Prenatal ultrasonographic morphologic assessment of the umbilical cord: a review. Part I

Current Perspectives of Prenatal Sonography of Umbilical Cord Morphology

The umbilical cord constitutes a continuation of the fetal cardiovascular system anatomically bridging between the placenta and the fetus. This structure, critical in human development, enables mobility of the developing fetus within the gestational sac in contrast to the placenta, which is anchored to the uterine wall. The umbilical cord is protected by unique, robust anatomical features, which include: length of the umbilical cord, Wharton’s jelly, two umbilical arteries, coiling, and suspension in amniotic fluid. These features all contribute to protect and buffer this essential structure from potential detrimental twisting, shearing, torsion, and compression forces throughout gestation, and specifically during labor and delivery. The arterial components of the umbilical cord are further protected by the presence of Hyrtl’s anastomosis between the two respective umbilical arteries. Abnormalities of the umbilical cord are uncommon yet include excessively long or short cords, hyper or hypocoiling, cysts, single umbilical artery, supernumerary vessels, rarely an absent umbilical cord, stricture, furcate and velamentous insertions (including vasa previa), umbilical vein and arterial thrombosis, umbilical artery aneurysm, hematomas, and tumors (including hemangioma angiomyxoma and teratoma). This commentary will address current perspectives of prenatal sonography of the umbilical cord, including structural anomalies and the potential impact of future imaging technologies.

Current Perspectives of Prenatal Sonographic Diagnosis and Clinical Management Challenges of True Knot of the Umbilical Cord

Umbilical cord accidents preceding labor are rare. Single and multiple nuchal cords, and true knot(s) of the umbilical cord, are often incidental findings noted at delivery of non-hypoxic non-acidotic newborns without any evidence of subsequent adverse neonatal outcome. In contrast to single nuchal cords, true knots of the umbilical cord, which occur in between 0.04% and 3% of all deliveries, have been associated with a reported 4 to 10 fold increased risk of stillbirth. First reported with real-time ultrasound, current widespread application of color Doppler, power Doppler and three-dimension sonography, has enabled increasingly more accurate prenatal sonographic diagnoses of true knot(s) of the umbilical cord. Reflecting the inability to visualize the entire umbilical cord at prenatal ultrasound assessment, despite detailed second and third-trimester scanning, many occurrences of incidental true knot of the umbilical cord remain undetected and are noted only at delivery. Although prenatal sonographic diagnostic accuracy is increasing, false positive sonographic diagnosis of true knot of the umbilical cord cannot be ruled out with certainty, and must continue to be considered clinically. Notwithstanding the inability to diagnose all true knots, currently there is a clear absence of clinical management guidelines by governing bodies regarding patients in whom prenatal sonographic diagnosis of true knot(s) of the umbilical cord is / are suspected. As a result, in many prenatal ultrasound units, suspected sonographic findings suggestive of or consistent with true knot of the umbilical cord are often disregarded, not documented, and patients are not uniformly informed of this potentially life-threatening condition, which carries an associated considerable risk of stillbirth. This commentary will address current perspectives of prenatal sonographic diagnostic and management challenges associated with true knot(s) of the umbilical cord in singleton pregnancies.

Altered fetal growth, placental abnormalities, and stillbirth

Background

Worldwide, stillbirth is one of the leading causes of death. Altered fetal growth and placental abnormalities are the strongest and most prevalent known risk factors for stillbirth. The aim of this study was to identify patterns of association between placental abnormalities, fetal growth, and stillbirth.

