Wesley Lee, MD*, Mary Rice, RDMS, Janet S. Kirk, MD, Christine H. Comstock, MD, S. Samuel Yang, MD+

Synonyms: Two-vessel cord.

Prevalence: 1% (M1:F1).

Definition: Absence of one umbilical artery.

Etiology: Two-vessel cords are more prevalent with advanced maternal age, high parity, multiple gestations, IUGR, diabetes.

Pathogenesis: Two theories have been proposed: 1) atrophy of one artery, 2) failure of division of the transient primordial umbilical artery, which derives from the single allantoic artery (in the body stalk).

Associated anomalies: see Fig. 1.

Differential diagnosis: None.

Prognosis: Dependent on the associated anomalies if present.

Recurrence risk: Unknown.

Management: Dependent on the associated anomalies when present.

MESH Umbilical-Arteries-abnormalities, -pathology, -physiopathology; Umbilical-Cord-pathology BDE 2500 ICD9 747.5 CDC 747.500

(*) Address correspondence to Wesley Lee, MD, William Beaumont Hospital, Department of Obstetrics and Gynecology, and ⁺Department of Pathology, 3601 West Thirteen Mile Road, Royal Oak, MI 48073-6769 Phone (313) 551-2071, Fax (313) 551-5128

Introduction

Approximately 30,000 babies are born annually with a single umbilical artery (SUA) on the basis of recent birth statistics from the United States¹. Several studies have clearly established an association between SUA with a higher incidence of structural abnormalities, fetal demise, growth retardation, and chromosomal defects (fig. 1)^2-5.

Fig. 1: Organ system anomalies associated with liveborn infants with single umbilical artery. (Adapted from Leung⁵).

The purpose of this study is to describe our experience with the antenatal diagnosis of SUA and to describe a potentially useful method for improving the diagnostic accuracy for this common vascular anomaly.

Material and methods

The study population consisted of patients referred to William Beaumont Hospital during the time period beginning July 1984 through October 1989. Approximately 24,052 obstetrical ultrasound scans were performed during this time period. Acuson 128 (Mountain View, CA), General Electric 3000 (Milwaukee, WI), and Aloka 650 (Corometrics, Wallingford, CT) ultrasound imaging systems were used for this study. Real-time ultrasound imaging was used to make the SUA diagnosis from a cross-sectional view of the umbilical cord. Sector and/or linear transducers were employed. Obstetrical and neonatal follow-up were performed by retrospective chart review or by placental pathology report if sufficient medical record information was not available. A second group of fetuses (1,018 ultrasound exams between February 20^th through June 8^th, 1990) was also examined to determine how likely gray-scale ultrasound could satisfactorily detail umbilical vascular anatomy by 15 to 20 weeks gestation. The maternal abdominal wall thickness (e.g., midline subcutaneous tissue between skin line and abdominal wall musculature) was measured by ultrasound in patients who had poorly visualized umbilical cords. The amniotic fluid amount was also noted. Color Doppler ultrasound were performed with the Acuson system.

Results

Thirty-six fetuses were prospectively identified with single umbilical artery by antenatal ultrasound during the 5.3 year study period. Thirty-two cases (89%) were found to have SUA by postnatal clinical examination or pathology report. Twenty of these SUA fetuses had confirmed two-vessel umbilical cords by pathology report. The other 12 SUA cases were clinically confirmed at delivery according to the medical record alone. SUA fetuses had a broad range of abnormalities involving the CNS (4), pulmonary (3), cardiac (8), gastrointestinal (8), genitourinary (9), and skeletal (7) systems. By contrast, 14 SUA fetuses (44%) had normal postnatal examinations. Four newborns (11%) who were originally thought to have two-vessel cords on ultrasound actually had three-vessel cords by newborn exam or placental pathology. The neonatal examination was normal in 3 of 4 of these fetuses. The fourth fetus with a three-vessel umbilical cord had gastroschisis with mild oligohydramnios noted at 28 weeks gestation by ultrasound. Figure 2 summarizes the ability of sonographers to satisfactorily visualize umbilical cord vessel anatomy between 15 and 20 weeks gestation (1,018 scans) by gray-scale ultrasound.

