ORIGINAL RESEARCH |
https://doi.org/10.5005/jp-journals-10006-2121
|
Comparison of Ultrasonic Measurement of Fetal Kidney Length with Other Fetal Biometric Indices in Determining Gestational Age in Third Trimester in South Indian Population
1–3Department of Obstetrics and Gynecology, Mahatma Gandhi Medical College and Research Institute, SBV University, Puducherry, India
Corresponding Author: Aaina Garg, Department of Obstetrics and Gynecology, Mahatma Gandhi Medical College and Research Institute, SBV University, Puducherry, India, Phone: +91 8951567840, e-mail: aainagarg008@gmail.com
How to cite this article: Garg A, Habeebullah S, Rathod S. Comparison of Ultrasonic Measurement of Fetal Kidney Length with Other Fetal Biometric Indices in Determining Gestational Age in Third Trimester in South Indian Population. J South Asian Feder Obst Gynae 2022;14(5):587–591.
Source of support: Nil
Conflict of interest: None
Received on: 23 March 2022; Accepted on: 23 August 2022; Published on: 16 November 2022
ABSTRACT
Aim: To investigate the accuracy of fetal kidney length (FKL) in determining gestational age (GA) in the third trimester by ultrasound and comparing it with existing parameters.
Materials and methods: This is an observational and comparative study conducted over a period of 1 year in the Department of Obstetrics and Gynecology of a tertiary care teaching hospital located in Puducherry, India. The study included pregnant women between 28 weeks and 34 weeks. All had a dating scan (DS) in the first trimester. The total study population was 100. All patients underwent a growth scan, and FKL was determined during the same. This value was compared with a normogram, and gestational age was determined. This was compared with the gold-standard gestational age derived from crown-rump length (CRL). The software used was MedCalc software.
Results: The average FKL was positively correlated with CRL which was statistically significant (p <0.001). The Kappa analysis showed agreement of average FKL with CRL. Both sides of the kidney had comparable values, but the average length showed the best correlation to determine gestational age in the third trimester.
Conclusion: The study showed that FKL is an accurate parameter to date pregnancy. It correlated well in combination with the other routinely used parameters for the estimation of gestational age in the third trimester.
Clinical significance: The present study is one of the few studies conducted in the third trimester to date the pregnancy. It is applicable in low-resource settings where DS might not be available to accurately date the pregnancy.
Keywords: Crown-rump length, Dating pregnancy, Fetal kidney length, Gestational age, Observational study, Third trimester.
INTRODUCTION
Accurate GA estimation is critical for effective obstetric management and outcome, particularly in high-risk pregnancies. Failure to do so can lead to fetal complications like inadvertent pre- or post-term delivery.1 Furthermore, accurate GA assessment is required for induction of labor and planning the timing of a cesarean section.2 In developing countries, a significant number of women find it difficult to recall their precise last menstrual period date and often present in later trimesters where GA assessment becomes highly unreliable. Based on physical examination, GA can be estimated. Factors like uterine fibroids and maternal obesity, on the other hand, can affect the accuracy of this method. Multiple studies have been done comparing the traditional methods of clinically dating pregnancies with ultrasound-based dating and have come to a conclusion that the latter is a better choice in case of suboptimally dated pregnancies.3
In the first trimester, during DS, measurement of the CRL has shown accuracy in predicting GA with least discrepancy accounting to around 3–5 days (±2 SD).4 After 12–13 weeks of gestation, the CRL becomes less reliable because it is affected by fetal position. The biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), and femur length (FL) are found to be helpful in fetal evaluation in the second and early third trimesters. Because of the biological variability of size in relation to age, these biometric parameters become increasingly unreliable as gestational age progresses. Beyond 26 weeks, BPD has a standard error of ±3 weeks increasing near-term.5 Femur length is found to be a more accurate parameter in the third trimester.
Other indices have been used as well, including transcerebellar diameter, humeral length, occipito-frontal diameter, clavicular length, orbital diameters, binocular distance, fetal heel ossification, chest circumference, and foot length.6 The FKL was one such parameter studied in the third trimester with promising results. Thus, a sonographic measurement of FKL in normal singleton pregnancies served as the basis for this study. However, while the width and antero-posterior dimensions of the fetal kidney can vary in some conditions, such as intrauterine growth restriction (IUGR), the length of the fetal kidney grows steadily throughout pregnancy with no significant change associated with any underlying growth abnormalities.7 There have been several western studies on normal FKL, but a review of the literature has revealed that there have been very few such studies conducted in people of South Asian descent. Thus, the present study was undertaken to predict the efficacy of FKL to predict gestational age in the third trimester and compare it with other existing biometric indices.
