Accurate determination of gestational age (GA) forms the cornerstone of effective obstetric management, influencing decisions on fetal monitoring, timing of delivery, and management of high-risk pregnancies. Conventionally, biometric parameters such as biparietal diameter, head circumference, abdominal circumference, and femur length are used to estimate GA. However, these indices may show variability in the second and third trimesters due to intrauterine growth restriction or constitutional differences. This highlights the need for alternative parameters with greater reliability across gestational stages.

Fetal kidney length (FKL) has emerged as a promising adjunctive marker due to its steady, linear growth pattern during mid- and late gestation [1, 2, 4, 6,7]. The kidney develops progressively from the mesonephric to metanephric stage, with measurable sonographic dimensions from the second trimester onward [8, 12, 13, 15]. Several studies across diverse populations, including Nigeria, Bangladesh, and India, have established a strong correlation between FKL and GA, underscoring its reproducibility and predictive value [9–11].

Furthermore, preterm birth remains a significant global challenge, with approximately fifteen million cases annually, reinforcing the importance of accurate GA estimation for neonatal survival [3]. By integrating FKL into obstetric ultrasound, clinicians can enhance diagnostic accuracy and strengthen perinatal outcomes, particularly when conventional parameters are limited [5, 14].

The fetal kidney undergoes a complex developmental sequence, beginning from the pronephros and mesonephros to the definitive metanephros, with contributions from the intermediate mesoderm [Chung KW, Chung HM (16); Nigam SK, Aperia AC, Brenner BM (17)]. Standard anatomical descriptions, as outlined in Gray’s Anatomy, highlight its maturation and ascent during early gestation, enabling sonographic visualization by the second trimester [Standring S (18)]. Ultrasonography has become a vital diagnostic tool in obstetrics, providing non-invasive and reliable imaging for fetal growth assessment [Middleton WD (19); Beckmann CRB (20)].

The application of obstetric ultrasound for dating pregnancies and detecting anomalies has been well-established, with structured protocols guiding its use in modern practice [Chudleigh T, Thilaganathan B (21); ACOG Practice Bulletin (22)]. Specific to renal imaging, early works demonstrated the value of ultrasonic evaluation of fetal kidneys in estimating gestational age [Lawson TL (23); Gonzales J (24)]. Quantitative studies confirmed a strong correlation between kidney length and gestational milestones [Bertagnoli L (25)], while subsequent research highlighted the progressive sonographic appearance of fetal kidneys across pregnancy [Bowie JD (26); Sagi J (27)].

Later investigations emphasized the consistency of fetal kidney length as a predictor of gestational age in both the second and third trimesters [Sampaio FJ (28); Campbell S (29); Wladimiroff JW, Eik-Nes SH (30)]. Collectively, these findings underscore its reliability as an adjunct parameter, supporting its integration into routine obstetric ultrasonography.

Aims & Objectives:

Aim-
To evaluate the efficacy of fetal kidney length (FKL) in predicting gestational age (GA) during the second and third trimesters at a tertiary care centre in Indore.

Objectives -

Primary Objective

• To determine the correlation between fetal kidney length and gestational age and compare its accuracy with conventional biometric parameters such as biparietal diameter, femur length, head circumference, and abdominal circumference.

Secondary Objectives

1. To assess the reliability of FKL as an independent marker of gestational age, particularly in late pregnancy when traditional parameters may lose precision.
2. To analyze the growth pattern of fetal kidneys across gestational weeks using ultrasonography

Study Design: Descriptive observational

Study Type:  Prospective cross-sectional study

Study Setting: Department of Radio diagnosis, Mahatma Gandhi Memorial Medical College & M.Y. Hospital, Indore, Madhya Pradesh, India.

Study Population:Pregnant women with singleton pregnancies between 20–40 weeks referred for routine antenatal ultrasonography.

Sample Size:A total of 370 antenatal women were included.

Where:

• Z = 1.96 (95% confidence level)
• Sensitivity = 77%
• Specificity = 92%
• d = 5% margin of error

Final calculated sample size = 370.

Inclusion Criteria:

• Singleton pregnancies between 20–40 weeks.
• Mothers with a reliable last menstrual period (LMP).
• Normal antenatal cases without maternal or fetal risk factors.

Exclusion Criteria:

• Gestational diabetes mellitus.
• Renal pelvic dilatation ≥ 5 mm.
• Multiple gestations.
• Congenital renal anomalies or maternal renal diseases.

Data Collection Methods:

• Each participant underwent ultrasonographic examination using a 2–5 MHz curvilinear transducer.
• Fetal kidneys were measured bilaterally from the upper to the lower pole in longitudinal section; the mean value was taken as FKL.
• Standard parameters (BPD, FL, HC, and AC) were also recorded for comparison.

Procedure:
FKL was correlated with gestational age (GA) derived from LMP and composite biometric indices. Care was taken to avoid oblique or partial views.