Methods and findings

Population-based case-control study of all stillbirths and a representative sample of live births in 59 hospitals in 5 geographic areas in the U.S. Fetal growth abnormalities were categorized as small (<10th percentile) and large (>90th percentile) for gestational age at death (stillbirth) or delivery (live birth) using a published algorithm. Placental examination by perinatal pathologists was performed using a standardized protocol. Data were weighted to account for the sampling design. Among 319 singleton stillbirths and 1119 singleton live births at ≥24 weeks at death or delivery respectively, 25 placental findings were investigated. Fifteen findings were significantly associated with stillbirth. Ten of the 15 were also associated with fetal growth abnormalities (single umbilical artery; velamentous insertion; terminal villous immaturity; retroplacental hematoma; parenchymal infarction; intraparenchymal thrombus; avascular villi; placental edema; placental weight; ratio birth weight/placental weight) while 5 of the 15 associated with stillbirth were not associated with fetal growth abnormalities (acute chorioamnionitis of placental membranes; acute chorioamionitis of chorionic plate; chorionic plate vascular degenerative changes; perivillous, intervillous fibrin, fibrinoid deposition; fetal vascular thrombi in the chorionic plate). Five patterns were observed: placental findings associated with (1) stillbirth but not fetal growth abnormalities; (2) fetal growth abnormalities in stillbirths only; (3) fetal growth abnormalities in live births only; (4) fetal growth abnormalities in stillbirths and live births in a similar manner; (5) a different pattern of fetal growth abnormalities in stillbirths and live births.

Conclusions

The patterns of association between placental abnormalities, fetal growth, and stillbirth provide insights into the mechanism of impaired placental function and stillbirth. They also suggest implications for clinical care, especially for placental findings amenable to prenatal diagnosis using ultrasound that may be associated with term stillbirths.

Umbilical artery aneurysm: a case report, literature review, and management recommendations

BACKGROUND

Umbilical artery aneurysm is a rare and often lethal condition frequently associated with fetal anomalies, fetal demise, and neonatal complications.

CASE

We report a case of umbilical artery aneurysm discovered at 21 weeks 2 days of gestation in a fetus of normal karyotype. Maternal hospitalization occurred at 28 weeks for antenatal testing, betamethasone administration, and monitoring for expansion of the aneurysm. Delivery of a live neonate by repeat cesarean delivery was performed at 32 weeks 2 days. Pathology confirmed a 3-vessel cord with an umbilical artery aneurysm. Neonatal course was complicated by respiratory distress of the newborn, hyperbilirubinemia, anemia, difficulty feeding, and cardiac defects. The newborn was discharged from the neonatal intensive care unit on day of life 19.

CONCLUSIONS

Umbilical artery aneurysm is highly associated with fetal complications including trisomy 18, single umbilical artery, cardiac anomalies, and intrauterine fetal demise. A normal karyotype, antenatal monitoring, and early delivery have been suggested to impact the likeliness of survival. Antenatal management strategies include consideration of nonstress testing 3 times daily, serial ultrasound assessments, testing to identify intrauterine growth restriction, and delivery by planned cesarean delivery between 32 and 34 weeks. We recommend that patients be counseled on the high risks associated with umbilical artery aneurysm and be included in discussions regarding antenatal management and delivery planning.

Multiple true umbilical knots: a silent risk for intrauterine growth restriction with anomalous hemodynamic pattern

True knots of the umbilical cord can represent a serious complication for the fetus due to the possible alteration in the fetal circulation with consequent intrauterine growth restriction or fetal death. We report a case of 5 true umbilical cord knots associated with severe fetal growth restriction and an abnormal hemodynamic pattern. The Doppler examination showed a hemodynamic pattern characterized by an early alteration in the waveform profile in the fetal venous districts with normal impedance to flow values in both uterine and umbilical arteries. This normal profile of the umbilical arteries remained unchanged until the last stage of hemodynamic decompensation, while the profiles of the uterine arteries remained normal until delivery. This case report suggests that it is important to pay close attention to the evaluation of the fetal cord in situations in which the above described hemodynamic pattern is noted. Although the ultrasound diagnosis of true knots is extremely difficult, the presence of a true knot should always be suspected in the presence of an intrauterine growth restriction fetus when the venous district is altered before the fetal arterial districts after exclusion of other detectable reasons for growth restriction.

Elevated maternal serum alpha-fetoprotein, umbilical vein varix, and mesenchymal dysplasia: are they related?

OBJECTIVES AND METHODS

Variceal dilatation of the umbilical vein is a rare vascular anomaly of the umbilical cord. We present a patient case where dilatation of the umbilical vein was associated with an elevated maternal serum alpha-fetoprotein on prenatal testing and mesenchymal dysplasia on pathological evaluation of the placenta.