MATERIALS AND METHODS
The present observational and comparative study was conducted in the Department of Obstetrics and Gynecology, Mahatma Gandhi Medical College and Research Institute (MGMCRI), Puducherry, India, between January 2020 and January 2021 in low-risk pregnant women between 28 and 34 weeks of gestation. The study population included 100 pregnant women in the third trimester attending the antenatal clinic of MGMCRI, Puducherry, India.
Inclusion Criteria
Pregnant women in the third trimester above 18 years of age with first-trimester DS.
Exclusion Criteria
Maternal exclusion criteria: Multiple pregnancies and medical complications in pregnancy like renal disease, hypertension, diabetes mellitus, or chronic illnesses.
Fetal exclusion criteria: Suspected IUGR, polyhydramnios, indistinct adrenal and renal borders, abnormal renal morphology, absent kidneys, gross fetal hydronephrosis, and fetal structural abnormality.
The normogram was taken from Shivalingaiah et al. study8 and the sample size was calculated by applying the mean standard deviation in the following formula: n = Z2(1 − α/2) p(1 − p)/d2 where n = required sample size, estimated error (d) = 0.1, α = 0.05, estimated standard deviation (σ) = 0.51, and sample size n = 100.
Flowchart 1: Methodology for study analysis
Abdominal Ultrasound
A transverse plane of the fetus was defined at the level of the four chambers of the heart, following which transverse scanning in a cephalo-caudal direction was done until the fetal kidneys were visualized, often at the level of the stomach, or just below it. The probe was then oriented through 90° to obtain the longitudinal axis of each kidney on either side of the midline tubular anechoic abdominal aorta. The largest longitudinal image showing both upper and lower poles of each kidney was obtained and frozen on the monitor screen. Using electronic calipers, kidney length was measured from the upper to the lower pole. Three measurements were taken per kidney to minimize intraobserver error, and the average value in millimeters was recorded in a worksheet. Care was taken to exclude the adrenal glands (AGs) from the measurements (Fig. 1). Both the fetal kidneys were measured, and an average was calculated, which was used in determining GA.
Fig. 1: FKL measurement
METHODOLOGY
Statistical Analysis
The data were entered with an excel sheet and were exported to MedCalc version 19.0 for further processing. The Pearson correlation and regression analysis have been performed to find the relationship between CRL with study parameters. The receiver-operating curve (ROC) analysis was done for the diagnostic accuracy of FKL. All values were considered significant, if the p-value was <0.05. This research was strictly fulfilling the guidelines and done after approval of Institutional Ethical Research Committee.
RESULTS AND DISCUSSION
The present study was undertaken with a view to find out the efficacy of ultrasound measurement of FKL in assessing the GA in the third trimester of pregnancy and to compare it with that of other fetal biometric parameters. One hundred women with a mean age of 25.60 ± 3.97 years having uncomplicated pregnancies were recruited in the third trimester.
The observations were as follows:
Age, parity, and BMI (mean = 27.07 ± 5.19 kg/m2) had no significant impact in the measurement of kidney length and further estimating gestational age.
The BPD, HC, AC, and FL, which are the routinely used parameters showed positive correlation with CRL, which is the gold standard measurement to estimate gestational age with a statistical significance of p <0.001 (Fig. 2).
The right and the left FKL was positively correlated with CRL, which was statistically significant (p <0.001) (Figs 3 and 4).
The average FKL was positively correlated with CRL, which was statistically significant (p <0.001) (Fig. 5).
Fig. 2: Regression plot for FL and CRL
Fig. 3: Regression plot for right FKL and CRL
Fig. 4: Regression plot for left FKL and CRL
Fig. 5: Regression plot for average FKL and CRL
Fetal kidney length measurements show a strong correlation with gestational age, with a correlation coefficient (r) of 0.939 and a p-value <0.05 in our study, which was comparable to the study done by Bardhan et al.9 They had a correlation coefficient of 0.99 with p-value <0.001. The difference in the left and right kidney length to establish GA was minimal with individual correlation coefficient being (r) 0.82 and 0.84, respectively, and area under the curve being 0.44 and 0.45, respectively. It was observed that the average of the two gave the best correlation coefficient of 0.90 with GA and maximum area under the curve of 0.46, similar to the findings noticed by Cohen et al. in their study.10
Receiver-operating Curve (ROC) Analysis
The diagnostic accuracy of study variables was compared with CRL to predict the gestational age (Fig. 6). Among the study variables, almost all the study variables were routine parameters for estimating gestational age, which showed significant accuracy. However, the FKL was not a routine parameter used for GA determination. The ROC analysis revealed that average FKL showed area under the curve of 0.469 as seen in Table 1.
