Doppler assessment of the placental circulation plays an important role in screening for impaired placentation and its complications of pre-eclampsia, fetal growth restriction (FGR), and perinatal death.
Abstract: Placental dysfunction is a major contributing factor to FGR. Umbilical artery Doppler can provide both diagnostic and prognostic information for management of the growth -restricted fetus.The clinical application of the UA Doppler is relevant in the diagnosis of this fetal condition and can provide insights into the severity of disease in fetuses with IUGR.
In fetuses with low estimated weight for gestational age, once genetic, structural, or infectious causes have been ruled out, the evaluation of the UA Doppler can detect placental insufficiency in early forms. Once the diagnosis is made, the degree of resistance in diastolic flow will be an indicator of the degree of fetal compromise and is an important tool to help guide management of this condition.
Authors: Patricia Galati1, Lucía Mandiá1, Daniel Cafici1
- SAUMB; Hospital Británico de Buenos Aires; Halitus, IM
Reviewer: Karen Fung-Kee-Fung
Doppler assessment of the placental circulation plays an important role in screening for impaired placentation and its complications of pre-eclampsia, fetal growth restriction (FGR), and perinatal death. Assessment of fetal circulation with Doppler, a technique first described in 1977, is essential for a better understanding of the pathophysiology of a wide range of pathological pregnancies and their clinical management.1
In general, the evaluation of the umbilical artery flow velocity waveform is not a complex task, however, it is required to know and apply certain basic knowledge of the technique so as not to generate false results.
The fundamental requirements for acquiring a correct umbilical artery waveform are described below, including the ISUOG recommendations.2
● Equipment should have colour flow and spectral wave Doppler capabilities, with the onscreen display of flow velocity scales or pulse repetition frequency (PRF) and Doppler ultrasound frequency (in MHz).
● The mechanical index (MI) and thermal index (TI) should be displayed on the ultrasound screen and the ALARA (as low as reasonably achievable) principle should be applied during the examination to ensure safety.
● The system software should be able to calculate the commonly used Doppler indices, i.e. pulsatility (PI) and resistance (RI) indices and the peak systolic/end-diastolic velocity ratio (S/D ratio).
● Doppler measurements should be reproducible; therefore, obtaining more than one Doppler recording is recommended. If there is an obvious discrepancy between two measurements, another recording is recommended, and use the one with the lowest impedance.
● The use of triplex mode (simultaneous use of grayscale, color flow mapping, and spectral Doppler) is not recommended as affects the quality of acquired data.
● Doppler horizontal sweep speed should be fast enough to separate successive waveforms. The ideal is a display of four to six (but no more than eight to ten) complete cardiac cycles (Fig 1).
● The PRF should be adjusted to avoid aliasing and the waveform should fill at least 75% of the Doppler screen (Fig 1).
● The filter should be set as low as possible to avoid hiding diastolic velocities. (Fig 2)
● The patient should be placed in a semi-seated position, to avoid the compression of the abdominal aorta and/or iliac arteries, which would reduce the blood flow to the placenta.
● Preferably, the study will be carried out in the absence of uterine contractions.
● Recordings should be obtained in the absence of fetal breathing and body movements and, if necessary, during temporary maternal breath-holding (Fig 3)
● The insonation angle should be less than 30º, and the size of the sample volume must be appropriate to the diameter of the vessel.
● The impedance is highest at the fetal end of the cord (intra-abdominal), and at the placental end, with lower impedance in the free loop cord.3
● For the sake of simplicity and consistency, by convention, measurements should be made in a free cord loop. However, reference ranges for umbilical artery Doppler indices at each of these sites have been published.4,5
● In a two-vessel cord, at any gestational age, the diameter of the single umbilical artery is larger than the arterial diameter would be if there were two arteries. Due to the different hemodynamics, the recorded velocity waveform in such cases should be interpreted with caution when using conventional reference ranges.6
● In multiple pregnancies, assessment of umbilical artery blood flow can be challenging, since there may be difficulty in assigning a cord loop to a particular fetus. It is recommended to measure the umbilical artery at the site of its exit from the fetal abdomen or at the edges of the bladder and compare the obtained values with reference tables.2
The umbilical artery waveform has a characteristic low resistance pattern with high forward flow velocities in both the systolic and diastolic phases of the cardiac cycle. With advancing gestation, umbilical arterial Doppler waveforms demonstrate a progressive rise in the end-diastolic velocity and a decrease in the impedance indices. When the high-pass filter is either turned off or set at the lowest value, end-diastolic frequencies may be detected from as early as 10 weeks, and in normal pregnancies, they are always present from 15 weeks. The PI, RI, and S/D ratio decrease with advancing gestations due to a decrease in placental vascular resistance (Fig 4).
