Normal length of fetal kidneys: sonographic study in 397 obstetric patients.

This study was done to measure normal lengths of fetal kidneys sonographically during pregnancy. Knowledge of these measurements may allow earlier diagnosis of a variety of abnormalities. The greatest length of each of 498 kidneys in 397 consecutive fetuses between 18 and 41 weeks gestation was measured on sonograms. Gestational ages were determined by last menstrual period and biometry; significant discrepancies led to case exclusion. Abnormal fetuses, twins, offspring of diabetic mothers, and fetuses with renal pelvic dilatation of 4 mm or greater were excluded to avoid any questionable measurements. The results show that mean lengths are greater and confidence intervals are wider than previously reported. Renal lengths are similar to those reported in premature and full-term neonates. Strong correlation exists between renal length and gestational age, determined by biparietal diameter, femoral length, and abdominal circumference, and an average of the three. No significant difference was found between right and left renal lengths in fetuses in whom both kidneys were imaged. Average renal lengths are significantly different when compared across the range of gestational ages (p less than .001). No correlation is seen (r = .00) between parental height or weight and fetal renal length. Our results show that fetal renal lengths are longer than previously reported.