Preliminary Study On The Method Of Predicting Birth Weight With Fetal Kidney Volume Assessed By Two-dimensional Ultrasound

Objective: To investigate the methods of predicting fetal birth weight and fetal growth with fetal kidney size assessed by two-dimensional ultrasound.Methods: The biparietal diameter, head circumference, abdominal circumference, femur length, bilateral kidney length, kidney width and kidney thickness of 319 singleton fetuses were measured by two-dimensional ultrasound. Bilateral kidney volume of all fetuses was calculated. All infants were delivered within 72 hours after the ultrasound examinations. The correlation between the kidney volume and the birth weight was analyzed. Using kidney volume as the independent variable and birth weight as the dependent variable, scattergrams was made by Excel. A best-fit formula was generated for the kidney volume to predict birth weight from polynomial regression analysis. The accuracy of the kidney-volume formula was compared with those of two formulas commonly used in clinical practices. Comparing kidney's every diameter of appropriate for gestational age fetal with that of small for gestational age fetal to look for an age-independent predictor of small for gestational age infant. In addition, another group of 150 fetuses was measured for prospective validation.Results: The fetal kidney volume assessed by two-dimensional ultrasound was highly correlated with birth weight (r=0.89, n=319, p<0.05). The best-fit formula for kidney volume to predict birth weight was linear. Its predicting absolute error (191-.1g) and absolute percent error (5.9%) were all smaller than those of the othertwo formulas. It was superior to the other commonly used two-dimensional formulas in predicting macrosomia birth weight (absolute error was 157.0g and absolute percent error was 3.7%). The result that small for gestational age fetal always has altered renal morphology, producing 'sausage-shaped' kidneys is similar as other literatures'. Kidney length to kidney width ratio can predict small for gestational age infant as an age-independent index. The cut-off value was 1.78. Its sensitivity and specificity of predicting small for gestational age infant was 80% and 99.3%, respectively. In addition, these results were also confirmed in the prospective-validation group.Conclusions: It is possible of predicting birth weight with kidney volume assessed by two-dimensional ultrasound, and it may improve the accuracy of macrosomia weight prediction in clinical practice. However, a large-scale prospective validation study may be needed to reconfirm the conclusion of kidney length to kidney width ratio as an age-independent predictor for small for gestational age infant.