hyperbilirubinemia , during neonatal period , is considerably frequent , and generally a benign problem . during the first week of life , approximately 60% of term babies have jaundice . thus most neonates are exposed to higher than normal levels of bilirubin in the first two weeks of life . the level of indirect bilirubin in the umbilical cord is 1 - 3 mg / dl normally . indirect bilirubin level increases to 5 - 6 mg / dl between 2 and 4 day and decreases to less than 2 mg / dl on 5 - 7 day . it reaches adult levels approximately 10 - 14 day of life in full - term babies . antenatal hydronephrosis is the most common urinary tract anomaly detected by prenatal ultrasound screening studies[2 , 3 ] . most neonates with antenatal hydronephrosis have a good prognosis with only 15 - 25% of patients requiring surgery during 4 years of follow - up . mallik and watson reported 350 infants with various antenatally detected urinary tract abnormalities between 1989 and 2003 , of which 48.6% ( 170/350 ) was non - specific renal pelvic dilatation . the most common final diagnosis of these patients is either transient or physiological hydronephrosis , secondary to large non - obstructive renal pelvis . etiology of transient hydronephrosis is uncertain but it may occur due to insufficient maturation of the pelviureteric junction muscles ; changes in ureteral and bladder muscle activity may also play a role especially in non - obstructive pelvis dilatation . it is becoming apparent that many of the antenatal hydronephroses spontaneously improve or resolve without any surgical treatment[4 , 6 ] . in a recent study , it was demonstrated that high bilirubin concentrations cause decrease in basal vesical tonicity and relaxation in both pre - contracted ureteral and vesical smooth muscles in vitro . the authors thought that hyperbilirubinemia may have probably contributed to spontaneous improvement of pelvic dilatation . so , we aimed to investigate whether there was any relationship between serum bilirubin levels and ap renal pelvic diameters or pelvicaliceal dilatation in newborn babies . thirty three jaundiced neonates ( group 1 ) and 22 healthy neonates ( group 2 ) were randomly selected . ekrem hayri stnda women 's diseases and maternity hospital in turkey , between may - december 2007 . our study has followed principles in the declaration of helsinki , and written consent was obtained from the patients ' parents . healthy term babies with ( group 1 ) and without ( group 2 ) jaundice were taken in the study . babies with onset of hyperbilirubinemia after 24 hours of birth were included in the study . preterm infants , infants with small for gestational age , with urinary tract infection ( uti ) , direct hyperbilirubinemia , those known to have antenatal hydronephrosis or another systemic health problem were not included . urinary system ultrasound ( us ) was done in both study groups by an experienced radiologist simultaneously with micro - bilirubin measurement . dilatation of ureter and/or pelvicaliceal system , renal length , renal pelvis ap diameter and in co - existence of hydronephrosis , its grading according to the society of fetal urology , were recorded . the statistical analysis was performed in spss 13.0 ( chicago , il ) in windows . student - t and chi - square tests were used to compare the difference between groups . relationship between bilirubin levels and ultrasonographic kidney measurements was tested using spearman 's rho test . group 1 and group 2 were identical regarding postnatal age , gender and weight ( p>0.05 ) ( table 1 ) . demographic properties of the study groups 
 p<0.01 / sd : standard deviation neither hydronephrosis nor hydroureter was detected in the study groups . left renal agenesis renal length and renal pelvis ap diameters were not found to be different between the study groups ( table 2 ) . there was no correlation between bilirubin levels and renal length and renal pelvis ap diameters ( p>0.05 ) . renal lengths and renal pelvis antero - posterior diameters in study groups sd : standard deviation renal length and renal pelvis ap diameters were not found to be different between the study groups ( table 2 ) . there was no correlation between bilirubin levels and renal length and renal pelvis ap diameters ( p>0.05 ) . renal lengths and renal pelvis antero - posterior diameters in study groups sd : standard deviation several studies pointed out that bilirubin has relaxant effect on smooth muscles such as arterial vessels and gastric fundus[810 ] . in a recent study , pflueger et al reported that pressor and prooxidant effects of angiotensin ii were attenuated in the hyperbilirubinemic gunn rats which did not have uridine diphosphate glucuronosyl transferase enzyme . they speculated that these effects may have arisen from the quenching of oxidative stress by bilirubin . in normal renal physiology , urine accumulates in the renal pelvis up to a certain volume and pressure , and then ureteropelvic junction ( upj ) opens . pelviureteral inhibitory reflex. this reflex is believed to regulate the passage of urine from the renal pelvis to the ureter . in addition ureteric distension closes the upj ; this reflex action is called the   ureteropelvic excitatory reflex as it seems to prevent reflux of urine through the upj and thus protects the kidney . the concept that the upj acts as a physiologic sphincter