Data Analysis:

• Data were entered in Microsoft Excel and analyzed using SPSS v25.0.
• Continuous variables were expressed as mean ± SD.
• Pearson’s correlation coefficient and regression analysis assessed the association of FKL with GA.
• Comparative analysis with BPD, FL, HC, and AC was performed using ANOVA/t-tests.
• A p-value <0.05 was considered statistically significant.

Demographic and Clinical Characteristics

Table 1: Demographic and Clinical Characteristics of Study Participants (n = 400)

Statistical Tests Used: Chi-square test     , >0.05 = Non-significant

Interpretation:

 The demographic and clinical profile of the study population highlights a predominantly young antenatal cohort, with the majority (61.3%) aged 19–25 years, reflecting early maternal age at pregnancy. Most women were multiparous, with gravida 2 and 3 constituting 68% of the group, indicative of representative obstetric diversity. Nutritional status was favorable, as nearly 70% had a normal BMI, while underweight, overweight, and obesity remained less common. Gestational age distribution revealed maximum enrollment in the mid-second trimester (21–25 weeks, 48.8%), which is the optimal period for reliable ultrasonographic fetal growth assessment. The highly significant p-values (<0.001) across all domains underscore the structured and non-random characteristics of this cohort, strengthening its reliability for evaluating fetal kidney length as a predictor of gestational age.

Graph Interpretation:

Graphical analysis illustrates the strong clustering of participants in the younger maternal age group and in women with normal BMI, reinforcing the representative nature of the study cohort. Parity distribution shows a balanced mix, with a predominance of gravida 2, highlighting the inclusion of both primigravida and multiparous women. Gestational age peaked in the second trimester, aligning with the period of consistent fetal growth patterns. These visual patterns, supported by statistically significant results, affirm that the study population was well-suited for evaluating fetal kidney length as a reliable biomarker of gestational age.

Table-2: Mean values of key fetal biometric parameters across different gestational age groups.

Statistical Tests Used: Chi-square test        , >0.05 = Non-significant

Interpretation:

The demographic and clinical data reveal that the majority of mothers were in the younger age group of 19–25 years, reflecting early childbearing common in the region. Parity distribution showed a predominance of gravida 2, followed by gravida 3, ensuring balanced representation of primigravida and multigravida cases. Nutritional status was favorable, with nearly 70% of women maintaining a normal BMI, while underweight and obesity were less frequent. Gestational age distribution demonstrated that almost half of the women were in the 21–25 weeks group, highlighting optimal recruitment during the mid-trimester, a crucial period for fetal biometric assessments including kidney length.

Graph Interpretation:

The graphical trends highlight that the study population was predominantly composed of younger women with normal BMI, ensuring a representative antenatal cohort. Parity distribution demonstrated adequate inclusion of both first-time and experienced mothers, adding strength to the clinical spectrum. Gestational age clustered around the second trimester, coinciding with the period of steady fetal organ development and reliable sonographic measurements. Together, the graphs visually reinforce the balanced and well-distributed study population, supporting the positive utility of fetal kidney length in gestational age prediction across varying maternal and fetal conditions.

Table 3: Correlation between Fetal Kidney Length and Gestation Age (USG).

Statistical Tests Used: Chi-square test            , >0.05 = Non-significant

Interpretation:

 The correlation analysis clearly demonstrates the strong predictive value of fetal kidney length in estimating gestational age. Both right and left kidney lengths showed high positive correlations with gestational age, with Pearson coefficients of 0.948 and 0.932 respectively, indicating excellent reliability. The mean kidney length also exhibited a highly significant correlation (r = 0.944, p < 0.001), further validating its role as a dependable marker. These consistent findings highlight that fetal kidney length can be considered a robust and reproducible parameter for gestational age assessment, particularly in later trimesters when conventional measurements may show greater variability

Graph Interpretation:

The scatter plot with regression lines illustrates a clear and consistent linear association between gestational age and fetal kidney length. Both right and left kidneys display parallel growth patterns, while the mean kidney length strengthens predictive accuracy by reducing inter-lateral variability. The upward trajectory across gestational weeks highlights uniform renal development, particularly valuable in later trimesters. These findings visually reinforce the statistical correlations, demonstrating that fetal kidney length is a dependable and reproducible parameter for gestational age estimation in obstetric ultrasonography.

Table 4: The strength of correlation between fetal kidney length and conventional biometric indices.

Interpretation:

The merged correlation table demonstrates that fetal kidney length shows consistently strong associations with standard biometric parameters used to estimate gestational age. Among them, abdominal circumference (AC) revealed the highest correlations (right r=0.975, left r=0.962, mean r=0.972; p<0.001), highlighting the close relationship between somatic growth and renal development. Head circumference (HC), femur length (FL), and biparietal diameter (BPD) also displayed highly significant correlations (r≈0.94–0.96), reinforcing the robustness of kidney length as a gestational marker. In contrast, fetal kidney width showed only a weak yet statistically significant correlation (r≈0.155, p=0.002). These findings confirm kidney length as a reliable adjunct for gestational age estimation, particularly in later trimesters.

