Ultrasonographic evaluation of fetal kidney length as a reliable parameter for estimation of gestation age in 2nd & 3rd trimester

Abstract The fetal Kidney length can be used in estimation of gestational age. In late 2 and 3 trimester both kidneys can be visualized easily and can be measured accurately. In late 2 and 3 trimester the fetal kidney length can be used as an investigational tool alone or may be in addition of other parameter in estimation of gestational age. The most commonly used parameters used by sonologist can predict gestational age in 1 and 2 trimester. But as pregnancy advances, the accuracy of these parameters decreased. The length of fetal kidneys in mm collaborates very nicely with gestational age in weeks


Introduction
The accurate knowledge of gestational age is very important for obstetrician as well as mother (Kaul et al., 2012). Estimation of gestational age is also very helpful to categorize the fetus in normal fetus (normal for gestational pregnancies (chronic hypertension, severe preeclampsia, central placenta previa, severe IUGR, sensitized Rhnegative mother etc.) where in some times early termination may become compulsory as soon as fetus becomes mature (Kaul et al., 2012;. Any inaccurate estimation of gestational age may lead to perinatal morbidity and mortality due to iatrogenic pre-or postmaturity (Kansaria and Parulekar, 2009).
There are many parameters to measure gestational age. Some parameters are pre-nataly and some are post-nataly. Pre-natally, the date of the last menstrual period (LMP) and abdominal ultrasound scan are commonly used. Post-nataly, examination of the anterior vascular capsule of the lens, physical and neurological assessment is most commonly used (Opara, 2009).
As the technology advanced in sonography, the accuracy in estimatation of fetal age has become more demanding. Due to advancement in medical technology the expectation of people have increased and they expect exact estimation to plan for their pregnancy (Gupta et al., 2013).
Many researches show that fetal kidney length is very accurate assessment of gestational age estimation than bi parietal diameter (BPD), femur length (FL), head circumference (HC) and abdominal circumference (AC) after 24th week of gestation. 8 Fetal kidney has been shown a steady growth of 1.7mm fortnightly throughout pregnancy and is unaffected by growth abnormalities (Konje et al., 2002).

Methodology
Study was conducted at Gilani ultrasound centre, Ferozpur road, Lahore Pakistan. 371 female with a history of regular menstrual cycles, known date of last menstrual period and previous live normal neonates in multipara and having singleton pregnancy without any maternal complications or fetal anomalies were included in the study.
All pregnant women with any maternal complication e.g. hypertension, pre-eclampsia, maternal diabetes, central placenta previa, cases of oligohydramnios or polyhydramnios were excluded from study.
Fetal Complications such as twins pregnancy, severe IUGR, chromosomal, congenital abnormalities, dilated renal pelvis (>4mm) were also excluded from study. Toshiba Xario with 3-5MHZ curvilinear probe was used. Transabdominal scan was performed on every included woman whose gestational age was confirmed by early ultrasound scan and by her LMP. Scan was performed in supine position. Data was recorded on predesigned questionniare forms and data collection sheets. These were then tranformed to EXCEL. SPSS 21 version was used. Regression analysis between the values of fetal right and left kidney was used. A normogram of these values was made using the available data and it was compared with previous studies using t test. P value of 0.5 was considered as significant.

Results
A total 371 pregnant females between 18 to 40 years of age regadless of parity were examined with ultrasound BPD, FL and length of fetal both kidneys were measured. The present study showed strong correlation between mean fetal kidney length and gestational age. Mean length of both kidneys were presented in milimeters (mm) with 95% confidence interval. There was no association between gestationl age, maternal weight, height, parity or socioeconomic status. Table 1 shows Gestational age estimated from FL, Rt and Lf kidney, range, mean and standard deviation. Table 2 shows Strong correlation between gestational age according to FL, right and left kidney. The Pearson correlation coefficient r= 0.988 which shows a strong relation between right kidney length and gestational age according to FL (p value=0.000).    The scatter plot analysis shown in Fig. 1 and 2 show a correlation of gestational age estimated from FL with left kidney (Fig. 1) and a linear relationship between GA according to FL (weeks) and left kidney length of fetus (Fig.  2).  Table 2, Pearson correlation shows a strong correlation between right and left kidneys length of fetuses and gestational age as the P value is 0.000 which is much smaller than the level of significance (=0.05).

