PURPOSE: In infancy it has been reported that intravesical pressure is generally 
higher in males than in females. We investigated whether there are significant 
differences in the sonographic characteristics of the urinary tract in male and 
female neonates.
MATERIALS AND METHODS: The study comprised 280 consecutive full-term newborns, 
including 146 males and 134 females. Kidneys were assessed for hydronephrosis 
and graded according to Society for Fetal Urology guidelines. Kidney 
measurements included maximum longitudinal length and largest longitudinal area. 
Bladder measurements included bladder dimensions and wall thickness. Bladder 
wall thickness was measured only in neonates with an estimated bladder volume of 
greater than 10 ml.
RESULTS: Of the 280 neonates 114 had mild renal pelvic dilatation in at least 1 
kidney and the incidence in males was significantly higher than in females (53% 
versus 27%). Kidney longitudinal length and largest area were significantly 
greater in males on each side. Mean bladder volume was essentially equal in the 
169 male and female neonates with an estimated bladder volume of greater than 10 
ml. but mean bladder wall thickness was significantly greater in males than in 
females (1.63 versus 1.38 mm.).
CONCLUSIONS: Our study shows that there are marked differences in sonographic 
findings in male and female neonates in regard to renal pelvic dilatation, renal 
size and bladder wall thickness. These differences should be considered when 
sonography is done for screening for urinary tract anomalies in newborns.