Graph Interpretation:

The scatter plot with regression line visually highlights the strength of correlation between fetal kidney length and conventional biometric indices. A clear upward trend is observed across BPD, FL, HC, and AC, with AC demonstrating the strongest correlation. The consistently high r-values indicate the uniform reliability of kidney length in parallel with established parameters. In contrast, kidney width shows only a minor contribution, confirming its limited clinical utility. The regression line emphasizes the linearity of these associations, supporting the integration of fetal kidney length into obstetric ultrasound protocols for gestational age assessment.

The present study demonstrates that fetal kidney length (FKL) is a reliable and reproducible sonographic parameter for estimating gestational age in the second and third trimesters. Our findings align with earlier research emphasizing the role of renal dimensions as a consistent growth marker when conventional indices such as biparietal diameter, femur length, head circumference, and abdominal circumference may lose accuracy in late pregnancy. Jeanty and Romero highlighted the clinical importance of developing adjunct tools in sonographic practice to improve accuracy in GA assessment, particularly in high-risk pregnancies³¹. Similarly, Sebire and Nicolaides emphasized the value of additional markers in screening for fetal development³². Several Indian and international studies, including those by Shivalingaiah et al. and Das et al., have demonstrated strong correlations between FKL and GA, particularly in the third trimester, reinforcing its predictive value³³˒³⁴.Classical anatomical and biometric works, such as those by Gonzales and Sagi, first established the direct relationship between kidney size and gestational age³⁵˒³⁶. More recent studies by Gandhi et al., Eissa et al., and Francis et al. confirmed that FKL provides comparable or superior accuracy to conventional parameters, especially after 20 weeks of gestation³⁷–⁴⁰. These findings mirror our results, where mean kidney length displayed a highly significant correlation with GA, underscoring its clinical robustness. Supporting literature by Bertagnoli et al., Chudleigh and Thilaganathan, and Middleton further highlight the technical feasibility and reliability of renal sonography in obstetric practice⁴¹–⁴³. Collectively, the evidence suggests that integrating FKL into routine ultrasonography could enhance the precision of GA estimation, particularly in late pregnancy, thereby contributing positively to perinatal care outcomes.

Implications for Clinical Practice:

 The findings of this study demonstrate that fetal kidney length (FKL) is a highly reliable parameter for estimating gestational age (GA) in the second and third trimesters. Its strong correlation with GA, along with consistent growth across gestational weeks, positions FKL as a valuable adjunct to conventional biometric indices such as biparietal diameter, femur length, head circumference, and abdominal circumference. Importantly, FKL retains accuracy in late pregnancy when traditional parameters may lose precision due to intrauterine growth restriction, oligohydramnios, or constitutional variation.

For clinicians, incorporating FKL into routine ultrasonography can enhance diagnostic confidence, particularly in high-risk pregnancies where accurate dating is crucial for decision-making regarding antenatal monitoring, timing of delivery, and neonatal preparedness. Its reproducibility and linear growth pattern make it a dependable tool even in resource-limited settings, where advanced imaging may not always be feasible. Additionally, the use of FKL may reduce misclassification of gestational age, thereby minimizing unnecessary interventions and optimising maternal and neonatal outcomes.

Overall, integrating FKL into obstetric ultrasound protocols can strengthen the accuracy of GA estimation, improve perinatal care strategies, and support evidence-based clinical practice in both routine and high-risk obstetric populations.

Limitations:

 Although this study provides strong evidence supporting fetal kidney length (FKL) as a reliable marker of gestational age, a few limitations should be acknowledged. Being a single-centre, hospital-based study, the findings may not fully reflect community-level populations. The sample, though adequate, excluded multiple gestations and pregnancies with fetal or maternal complications, slightly narrowing generalizability. Inter-observer variation in sonographic measurement, though minimized through standardized protocols, cannot be entirely excluded. Despite these constraints, the large sample size, rigorous methodology, and consistent correlations affirm the robustness of results and their practical clinical utility.

This study highlights fetal kidney length (FKL) as a highly reliable and reproducible sonographic parameter for estimating gestational age in the second and third trimesters. The findings demonstrate strong correlations between FKL and gestational age, often comparable or superior to conventional biometric indices. Its accuracy remains consistent even in late pregnancy, where traditional parameters may lose precision. The large sample size and robust methodology strengthen the validity of results. Incorporating FKL into routine ultrasonography can improve diagnostic accuracy, optimize pregnancy monitoring, and support timely clinical decision-making, ultimately contributing to better maternal and neonatal outcomes

Acknowledgment 

  The authors sincerely thank the Department of Radiodiagnosis, Mahatma Gandhi Memorial Medical College & M.Y. Hospital, Indore, for providing the infrastructure and support essential to this study. We express our gratitude to the faculty and colleagues for their guidance and constructive inputs throughout the research process. Special appreciation is extended to the participating patients and their families, whose cooperation made this work possible. Their contribution remains invaluable in advancing clinical knowledge and improving obstetric care practices.

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