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 Table of Contents  
Year : 2021  |  Volume : 2  |  Issue : 2  |  Page : 79-85

Umbilical artery doppler reference indices in normal pregnancy

1 Department of Obstetrics and Gynaecology, University of Abuja Teaching Hospital, Abuja, Nigeria
2 Department of Radiology, University of Abuja/University of Abuja Teaching Hospital, Gwagwalada, Abuja, Nigeria

Date of Submission04-Jun-2021
Date of Decision09-Sep-2021
Date of Acceptance28-Sep-2021
Date of Web Publication30-Nov-2021

Correspondence Address:
Hadijat Oluseyi Kolade-Yunusa
Department of Radiology, University of Abuja/University of Abuja Teaching Hospital, Gwagwalada, Abuja
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/jrmt.jrmt_10_21

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Background: Umbilical artery Doppler (UAD) studies offer a noninvasive method of indirectly assessing the fetal and uteroplacental circulation. However, the umbilical artery (UA) velocimetry reference currently in use was established in developed countries and derived from pregnant women that are likely to have had different biophysical characteristics. There is a need to develop an UA index nomogram for the indigenous pregnant population. Aim: The aim of this study was to determine the UAD reference indices among women with normal singleton pregnancy. Materials and Methods: This was a cross-sectional descriptive study of 422 women with uncomplicated pregnancies who attended the antenatal clinic of the University of Abuja Teaching Hospital with pregnancy ages between 28 and 40 weeks' gestation. The systolic and diastolic (S/D) ratio, the resistance index (RI), and the pulsatility index (PI) of the UA were obtained. Results: The mean UAD showed a progressive decrease with increasing fetal gestational age with S/D ratio, PI, and RI being 3.05 ± 0.28, 1.07 ± 0.1, and 0.68 ± 0.05 at 28 weeks and 2.10 ± 0.17, 0.73 ± 0.09, and 0.52 ± 0.04 at 40 weeks, respectively. Estimated reference values for relatively normal singleton fetuses S/D ratio, PI, and RI were 2.98, 1.03, and 0.67 and 2.07, 0.72, and 0.52 at 28 and 40 weeks of gestation, respectively. The S/D ratio (r = ‒0.741, P ≤ 0.002), PI (r = ‒0.694, P ≤ 0.001), and RI (r = ‒0.366, P ≤ 0.001) were significantly different from the existing reference value of 3.02 (S/D ratio), 1.05 (PI), and 0.62 (RI) and 2.18 (S/D ratio), 0.82 (PI), and 0.52 (RI). The respective existing values were at 28 weeks and 40 weeks of gestation. Conclusion: The UA index nomogram obtained from this study showed significantly lower values compared with the existing values, and therefore, it is advocated for use among African pregnant population.

Keywords: Doppler indices, nomogram, normal pregnancy, reference ranges, umbilical artery

How to cite this article:
Okochi OD, Isah AY, Kolade-Yunusa HO, Abdullahi HI. Umbilical artery doppler reference indices in normal pregnancy. J Radiat Med Trop 2021;2:79-85

How to cite this URL:
Okochi OD, Isah AY, Kolade-Yunusa HO, Abdullahi HI. Umbilical artery doppler reference indices in normal pregnancy. J Radiat Med Trop [serial online] 2021 [cited 2023 Mar 26];2:79-85. Available from: http://www.jrmt.org/text.asp?2021/2/2/79/331547

  Introduction Top

Fetal health is one of the important subjects in public health bearing significant impact on patients, their families, and the society.[1],[2] The assessment of fetal circulation is essential in the better understanding of the pathophysiology of a wide range of pathological events in pregnancies and their clinical management.[3],[4],[5] Antepartum fetal surveillance in obstetrics with Doppler sonography is a widely acceptable and functional method for evaluating fetal well-being, and has been an invaluable tool over the past 30 years.[2],[6],[7],[8] The commonly used Doppler assessment in obstetrics encompasses the umbilical artery (UA), middle cerebral artery, uterine artery, and ductus venosus (DV) assessments. More specialized and uncommonly performed Doppler studies include the umbilical vein (UV), aortic isthmus, and atrioventricular valves.[6],[9],[10]