Discussion
The standard fetal biometric parameters used for estimation of GA during 2 nd and 3 rd trimesters despite the fact that as with advancing gestational age the accuracy of these parameters decreased. We evaluated the role of length of fetal kidneys for estimation of gestational age. The main aim of the study was to measure the length of normal length of fetal kidneys sonographically in the all cases of study during late 2nd and 3 rd trimester of pregnancy. We compared its accuracy with other standard biometric parameter.
Our measurements were at 27 weeks 27.6mm, at 30 weeks 30.4mm, at 34 weeks 34.7mm and at 38 weeks it was 39.3mm. Gupta et al. (2013) had also found strong correlation between FKL and GA. They also found length of FK 28.6mm at 27 weeks, 30.85mm at 30 weeks, 33.45 mm at 34 weeks and 39.45 mm at 38 weeks. Rehman et al. (2008) also reported the mean of fetal kidney length at 34 weeks it was 34.30mm and at 38 weeks was 37.30mm. Jayati Bardhan et al. (2016) also reported that fetal kidney length has strong correlation with gestational age of fetus and throughout pregnancy has steady growth rate.
JJ Kansaria (2009) in his study between 22 to 38 weeks of pregnancy found that mean kidney length increased at 24 weeks of gestation 23.87mm and at 38 weeks of gestation to 36.25.
There are number of factors that effect on readings of fetal kidney length i.e numbers of operators for example multiple versus double or single skilled operator, Skilled or non skilled operator, quality of ultrasound machine( newer vs older), study type (longitudinal vs cross sectional), and estimation of gestational age (exact or rounded off).
In our study a single skilled and well experienced sinologist performed all the sonographic measurements. The present study was non random study and new Toshiba machine was used.
The study population was from Lahore division, Pakistan. The length of fetal kidney better predict the gestational age than the biometric index model of FL. In all cases, with little manipulation in the position of the transducer both kidneys were easily visualized. Also, by angle insonation relative to the plane of kidney both fetal kidneys could be easily identified, which is in agreement with Gupta et al. (2013).
In present study, fetal kidney length showed strong correlation with gestational age and the correlation coeffient showed 98.8% association between gestational according to FL and both kidneys of fetuses and P<0.0001. For estimation of gestational age regression model with fetal kidney length was (R 2 =0.97). Regression model for the right and left kidney length and GA-FL is statistically significant (p-value = 0.000). Another study conducted by Sunipa Chatterjee et al. (2017) in Rajastan, india stated that fetal kidney length is much reliable parameter in dating labor that it can be used as individual parameter, especially when other parameters are not effective and much reliable.
Moreover, Ahmadi et al. (2015) investigated fetal kidney length in 557 fetuses between 26 to 39 weeks of gestational age and predicted that even when exact date of LMP is not confirmed by female then the fetal kidney length can be used as parameter for estimation of gestational age. The study showed strong correlation between gestational age and fetal kidney length.

Conclusion
Fetal kidney is easy to identify and to calculate its length due to lack of intestinal gases. Fetal kidney length is therefore a reliable parameter for the determination of gestational age in second and third trimesters of pregnancy.
In the late second and third trimester fetal kidney length is the most perfect and accurate parameter than other traditional parameters. Fetal kidney length can be used as an individual parameter for gestational age estimation. It could prove to be a valuable tool in cases where other established biometric indices are difficult to obtain show gross discrepancies with each other or with GA. The length of fetal kidney in mm collaborates very nicely with gestational age in weeks.