RESULTS

Alterations in the villous structure of the placenta as in mesenchymal dysplasia may lead to increased placental permeability causing an elevated maternal serum alpha-fetoprotein on antenatal testing.

CONCLUSION

Abnormal testing should be an indication for close evaluation of the placenta and placental structures, as well as the fetus.

Ultrasound abnormalities of the amniotic fluid, membranes, umbilical cord, and placenta

Prenatal ultrasound has expanded the ability to assess the umbilical cord, fetal membranes, amniotic fluid volume, and placenta. Evaluation of these structures provides information regarding the intrauterine environment. Umbilical cord abnormalities may be associated with fetal aneuploidy, structural anomalies, and fetal compromise. Estimating the amniotic fluid volume has become an integral part of a sonogram and provides immense information regarding possible fetal anomalies and perinatal outcome. Likewise, placental location or abnormalities may significantly impact obstetric management and prognosis. Early detection of several of these conditions may lead to increased vigilance that may improve perinatal outcome.

Umbilical artery aneurysm: prenatal identification in three fetuses with trisomy 18

Aneurysm of the umbilical artery is an extremely rare anomaly of the umbilical cord, with only two cases being documented in the English language literature. We report three cases diagnosed prenatally by ultrasound in the third trimester, all associated with single umbilical artery and multiple structural fetal anomalies. Prenatal karyotyping revealed trisomy 18 in all three cases. Umbilical cord anomalies, although rare, are associated with significant fetal morbidity and mortality. This report identifies umbilical artery aneurysm as an additional prenatal feature of trisomy 18.

Umbilical cord pseudocyst in a fetus with trisomy 18

An umbilical cord pseudocyst was detected in the 28th week of gestation in a fetus complicated with growth restriction and polyhydramnios. The combination of cord pseudocysts, growth restriction, and polyhydramnios prompted us to perform a detailed ultrasonographic examination (gray scale and three-dimensional), which revealed the presence of micrognathia, overlapping fingers, and congenital heart defects, features characteristic of trisomy 18. Karyotyping confirmed a diagnosis of trisomy 18. After spontaneous labor onset, the infant was delivered at 31 weeks of gestation, and died soon after delivery. An umbilical cord pseudocyst is a good marker for the prenatal detection of trisomy 18.

A transfer function-based mathematical model of the fetal-placental circulation

The circulation of a human fetus has been modeled using a transfer function that is based on the arterial dimensions at 28 weeks gestational age (GA). These dimensions have then been adapted for growth between 28 and 40 weeks GA. The input to the model is a series of current pulses at the fetal heart rate, where current in the model is analogous to volume blood flow in the fetus. The arterial system is divided into short segments that are cascaded together. The respective transfer functions are based on the dimensions, wall properties and fluid characteristics at each frequency and GA. Bleed off conductances distribute current to circuits representing the various anatomical regions. In particular, the placenta is simplified to a symmetrically distributed network of branching vessels, each represented by a transfer function. All calculations are performed in the frequency domain, after which the inverse Fourier transform is used to calculate the currents that represent the time-domain blood flow waveforms. Simulated flow waveform resistance index and pulsatility index values are within 8% of those reported for human clinical studies, at all gestational ages.

A transmission line model of the human foetal circulatory system

A transmission line model of the human foetal circulatory system is presented. The model has been developed in the frequency domain with the cardiac input modeled as a flow rather than as a pressure pulse and is structured upon electrical transmission line analogies. The model is formed by cascading solutions to the two-dimensional Navier-Stokes equations for both oscillatory and steady, laminar viscous fluid flow in isotropic visco-elastic tubes with thick walls, which are constrained by surrounding tissues. Simulations allow for representation of both forward and retrograde travelling flow and pressure waves in all of the main foetal arterial vessels. The solution is verified by a comparison of model generated Doppler indices in the thoracic aorta, abdominal aorta, iliac artery and both ends of the umbilical arteries with previously published indices obtained by clinical measurements in these arteries. For simulations of blood flow in a healthy foetus, the model generated Pulsatility and Resistance indices were on average within 8% of the corresponding clinical measurements. The model results also demonstrates that placental resistance must increase by a factor of three, corresponding to a 60% decrease in flow to the placenta, before umbilical arterial absent end diastolic flow is observed. Differences between indices obtained from simulations at opposite ends of the umbilical arteries increase with increasing placental resistance.