The UA was the first fetal vessel to be evaluated by Doppler velocimetry and has since become the most widely investigated component of fetal circulation.[2],[11],[12] The attention paid to this vessel may be explained partly by its ready accessibility to Doppler interrogation, even without guidance by duplex imaging, and because it is a vital component of the fetal circulation, acting as a lifeline between the fetus and placenta.[2],[6],[13] Reference indices used in these valuable assessments are, however, derived from European-based studies and where their use originated with very little contribution from pregnant women from African origin.[3],[4]

The use of Doppler ultrasound in obstetrics to evaluate the umbilical arteries offers a noninvasive method of indirectly assessing the fetal and uteroplacental circulation with significant diagnostic efficacy for hemodynamic complications related to uteroplacental insufficiency in intrauterine growth restriction, preeclampsia, intrauterine fetal death, and placental abruption.[5],[14],[15],[16],[17] UA velocimetry has proven to be an effective invaluable obstetric tool as it has been used due to its significant diagnostic efficacy to access both fetal and placental circulation, with the aim of facilitating diagnosis and monitoring high-risk pregnancies.[6],[18],[19],[20] Moreover, randomized clinical trials and meta-analysis have shown the effectiveness of UA velocimetry in decreasing perinatal mortality (Level 1 evidence) being the only intrapartum test to show this level of effectiveness.[15] The placenta helps in gaseous exchange through two umbilical arteries and one UV.[6] The umbilical arteries take origin from the fetal internal iliac arteries, and it courses posteriorly and cephalad to join the left branch of the portal vein. The oxygenated blood is then shunted through the liver to the inferior vena cava by DV. Hence, the utilization of umbilical artery Doppler to evaluate fetal compromise.[13],[21]

Interestingly, obstetric complications such as fetal growth restriction and preeclampsia that have a high prevalence in the developing world like ours, result in chronic fetal nutritive and respiratory deprivation. As the stress intensifies and/or lengthens, the fetus mobilizes defensive responses which include preferential preservation of fetal growth over placental growth, changes in fetal movement pattern, deceleration of the fetal growth rate, and eventually, chronic hypoxia and acidosis.[6] Doppler sonographic nomograms or references are used for differentiating normal from abnormal Doppler velocity waveforms and help to determine pregnancies at risk.[2] By taking threshold values of pathologic pregnancies into consideration, references are used to differentiate between normal and abnormal. While conforming to these nomograms, it must always be kept in mind that these references have limitations of sensitivity and specificity.[2]

A few numbers of investigators have published reference ranges for fetal UA. However, only a few of such reference ranges may have been published for the Nigerian population by Adekanmi et al. and Ayoola et al.[3],[4] In the former, UA Doppler (UAD) indices established were not gestational age specific, and they were deduced with gestational age intervals. Whereas in the latter, the benefits of the lower limit of the gestational age recruited (15 weeks) are still questioned. In the proposed study, umbilical Doppler reference values will be gestational age specific and shall be from the recommended threshold gestational age (≥28 weeks).

UA reference indices have been developed in other parts of the world.[3],[11],[12],[13],[14],[15],[16],[17] There are insufficient data in these areas of UA velocimetry in Africa to the best of the researcher's knowledge. The UA velocimetry reference range used in the developing part of the world was calculated or established in developed countries by enrolling normal healthy pregnant women who were sociodemographically different from Black women of African descent.[4] The use of these reference values from developed countries may not necessarily conform to what is obtainable in the African population with a higher burden of preeclampsia and intrauterine growth restriction. It is, therefore, imperative to develop an UAD nomogram for this part of the world.

