In the second and third trimesters of pregnancy, amniotic fluid volume is an indicator of fetal well-being and is an important measurement during antenatal fetal testing. The amniotic fluid dynamics are significantly influenced by renal perfusion.Using ultrasound imaging, the fetal renal circulation can be evaluated to determine the relation of renal artery flow velocity waveforms and amniotic fluid volume in normal pregnancies and those complicated by either polyhydramnios or oligohydramnios. The advent of fetal hemodynamic studies performed in conjunction with Doppler velocimetry of the umbilical arteries enabled clinicians to detect aberrantfetal blood flow and evaluate perfusion of the fetal organs. Technological advances have permitted the study of other vessels, including the renal artery, thereby improving fetal surveillance.

The fetal urogenital apparatus plays a major role in the formation of amniotic fluid throughout the second and third trimesters of pregnancy. Idiopathic polyhydramnios is more likely due to altered renal hemodynamics leading to increased filtration pressure. Amniotic fluid volumes have been described as oligohydramnios if the actual volume of the amniotic fluid is less than 500ml or as polyhydramnios if the volume is more than 2000ml^[1]. However, the definition of normal amniotic fluid has previously been made according to each gestational age. Rossi and Prefumo published some normative data for amniotic fluid index throughout pregnancy and noted that the mean amniotic fluid index changed weekly ^[2].

Oligohydramnios or polyhydramnios are characteristic features of structural and functional anomalies and signal to the health-care provider that additional assessments or antenatal testing is required ^[3].

Oligohydramnios has been associated with abnormalities, such as meconium staining, congenital anomalies, growth retardation, dysmaturity and fetal asphyxia ^[4,5]. Polyhydramnios has been associated with fetal structural abnormalities, aneuploidyand macrosomia ^[6]. Although polyhydramnios has a wide range of causes, including diabetes mellitus, isoimmunization, foetal sickness, & placental anomalies, the majority of cases (50–60%) seem to be idiopathic. An alteration in overall fetalhemodynamics in idiopathic cases of amniotic fluid disturbances may affect the peri-natal outcome which could be determined by the Doppler examination of the fetal RA and thereby identifying the subjects with increased risk of neonatal ICU admissions prior hand in the antenatal period.

The aim of this work was to study the relation of renal artery flow velocity waveforms and amniotic fluid volume in normal pregnancies and those complicated by either polyhydramnios or oligohydramnios.

This Prospective observational study was conducted during the period from February 2024 to January 2025.During the study period antenatal patients attending the OPD and IPD department of Obstetrics &Gynecology and were referred in the Department of Radiodiagnosis at M.Y .H &M.T.Hospital Indore.

Inclusion criteria:

In Pregnant women who were

1. Age: 18 Years and Above.
2. Single viable intrauterine pregnancy.
3. Who are coming for Target scan at Gestational age at 20 to 22weeks.
4. 4.Primigravida and multigravida.
5. 5.Who are willing to provide the informed consent.

Exclusion criteria:

1. Known fetal congenital malformations [like Anencephaly, Bowel Atresias, Spina Bifida, Bilateral renal agenesis etc.,]
2. Twin pregnancy.
3. Pregnant women already diagnosed with maternal complications [like Hypertension disorders, Diabetes and Cardiac diseases etc,.]

A written informed consent was obtained from all patients after full explanationof the study and handing over the patient information document.

• A detailed demographic and clinical history, including age, gravida, obstetric history was taken.
• All the information were recorded in the pre-formed Proforma.
• After PCPNDT registration detailed routine Target Obstetric scan was done(20to 22 wks) and eligible pregnant women were included in the study.
• At the end of the scan using curvilinear Transducer colour doppler of Fetal Renalartery Doppler is done.
• Pulsatility index(PI) of Fetal renal artery,Middle cerebral artery(MCA) andUmbilical artery were recorded along with the Amniotic fluid volume at 20 to 22wks and follow up done at 32 to 34wks of Gestational age.

Fetal renal artery doppler:

Pulsed wave Doppler sonographic studies was performed on theparticipants placed in the left lateral position with a 3.5-MHz convextransducer. The wall filter was set at the lowest available setting (50–75Hz) to preserve the end-diastolic component of the waveform. The anglebetween the ultrasound beam and the direction of blood flow wasmaintained below 30°.Evaluating renal artery blood flow, an axial imageof the fetal abdomen was obtained at the level of the fetal kidneys. Usingcolour flow Doppler, the renal arteries were evaluated at the level of theirorigin from the abdominal aorta. The Doppler gate then was placed withinthe lumen in a straight segment of the vessel.

Fig.1Method of measurement of Fetal renal artery doppler.

• To determine fetal well fetal wellbeing all patients were given a Non-stress test(NST). During labour fetal distress was determined by late deceleration dataon CTG(Cardiotocographic) .
• Pregnancy outcomes data were recorded in terms of gestational age at the time of delivery, mode of delivery.