Fig. 6: ROC analysis for study variables
Test result variable | Area under curve |
---|---|
LMP | 0.490 |
BPD | 0.434 |
HC | 0.658 |
AC | 0.648 |
FL | 0.704 |
RFKL | 0.459 |
LFKL | 0.444 |
AFKL | 0.469 |
The interrater reliability of the study variables with CRL was done using Kappa analysis. The Kappa analysis results were analyzed based on the following: 0.01–0.20 as none to slight, 0.21–0.40 as fair, 0.41–0.60 as moderate, 0.61–0.80 as substantial, and 0.81–1.00 as almost perfect agreement. The analysis results are described in Table 2. This analysis showed that the average FKL, right FKL, and left FKL has slight agreement with gestational age determination.
Values | Asymptotic standard error | Approximate T | Approximate significance | |
---|---|---|---|---|
AFKL | 0.141 | 0.035 | 8.101 | 0.000 |
LMP | 0.063 | 0.027 | 4.663 | 0.000 |
BPD | 0.004 | 0.014 | 0.316 | 0.752 |
HC | 0.026 | 0.019 | 2.166 | 0.030 |
AC | 0.041 | 0.024 | 2.951 | 0.003 |
FL | 0.025 | 0.020 | 1.965 | 0.049 |
RFKL | 0.080 | 0.029 | 4.608 | 0.000 |
LFKL | 0.121 | 0.033 | 7.036 | 0.000 |
Independent variables | Coefficient | Std. error | T | p |
---|---|---|---|---|
Constant | −0.4529 | – | – | – |
GA by DS | 0.1015 | 0.03549 | 2.859 | 0.0052 |
GA by LFKL | 0.4621 | 0.02684 | 17.216 | <0.0001 |
GA by RFKL | 0.4478 | 0.03000 | 14.924 | <0.0001 |
The ROC and Kappa analysis were carried out to determine the diagnostic accuracy and interrater agreement, respectively. The FKL shows better diagnostic accuracy for prediction of gestational age and slight interrater reliability to determine the gestational age. However, to the best of our knowledge, there are no research articles available using similar statistical tools. Hence, we could not discuss our results in these aspects and considered these research findings to be the first of its kind.
In our study, on follow-up, parameters for the fetuses who had growth restriction were studied and no significant difference in kidney length was noted on growth scan for them as shown by the multiple-regression analysis below. Similar findings were noted in studies done before. In case of growth restriction, macrosomia or fetal malformations, they noticed a change in other parameters, but the same was not true for kidney length. Hence, they concluded saying that FKL can be useful in such scenarios as an individual or adjunct parameter.9,11
Multiple-regression analysis was used to examine the relationship between AFKL as dependent variable and other variables as independent variables as shown in Table 3. It clearly demonstrated that the AFKL was influenced by CRL (dating scan) (p = 0.0052), left (p <0.0001), and right FKL (p <0.0001). All other study variables, including fetal growth restriction, did not influence the AFKL levels.
Shivalingaiah et al. found a strong correlation of FKL with the GA, including IUGR fetuses (r = 0.85), in late trimesters. Mean difference from GA was found to be the least with kidney length.8
Akram et al. in a cross-sectional study done on 399 pregnant women noticed that maternal conditions and fetal growth discrepancies directly influence the existing parameters (BPD, HC, FL, and AC), whereas the same is not true for FKL.11
In this study, sensitivity and specificity could not be determined as it was a cross-sectional study. More longitudinal studies are needed for the same. This study is one of the few studies conducted in the third trimester to date the pregnancy. Being a single-observer study, the interobserver bias was eliminated. Hence, the efficacy of FKL in assessing the GA in the third trimester of pregnancy was established by the findings of this study.
CONCLUSION
From the present study, it may be concluded that in the third trimester, FKL measurement is an accurate parameter to date the pregnancy and correlated well with other routinely used parameters for the estimation of gestational age.
ORCID
Aaina Garg https://orcid.org/0000-0001-6634-4963
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