  Materials and Methods Top

This was a cross-sectional study of a cohort of 422 normal pregnant women at 28 weeks and 42 weeks of gestation that had their antenatal care at the Department of Obstetrics and Gynecology, University of Abuja Teaching Hospital, Abuja, between June and December 2019 (inclusive). The study location provides health-care services to the inhabitants of Abuja and serves as a major referral center to neighboring states such as Niger, Kaduna, Kogi, and Nasarawa states, Nigeria. Women with uncomplicated singleton pregnancies whose gestational age was calculated from the last menstrual period and corroborated with an ultrasound scan carried out at ≤20 weeks' gestation. Those who were unsure of date and had diabetes, anemia, chronic kidney, or cardiac disease in pregnancy as well as hypertensive disorders in pregnancy were excluded. Those with multiple pregnancy, who smoke cigarette, and who ingest alcoholic beverages were also excluded.

Umbilical artery Doppler

The Doppler sonography was performed using a general electric health-care Voluson (P8 2016 version, U.S.AR) ultrasound machine. The wall filter settings were kept as low as possible (50–100 Hz). Using a 3.5-MHZ curved transducer, an initial obstetric scan was carried out to document obstetric parameters, number of fetuses, to rule out any gross fetal malformation. Color and pulsed Doppler was carried out on the subjects. A free loop of the umbilical cord was located on the B mode for simplicity and consistency of measurements, and color Doppler was applied to it. The UA was identified using Doppler interrogation, and a Doppler gate (2–3 mm) wide was applied on the section of interest. The Doppler cursor line was aligned with the vessel axis at an angle of insonation ≤20°. The interrogation was obtained from the umbilical middle third where adequate waveform signal was visualized. The Doppler indices (resistance index [RI], pulsatility index [PI], and systolic/diastolic (S/D) ratio) were automatically generated from tracing of the spectral waveform by the ultrasound machine and values for each index were recorded [Figure 1]. These indices were measured from consecutive uniform UA waveforms, and measurement was done during fetal quiescence.
Figure 1: Pulsed Doppler of the Umbilical Artery showing the spectral waveform (saw-tooth pattern)

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The data obtained were analyzed using SPSS (Statistical Package for the Social Sciences) version 20 and software 2010 by IBMR USA. While descriptive tools were used to analyze quantitative variables, frequency tables and percentages were used to explain qualitative variables. Pearson's correlation coefficient was used to analyze the relationship between UAD parameters with gestational age. Linear regression equation was calculated and percentile fitted values were obtained for all the parameters at different gestational ages. The paired t-test was used to compare the newly constructed nomogram with the existing local and existing foreign nomograms. P <0.05 was considered statistically significant. Ethical clearance was obtained from the Health Research and Ethics Committee of the University of Abuja Teaching Hospital with reference number FCT/UATH/HREC/1085.

  Results Top

Four hundred and twenty-two uncomplicated singleton pregnancies were recruited and had UAD studies carried out with an average of 32/gestational week. The mean maternal age was 30.92 ± 4.79 while about 35% of the participants relatively had economic empowerment [Table 1].
Table 1: Sociodemographic characteristics of the subjects

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The gestational age-specific mean values for the UA indices obtained showed a progressive decrease in parameters with increasing fetal gestational age with S/D ratio, PI, and RI being 3.05 ± 0.28, 1.07 ± 0.1, and 0.68 ± 0.05 at 28 weeks and 2.10 ± 0.17, 0.73 ± 0.09, and 0.52 ± 0.04 at 40 weeks, respectively [Table 2].
Table 2: Mean value of umbilical artery Doppler indices at different gestational ages

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Gestational age-specific nomograms were obtained by plotting Doppler parameter against age of gestation in a scatter diagram and lines were fitted for the 5th, 50th, and 95th percentile values. All the plots show a continuous reduction of all the Doppler indices with increasing period of gestational age [Figure 2], [Figure 3], [Figure 4].
Figure 2: Scatter plot for S/D ratio (5th, 50th, and 95th) versus gestational age

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Figure 3: Scatter plot for pulsatility index (5th, 50th, and 95th) versus gestational age

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Figure 4: Scatter plot of resistance index (5th, 50th, and 95th) versus gestational age