Ethical Consideration

Approval from the institutional ethics committee was sought. Each study population was informed about the benefits and  risks of procedures of the study.

The age of the patients ranged from 18 to 40 years with highest proportion of participants in the 26-to-30-year age group.

Table 1. Comparison of Mean Values of Different Parameters Among AFI 20- 22 Wks Grades.

With comparision of PI of Fetal RA, MCA and UA among the different amniotic fluid index(AFI) groups Oligohydramnios, normal and Polyhydramnios.

• MCA PI values were fairly consistent across all groupsand are statistically non-significant (p =0.828).
• Similarly, UA PI showed a p-value of 0.538 and Fetal RA PI had a p-value of 0.339 both of which are also not statiscally significant.
• These findings suggest that at this gestational age window AFI status does not significantly affect Fetal Doppler indices.

Table 2. Comparison of Mean Values of Different Parameters Among AFI Grades At 32 – 34Wks.

At 32–34 weeks of gestation, a statistically significant difference was observed in Fetal Renal Artery Pulsatility Index (PI) among the three AFI groups — oligohydramnios, normal, and polyhydramnios. The mean Fetal RA PI was highest in the oligohydramnios group (2.81) and lowest in the polyhydramnios group (2.39). This difference was statistically significant (F = 190.610, p < 0.001).

In contrast, Middle Cerebral Artery PI (MCA PI) and Umbilical Artery PI (UA PI) did not show statistically significant differences among the AFI groups at this gestational age.

Table 3.Association Between AFI Grades and Mode of Delivery

On comparision of AFI follow up values and Fetal RA PI at 32 – 34 wks of gestational age between women who underwent caesarean section and those who had undergone Vaginal delivery.

AFI levels were significantly lower in the C-section group (mean 7.16 ± 5.126) compared to the vaginal delivery group (mean 9.52 ± 3.047), with a statistically significant difference (p = 0.001). Similarly, fetal RA PI was significantly higher in the C-section group (mean 2.68± 0.160) than in the vaginal delivery group (mean 2.56 ± 0.060), with a highly significant difference (p = 0.000).

Table 4. Association Between AFI Grades and Neonatal Outcome.

On comparision both Amniotic Fluid Index (AFI) and Fetal Renal Artery Doppler PI measured at 32–34 weeks showed a significant association with fetal distress at birth.

Neonates with fetal distress (11.3%) had a significantly lower mean AFI (4.65 ± 2.88) compared to those with normal outcomes (9.59 ± 5.75), with a p-value <0. 001.The mean renal artery PI was significantly higher in the fetal distress group (2.77 ± 0.16) than in the normal group (2.56 ± 0.06), also with a p-value < 0.001. 

Amniotic fluid volume, primarily maintained by fetal urine output in the second half of pregnancy, is a critical parameter in assessing fetal well-being. Fetal renal perfusion is closely tied to this process, and Doppler assessment of the fetal renal artery can provide insights into fetal hemodynamic adaptations. Specifically, the renal artery pulsatility index (PI) serves as a non-invasive marker of renal vascular resistance, which may reflect early changes in placental function or fetal hypoxia.

In our study population, the majority of pregnant women were aged 26–30 years (37.3%), followed by 33.3% in the 31–35 years group, 20.7% in the 18–25 years group, and 13.3% in the 36–40 years group.

At 20-22 wks of gestation, the distribution of amniotic fluid index (AFI) among 150 pregnant patients  was evaluated and the majority of participants (n = 136; 90.7%) exhibited normal AFI values, suggesting a predominantly healthy intrauterine environment during the mid-trimester. Oligohydramnios was noted in 9 participants (6%), while polyhydramnios was observed in 5 cases (3.3%).

When analysing the mean renal artery (RA) Doppler indices at 20 – 22wks among different AFI categories, the mean RA(PI) Doppler was slightly higher in the oligohydramnios group (2.788 ± 0.0172) compared to the normal group (2.770 ± 0.0451) and the polyhydramnios group (2.754 ± 0.0207). However, the difference was not statistically significant (F = 1.090, p = 0.339).Similar findings are seen in studies such as that by Kozuma et al. (1997), who reported no significant correlation between the renal artery Doppler indices , suggesting that in the second trimester, fetal renal hemodynamics remain relatively stable across AFI variations.