Placental dysfunction is a major contributing factor to FGR and umbilical artery Doppler can provide diagnostic and prognostic information for the management of FGR.7
Umbilical artery flow velocity waveform reflects placental impedance. In cases of reduction in the vascular territory of the placenta, there is an increase in resistance in its bed which at the same time determines an increase in resistance in the artery and its pulsatility index (PI).
There is evidence showing that an abnormal UA Doppler (PI >95th centile or AEDF/REDF) is strongly associated with adverse perinatal outcomes in small for gestational age (SGA) fetuses.8
It has been proposed, based on experiences using embolization models, that the drop in the diastolic velocity in the UA with an increase in the PI above 90-95 percentile occurs when there is a compromise of the placental vascular territory greater than 30%, on the other hand, when the compromise is greater than 60%, more severe patterns are seen with absence of end-diastolic flow or reverse diastolic flow.9,10
The clinical application of the UA Doppler is relevant in the diagnosis and knowledge of the severity of fetuses with IUGR.
In fetuses with low weight for gestational age, once genetic, structural, or infectious causes have been ruled out, the evaluation of the UA Doppler can diagnose placental insufficiency in early forms. Once the diagnosis is made, the degree of impedance in diastolic flow will be the indicator of fetal compromise.
Several meta-analyses have demonstrated that abnormal UA waveforms are associated with an increase in perinatal mortality and a decrease in obstetric interventions.11-14
There is sufficient evidence to recommend the use of the UA Doppler in pregnancies complicated with IUGR or PE. Umbilical artery Doppler measurement can reduce perinatal deaths with a reduction of up to 29% in high-risk pregnancies.12 However, it is important to keep in mind that routine use in low-risk or non-selected populations, does not confer maternal or fetal benefit, and can lead to unnecessary obstetric interventions.15
The UA Doppler is a simple tool to be used in high-risk pregnancies. It is useful to identify a group of low-weight for gestational age fetuses with a placenta-mediated etiology with a certain degree of hypoxia.
According to the gestational age at which the diagnosis is made FGR is classified in two forms. FGR presenting before 32 weeks is considered “early onset” and thereafter “late onset”.16
In early onset FGR, there is a high prevalence of villous perfusion abnormalities that results in a decreased UA end-diastolic velocity proportional to the degree of flow impairment. When the placental compromise reaches 30%, UAPI becomes abnormal (Fig 5). An affectation greater than 60% will result in absent and reverse diastolic flow (Figs 6 and 7).
Umbilical artery Doppler is crucial in the diagnosis of early onset FGR17 especially as it is difficult to interpret CTG findings in fetuses under 28 weeks of gestation.
In late-onset FGR vascular compromise of the placenta is usually less than 30% and for that reason, UAPI tends to remain normal or with mild compromise.18
Please reveiew the VISUOG chapter on FGR which addresses this issue in more detail.
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17. Figueras F, Gratacos E. Stage-based approach to the management of fetal growth restriction. Prenat Diagn 2014;34(7): 655e9.
18. Baschat AA, Planning management and delivery of the growth-restricted fetus, Best Practice & Research Clinical Obstetrics & Gynaecology (2018), DOI: 10.1016/ j.bpobgyn.2018.02.009.
This article should be cited as: Galati P., Mandiá L., Cafici D.: Umbilical Artery Doppler, Visual Encyclopedia of Ultrasound in Obstetrics and Gynecology, www.isuog.org, October 2022.
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