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Pearson's correlation was used to show the strength of association of the umbilical indices in relation to gestational age. All the three Doppler parameters (S/D ratio, PI, and RI) showed a statistically significant negative correlation with gestational age, i.e. as gestational age increased, all three Doppler parameters (S/D ratio, PI, and RI) decreased with S/D ratio (r = ‒0.741, P ≤ 0.002), PI (r = ‒0.694, P ≤ 0.001), and RI (r = ‒0.366, P ≤ 0.001) [Table 3].
Table 3: Correlation between umbilical artery Doppler indices and gestational age

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In attempt to test for confounding variables, Pearson's correlation was used to assess the relationship between Doppler indices, maternal age, and parity. Both maternal age and parity exert no significant relationship on the Doppler indices [Table 4].
Table 4: Correlation between maternal age, parity, and umbilical artery indices

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Established gestational age-specific linear regression model for each UAD indices revealed that Doppler parameters for the fetal UA had a strong linear relationship with gestational age at various percentiles [Table 5].
Table 5: Linear regression model equations for umbilical artery Doppler reference indices

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The estimated reference values from the model above at different gestation along with the 5th, 50th, and 95th percentile values are represented below. Estimated limits for S/D ratio, PI, and RI were 2.98, 1.03, and 0.67 and 2.07, 0.72, and 0.52 at 28 and 40 weeks of gestation, respectively [Table 6].
Table 6: Reference limits for the umbilical artery indices

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The comparison between the newly established indices and the existing local and foreign values at their 50th percentile showed consistently that the obtained S/D ratio values are relatively lower than the local existing values just as were marginally observed with PI and RI. All the values for the newly established indices were in close comparison but minimally lower than existing foreign [Table 7].
Table 7: Comparison of obtained umbilical artery index values with existing values

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The compared mean values for the UAD indices in this study with existing local and foreign indices using paired t-test showed a significant difference between the newly estimated indices and the existing values (both local and foreign) [Table 8].
Table 8: Comparison between obtained indices and existing indices using paired t-test

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  Discussion Top

This study involving 422 normal singleton pregnancies set out and has established reference intervals for the UAD nomograms for Black population attending our antenatal care at gestational ages from 28 to 40 weeks. The mean maternal age was 30.92 ± 4.79, with primigravida constituting 43.8%. The mean S/D ratio, PI, and RI at the 5th, 50th, and 95th percentiles were obtained to establish gestational age-specific reference range for the fetal UAD indices which were obtained.

All the three UAD indices in this study showed a continuous and significant reduction with increasing gestational age from 28 to 40 weeks (P < 0.01). This result is consistent with results of Adekanmi et al., Ayoola et al., and Acharya et al. who also showed a continuous decrease in Doppler indices with advancing gestational age (P < 0.01).[3],[4],[22]

This reduction in Doppler indices with gestational age was, however, not unexpected as it is a reflection of progressive low resistant placental flow with resultant increased vascular supply necessary for normal growth and development of the fetus.[13] Trudinger also postulated the continuous fall in the Doppler indices to the following mechanisms: continuous maturation of the fetal–placental villous system, increase in the cross-sectional area of the fetoplacental vessels causing a progressive decline of fetoplacental vascular resistance, increase in fetal cardiac output, the changing compliance and resistance of the vessel wall, and rise of fetal blood pressure.[23]

When the individual parameters were compared with existing literature on the subject matter, it was found that the mean UAD indices decreased progressively with gestational age from 3.05 ± 0.28 to 2.10 ± 0.17, 1.07 ± 0.10 to 0.73 ± 0.09, and 0.68 ± 0.05 to 0.52 ± 0.04 for S/D ratio, PI, and RI at 28 weeks and 40 weeks, respectively. This was consistent with Ayoola et al. who also reported a decline in the mean (UAD) indices from 4.068 to 2.365, 1.265 to 0.829, and 0.760 to 0.585 for S/D ratio, PI, and RI, respectively.[4]

The RI of the UA (UA-RI) in this study similarly showed a significant decrease with advancing gestational age (r = ‒0.366, P < 0.001). The UA-RI of 0.67 at 28 weeks reduced to 0.52 at 40 weeks' gestation which is similar to what was reported by Acharya et al.[22] at similar corresponding gestational ages.