Fig. 2: Visualization of umbilical cord anatomy by gray-scale ultrasound from 15 to 20 weeks of pregnancy. White bars = visualized cords, black bars = non-visualized cords.

These vessels could be satisfactorily examined in only 66% of 15-week fetuses, whereas they could be counted in approximately 97% of fetuses by 18 weeks gestation. Maternal wall thickness was an important contributing factor for poor cord visualization as follows: 16 weeks (7 of 28 cases), 17 weeks (7 of 13 cases), 18 weeks (3 of 6 cases), and 19 weeks (4 of 6 cases) gestation. The mean maternal wall thickness for fetuses with poorly visualized umbilical cords associated with obesity was 5.4 + 1.1 (1SD) cm (range 4 to 8 cm). Marked oligohydramnios also hindered umbilical cord visualization in one of the two 20-week fetuses. Similar findings have been observed by other authors¹⁰.

Discussion

Cord visualization

This study suggests that real-time ultrasound imaging will provide a correct antenatal SUA diagnosis in about 89% of cases but that some false-positive diagnoses can even be made by experienced examiners. Approximately 66% of fetuses at 16-17 weeks and 97% at 18-19 weeks gestation will have umbilical cords that can be satisfactorily examined with gray-scale ultrasound. The antepartum diagnosis of SUA depends upon several factors including maternal abdominal wall thickness, presence of lower abdominal scar, gestational age, fetal position, amniotic fluid amount, vessel tortuosity, scanning experience, and lateral resolution of the equipment. The diagnosis is also complicated by the fact that varying degrees of umbilical artery fusion may occur near the placenta⁶. Other investigators have also cited the difficulty of antepartum SUA diagnosis by real-time imaging^7,8. For instance, Nyberg and associates⁷ reviewed 107 consecutive fetuses with known CNS malformations and noted that 18% (20 fetuses) had a two-vessel umbilical cord. Only six of the 20 SUA fetuses were prospectively identified by antenatal ultrasound. Six more fetuses were identified retrospectively, and the remaining 8 SUA cases could not be evaluated due to oligohydramnios or other technical reasons. A prospective study of 450 high-risk patients also identified 9 fetuses with SUA⁸. The earliest SUA diagnosis was made at 23 weeks gestation. Two fetuses had suspected SUA, and 3 SUA cases were entirely overlooked by sonography. Seven of these 9 SUA fetuses had structural abnormalities. These two studies emphasized the need for better imaging techniques that could improve the likelihood of satisfactory examining cord vessels—especially during early pregnancy or under suboptimal scanning conditions.

Alternate identification criteria

It is sometimes difficult to image a cross section of umbilical cord that clearly demonstrates 3 vessels. Factors such as maternal obesity, early gestational age, or oligohydramnios may hinder this axial view. Alternative methods for visualizing the umbilical vessels within the fetal abdomen have been described⁹. For instance, a coronal view of the fetal aortic bifurcation can provide additional information about suspected SUA. The distal aorta normally bifurcates into two common iliac arteries of similar caliber. The distal aorta of SUA fetuses has a gentle curve toward an aorta-sized common iliac vessel contralateral to the absent umbilical artery, and the iliac vessel on the side of the absent umbilical artery is small or even barely visible. Therefore,a coronal view of the aortic bifurcation can assist the sonographer"s ability to detect SUA. Further, only one vessel is seen alongside the bladder.

Color Doppler

Our initial efforts to directly verify a three-vessel cord by color Doppler ultrasound were disappointing since umbilical vessel tortuosity made it difficult to “capture” a true cross section. This difficulty was also related to the fact that Doppler velocities are best detected when the angle of insonation is parallel to blood flow. An axial view of the umbilical cord places the ultrasonic beam at right angles to blood flow and will theoretically not result in any Doppler shift at all. More recently, we have begun to investigate a new use of color Doppler ultrasound for improving the diagnostic accuracy of suspected SUA by real-time imaging. This technique employs a relatively low Doppler velocity scale (approximately 20 to 40 cm/sec) to assure appropriate sensitivity and is based upon the fact that both umbilical arteries originate as extensions of the internal iliac arteries surrounding the fetal bladder.