Umbilical cord accidents: human studies

Research is needed to determine the cause of unexplained stillbirth. Sudden antenatal death syndrome is an important national issue that requires more scrutiny. Umbilical cord accidents as a causative factor of stillbirth need intensive investigation. Evidence supports a role of the umbilical cord in a portion of stillbirth cases, and theory suggests additional causes. This article summarizes the known information relating umbilical cord accidents and stillbirth and highlights the research needs.

Risk factors associated with true knots of the umbilical cord

OBJECTIVE

To determine obstetrical risk factors and pregnancy outcome of fetuses with true knot of the umbilical cord.

METHODS

Study population included 69,139 singleton deliveries occurring between the years 1990-1997. Data were retrieved from the database of the Soroka University Medical Center. Fetuses with malformations were excluded.

RESULTS

The incidence of true knots was 1.2% (841/69,139). In a multivariate analysis the following factors were found to be significantly associated with true knot of cord: grandmultiparity, chronic hypertension, hydramnios, patients who undergone genetic amniocentesis, male gender and cord problems (prolapse of cord and cord around the neck). The incidence of fetal distress and meconium stained amniotic fluid was significantly higher among patients with true knots of cord (7% versus 3.6%, P<0.001 and 22% versus 16%, respectively, P<0.0001). Moreover, there was a four-fold higher rate of antepartum fetal death among those fetuses (1.9% versus 0.5%, P<0.0001). In addition, fetuses with true knots of the umbilical cord were more often delivered by a cesarean section (130/841 versus 711/68,298, P<0.0001). The following obstetrical factors were found to be significantly correlated to true knots of the umbilical cord in a multiple logistic regression model: gestational diabetes, hydramnios, patients undergoing genetic amniocentesis, male fetuses.

CONCLUSIONS

Patients with hydramnios, who underwent genetic amniocentesis and those carrying male fetuses are at an increased risk for having true knots of the umbilical cord. Thus, careful sonographic and Doppler examinations should be seriously performed in these patients for detection of the complication of the umbilical cord.

Umbilical cord morphology and pregnancy outcome

Traditionally, the prenatal assessment of the umbilical cord (UC) is limited to the assessment of the number of vessels and to the evaluation of umbilical artery blood flow parameters. Morphologic aspects of the UC have usually been studies by pathologists and retrospectively correlated with the perinatal outcome. The introduction of more sophisticated imaging techniques have offered the possibility to investigate the UC characteristics during fetal life from early to late gestation. A number of investigations have demonstrated that an altered structure of the UC can be associated with pathologic conditions (i.e. Preeclampsia, fetal growth restriction, diabetes, fetal demise). Nomograms of the various UC components have been generated and allow the identification of lean or large umbilical cords, entities frequently associated with fetal growth abnormalities and diabetes. A Wharton's jelly reduction has also been invoked as a possible cause of fetal death in the presence of single umbilical artery. Prenatal morphometric UC characteristics as well as arterial and venous blood flow parameters in normal and pathologic conditions will be discussed.

Amniotic fluid and the umbilical cord: the fetal milieu and lifeline

Amniotic fluid volume should be routinely assessed in every second and third trimester case. A review of amniotic fluid physiology and techniques for ultrasound evaluation of fluid volume is presented. The causes and significance of oligohydramnios and polyhydramnios are stressed. Umbilical cord abnormalities are often incidently observed at the time of amniotic fluid evaluation. The clinical significance of some common umbilical cord abnormalities such as a two-vessel cord and nuchal cord are discussed. Other, more uncommon entities such as cord mass lesions are also reviewed. Finally, the role of cord Doppler interrogation in determining fetal well-being is discussed.