In Third trimester the renal artery (RA) Doppler values in relation to AFI grades,shows a statistically significant difference (F = 190.610, p = 0.000). The mean RA Doppler indices were highest in the oligohydramnios group (2.8100 ± 0.09124), followed by the normal AFI group (2.5599 ± 0.04796), and lowest in the polyhydramnios group (2.3900 ± 0.05148).This progressive decrease in RA resistance with increasing amniotic fluid volume indicates a strong inverse relationship between fetal renal vascular resistance and AFI status during late gestation.These findings are similar with previous studies like Acharya et al(2004) where the renal artery Doppler indices in healthy pregnancies decline slightly as gestation advances.Similarly Kessler et al.(2002) found that elevated renal artery resistance indices were significantly associated with oligohydramnios suggesting that reduced renal perfusion as a contributor to decreased amniotic fluid volume.

This pattern reflects the physiologic interplay between renal perfusion and urine output in the third trimester. In cases of oligohydramnios, elevated renal vascular resistance may reflect compromised placental perfusion, resulting in blood flow redistribution (centralization) to vital organs at the expense of the kidneys. 

In the present study,the relationship between amniotic fluid index (AFI) and the PI values of MCA and UA was evaluated at 20 to 22 weeks and 32 to 34 weeks of gestation and the findings revealed no statistically significant differences across the AFI categories,indicating that fetal circulatory dynamics remain largely stable during the mid and late trimester indicating that these parameters remain relatively stable in the absence of severe fetal compromise or placental dysfunction.

This MCA,whichreflectcs cerebral blood flow remains relatively stable regardless of amniotic fluid volume in the absence of overt fetal compromise. Similar findings have been reported by Griffin et al. (1994) and Gramellini et al. (1992), who noted that MCA PI changes are typically seen only in the presence of significant hypoxia or placental insufficiency. Thus, isolated AFI abnormalities may not significantly impact cerebral vascular resistance.

The UA PI, indicative of placental vascular resistance is not notably influenced by changes in amniotic fluid volume alone. These findings align with previous research by Wladimiroff et al. (1992), Arduini et al. (1993), and Baschat et al. (2003), who reported that significant UA Doppler alterations are more closely linked to placental insufficiency and fetal growth restriction rather than isolated AFI deviations.

We also evaluated the relationship between AFI, renal artery PI, and mode of delivery.It was observed that women who underwent Cesarean section (20.7%) had significantly lower mean AFI values (7.16 ± 5.126) compared to those who had vaginal deliveries (9.52 ± 3.047), with (p = 0.001) indicating that lower amniotic fluid levels are associated with higher caesarean rates.Similarly, the renal artery PI was significantly higher in the Cesarean group (2.68 ± 0.160) than in the vaginal delivery group (2.56 ± 0.060), with (p < 0.001).

When analyzed together, the coexistence of elevated RA PI and reduced AFI strongly predicts caesarean deliverylikely due to the greater incidence of fetal compromise, non-reassuring fetal heart rate patterns, or cord compression in such scenarios.Similar results are seen with Alfirevic et al. (2010) emphasized that low AFI in the third trimester was associated with increased rates of cesarean delivery, particularly in cases of oligohydramnios or IUGR, due to higher incidences of fetal distress and abnormal fetal heart rate patterns.

Our study also revealed a significant association between AFI, renal artery PI, and neonatal birth outcomes, particularly the incidence of fetal distress. Fetuses with distress (11.3%) had a significantly lower AFI (4.65 ± 2.876) compared to those with normal outcomes (9.59 ± 5.749) (p < 0.001). Similarly the mean renal artery PI was significantly higher in the fetal distress group (2.77 ± 0.158) than in those without distress (2.56 ± 0.060) (p < 0.001).This finding supports the well-established association between oligohydramnios and adverse perinatal events, including non-reassuring fetal heart rate patterns, meconium-stained amniotic fluid, and emergency caesarean deliveries. Similar findings are seen in studies like Parra-Cordero et al., 2011 where the integration of renal Doppler assessments in routine fetal surveillance, particularly for high-risk pregnancies is done to improve early detection of fetal compromise.

This study highlights the clinical significance of Fetal renal artery Doppler in predicting the Idiopathic oligohydramnios and polyhydramnios and perinatal outcomes during late gestation. In the 2nd trimester, no significant variation in Doppler was observed across different AFI categories, indicating relative circulatory stability. However in the 3rd Trimester there is a strong association between increased Fetal RA(PI)32wks and Amniotic fluid abnormalities and also the adverse outcomes like higher rates of cesarean delivery, preterm births, and increased incidence of fetal distress. In contrast, normal Fetal RA(PI) with AFI was consistently linked to favorable maternal and neonatal outcomes, reinforcing its clinical value as a key surveillance parameter.MCA and UA Doppler indices remained largely unaffected across AFI groups, reaffirming that renal artery Doppler may serve as a more sensitive marker for detecting subtle fetal compromise when traditional Doppler markers remain normal.

The study explains the importance of integrating fetal RA Doppler with routine AFI measurements at 32–34 weeks enhances the early identification of fetuses at risk,allowing for timely intervention and improved perinatal outcomes.

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