Assessment of fetal kidney size in normal gestation by comparison of ratio of kidney circumference to abdominal circumference.

With ultrasound one can outline fetal kidneys and evaluate intrarenal anatomy. In an effort to determine how fetal kidney size varies with gestational age, 89 patients not at risk for fetal kidney disease and whose pregnancies resulted in a normal outcome were studied from 12 weeks until term. Kidney diameter measurements were taken and found to increase linearly until term. Kidney circumference (KC) measurements were made and compared to abdominal circumference (AC) measurements at the level of the umbilical vein by means of a KC/AC ratio. This ratio was found to remain constant throughout pregnancy (0.27 to 0.30). In addition, three patients at risk for congenital kidney disease were studied and the fetal kidneys were found to have KC/AC ratios approximately 6 S.D. (KC/AC ratio greater than or equal to 0.45) above the normal mean. The diagnosis was confirmed by autopsy in two cases. These data are presented for reference in evaluating patients in the second and third trimester whose fetuses are at risk for kidney abnormalities.