Ayoola et al.[4] also reported a decrease in RI from 0.66 at 28 weeks to 0.58 at 39 weeks just as similar to the finding of this study. In all and as was described by Olofsson et al., these reported a decrease in the UA resistance indices of the fetus as pregnancy advances is not an aberration but rather a favorable and necessary change for optimal fetal development.[24]

The UA-PI similarly and significantly decreased more than the existing studies.[4],[22],[25] The reference values for UA-PI in this study decreased from 0.98 at 30 weeks to 0.72 at 40 weeks of gestation which was similar but lower than those reported by Chanprapaph et al. who had an UA-PI of 1.14 and 0.97 at the same 30 and 40 weeks, respectively.[11] The progressive decline in the UA-PI of 1.03 and 0.75 at 28 weeks and 39 weeks, respectively, reported in this study was similar to Ayoola et al., who reported an UA-PI linear decline from 1.02 at 28 weeks to 0.82 at 39 weeks.[4] These minimal variations may be due to racial, environmental, and/or geographical factors. A large multicenter trial may be a rejoinder to the disparity in these variations.

There was a significant decrease in the UA-S/D ratio with advancing gestational age (r = ‒0.741, P < 0.002) when compared with the existing studies.[4],[11],[22] The decline in the UA-S/D ratio in this study from 2.98 to 2.07 at 28 and 40 weeks' gestation, respectively, was comparable but had values lower than those reported by other European authors; Acharya et al. found an UA-S/D ratio of 3.02 and 2.18 at 28 and 40 weeks, respectively.[22] Chanprapaph et al. obtained an UA-S/D ratio of 3.32 at 28 weeks and 2.51 at 40 weeks of gestation.[11] This raises the question for a possibility of racial and geographical variations. Paradoxically, however, one would expect that “Caucasians” that had relatively higher values would have demonstrated more impediments to placenta blood flow with attendant fetuses with lesser weight for gestational age. However, this is not so in the literature. Even the definitions of macrocosmic babies are higher in Caucasians, insinuating that their fetuses may be better perfused. It may be more reassuring that by comparison, the values of this research should be higher than those obtained from the Caucasians to explain less “weighty” fetuses among Blacks if fetal perfusion is a function of wider vascular capacitance.

Site-dependent variations may contribute significantly in differences in Doppler indices.[2],[26] However, the referenced authors also used cord sampling from a free loop of cord as done in the index study and hence reducing the chance of sampling site variability. This may buttress the perception that they may be environmental, geographical, and/or racial factors responsible for these inter- and intraracial variations. Jacquemyn and Verdonk reported that only small statistically significant differences exist between ethnic groups for UAD indices, and for clinical practice, the use of different reference charts for UAD may not be warranted,[27] but with significantly lower values of UAD from this study, compared to an equally local study by Ayoola et al.[4] There may be a need to generate uterine artery doppler references among different ethnic groups.[4]

  Conclusion/Recommendations Top

Reference values for fetal UAD indices were established for our obstetric population, and it showed a continuous inverse relationship for all the Doppler indices with gestational age which is consistent with previous studies reported by various foreign and local authors. It is, however, notable that interracial, intraracial, and/or geographical variations may exist due to significant difference in foreign and local UAD reference indices from this study. Due to lower but significant values compared with previous studies (both local and foreign), it is hereby recommended for use in our population for clinical judgment, especially among fetomaternal units, general obstetricians, and gynecologists.


The UAD values in this study were measured only once in pregnancy, a true representation would have been at serial measurements and to observe the neonatal outcome at birth.

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Conflicts of interest

There are no conflicts of interest.

  References Top

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Adekanmi AJ, Roberts A, Adeyinka AO, Umeh EO, Anor F, Odo CJ, et al. Normal second and third trimester umbilical and uterine artery indices amongst healthy singleton gestation Nigerian women. West Afr J Radiol 2017;24:1-7.  Back to cited text no. 3
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  [Figure 1], [Figure 2], [Figure 3], [Figure 4]

  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6], [Table 7], [Table 8]


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