Medical Insights: Practical Summaries of Clinical Guidelines, 2025

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Professional Medical Disclaimer

This information is intended for healthcare professionals. Any medical decision-making should rely on clinical judgment and independently verified information. The content provided herein does not replace professional discretion and should be considered supplementary to established clinical guidelines. Healthcare providers should verify all information against primary literature and current practice standards before application in patient care. Dr.Oracle assumes no liability for clinical decisions based on this content.

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