These two branches pass through the fetal umbilicus and intertwine with the umbilical vein to form a three-vessel cord. The most useful view has been obtained from the axial plane of the bladder, where both ischial spines are present posteriorly (umbilicus anterior). This acoustic window allows the insonation angle to be parallel with blood flow, resulting in an easily detectable Doppler shift. The umbilical vessels have been detected by this color Doppler technique as early as 14.5 weeks. However, it has been our preliminary experience that umbilical arteries can be reliably identified and counted by this color Doppler technique by at least 16-weeks gestation when 18% of studies may be unsatisfactory by gray-scale ultrasound. Figure 4 (left) demonstrates the typical appearance of these vessels at 16.5 weeks by color Doppler ultrasound. By comparison, Figure 4 (right) was taken from a sixteen-week fetus with a SUA that was easily verified by gray-scale imaging.

Fig. 4: Left: This represents an optimal “umbilicus anterior” view of two umbilical arteries (orange) coursing around the fetal bladder (b) by color Doppler ultrasound. Note that the iliac crests (IC) can be seen bilaterally in this 16.5-week fetus. Right: Another axial color Doppler scan at the level of the fetal bladder (b) which is associated with single umbilical artery (orange) in a 16-week fetus.

Summary

The prognostic clinical significance of identifying a two-vessel umbilical cord (either as an isolated finding or when associated with other anomalies) during the antenatal period requires further investigation. We routinely screen all fetuses (greater than 16 weeks) for umbilical cord anatomy at our imaging facility. Nonetheless, the identification of this problem should prompt a thorough search for other associated abnormalities. Accurate diagnosis should minimize the number of mothers subjected to unnecessary detailed studies and anxiety in cases of “suspected” but not confirmed SUA. This report documents the appearance of umbilical cord vessels around the bladder by color Doppler ultrasound and correlates this information with normal fetal anatomy. This technique can also be applied to cases where bladder location may be unclear (e.g., bilateral renal agenesis or cloacal exstrophy). Future work at this institution will concentrate upon improving the accuracy of antenatal SUA diagnosis through the supplemental use of color Doppler ultrasound.

References

1. National Center for Health Statistics. Births, marriages, divorces, and deaths for February, 1988. Monthly Vital Stat Rep 37:3, 1988.

2. Benirschke K, Bourne GL. The incidence and prognostic implication of congenital absence of one umbilical artery. Am J Obstet Gynecol 79: 251-254, 1960.

3. Saller DN, Keene CL, Sun CC, et al. The association of single umbilical artery with cytogenetically abnormal pregnancies. Am J Obstet Gynecol 163: 922, 1990.

4. Froehlich LA, Fukikura T. Significance of a single umbilical artery. Report from the collaborative study of cerebral palsy. Am J Obstet Gynecol 94: 274-279, 1966.

5. Leung AKC, Robson WLM. Single umbilical artery. A report of 159 cases. Am J Dis Child 143: 108-111, 1989.

6. Szpakowski M. Morphology of arterial anastomoses in the human placenta. Folia Morphol (Warsz) 33: 53-60, 1974.

7. Nyberg DA, Shepard T, Mack LA, et al. Significance of a single umbilical artery in fetuses with central nervous system malformations. J Ultrasound Med 7: 265-273, 1988.

8. Herrman UJ, Sidiropoulos D. Single umbilical artery: prenatal findings. Prenat Diagn 1988; 8: 275-80.

9. Jeanty P. Fetal and funicular vascular anomalies: identification with prenatal US. Radiology 173: 367-70, 1989.

10. Nyberg DA, Mahony BS, Luthy D, et al: Single umbilical artery: prenatal detection of concurrent anomalies. J Ultrasound Med (in press).