has been put forward . in normal physiologic conditions , opening of the ureterovesical junction occurs synchronously with the upj . murat et al focused on effect of bilirubin to the ureteral and bladder smooth muscles . they investigated whether long - term exposure to higher bilirubin levels might have a role in dilatation of ureteral and vesical smooth muscles that may cause recovery of transient pelvicaliceal dilatation . they investigated the effects of normal and high bilirubin concentrations on both basal and pre - contracted ureteral and bladder muscles in vitro . they reported that normal and high bilirubin levels caused no change in the basal ureteral tension , normal bilirubin levels caused no difference in basal bladder tension but high bilirubin concentrations caused a decrease in bladder tension . after potassium chloride ( kcl ) application to tissues for contraction of ureteral and bladder muscles , normal bilirubin caused 86.47.2% relaxation , high bilirubin caused 133.917.4% relaxation , which was statistically significant . in pre - contracted bladder muscles , normal and high bilirubin concentrations caused 35.32.2% and 53.53.5% relaxation , respectively , which was significantly higher in high bilirubin concentrations . as a result , they did not find any effect of hyperbilirubinemia on basal ureteral tension . but , on contrary hyperbilirubinemia may also worsen the outcome of pelvicaliceal dilatation rather than improve it , because it causes relaxation of pre - contracted ureteral tension , namely ureteral peristalsis . on the other hand , they found that hyperbilirubinemia caused decreased bladder muscle activity , in a dose dependent manner . they speculated that neonates with hyperbilirubinemia might have a more relaxed bladder which might result in lower resistance for the renal pelvis and might contribute to the improvement of hydronephrosis . they discussed that recovery of antenatal hydronephrosis might be , not directly , but partly associated with decreased bladder resistance to the urine flow . we therefore planned this study to investigate whether relationship between serum bilirubin levels and pelvicaliceal dilatations in the newborn babies exists . however , in our study renal length , renal parenchymal thickness and renal pelvis ap diameter of both kidneys were not different between hyperbilirubinemic and normobilirubinemic newborns . so , it is thought that hyperbilirubinemia might not have a direct effect in outcome of the pelvicaliceal dilatation . on the other hand , more relaxed bladder and urinary stasis is one of the most important reasons for uti[13 , 14 ] . besides a lot of study pointed out that uti was relatively frequent in neonates with hyperbilirubinemia[1519 ] . bacteriuria in asymptomatic neonates is approximately 0.71.4% , however the prevalence of uti is between 5% and 11% in hyper - bilirubinemic neonates . uti in hyperbilirubinemic neonates shows co - existence with anatomical abnormalities of the urinary tract infection such as vesicoureteral reflux , hydronephrosis and pelvicaliectasis[17 , 18 ] . interestingly , in a recent study , xsinias et al reported that increased bilirubin level was related to renal damage in dmsa scintigraphy . besides these different observations , the relationship between uti and hyperbilirubinemia has not between clearly determined . in our study we were not able to demonstrate any relationship between serum bilirubin levels and renal pelvic diameters and pelvicaliceal dilatations in hyperbilirubinemic neonates . so , it is thought that hyperbilirubinemia might not have a direct effect on outcome of the pelvicaliceal dilatations . although we were not able to show association between hyperbilirubinemia and pelvic diameters , as a limitation of our study , our study sample was small and we did not have follow - up data of the included babies . further studies with larger sample size are required to draw more precise conclusions regarding this subject .
objectivea recent study reported association of high bilirubin concentrations with decrease in basal vesical tonicity and relaxation of pre - contracted ureteral and vesical smooth muscles in vitro , and authors discussed that recovery of antenatal hydronephrosis might partly be associated with decreased bladder resistance to the urine flow due to hyperbilirubinemia . 
 we aimed to investigate whether any relationship between serum bilirubin levels and antero - posterior renal pelvic diameters or pelvicaliceal dilatations exist during newborn period.methodsneonates with hyperbilirubinemia ( group 1 ) and healthy neonates ( group 2 ) were randomly selected to the study . 
 capillary blood samples were used to measure micro - bilirubin . 
 urinary system ultrasound ( us ) was performed in both groups by an experienced radiologist.findingsgroup 1 ( 31 neonates , 16 males , 15 females ) and group 2 ( 22 neonates , 11 males , 11 females ) were identical by means of postnatal age , gender and weight ( p>0.05 ) . 
 mean serum bilirubin levels were 11.13.1 mg / dl and 1.40.2 mg / dl in group 1 and 2 , respectively . 
 renal length and renal pelvis antero - posterior ( ap ) diameters were not different between study groups . 
 pelvis ap diameters of right kidney were 2.10.7 mm in group 1 and 1.90.7 mm in group 2 , and of left kidney were 2.40.8 mm in group 1 and 2.30.6 mm in group 2 . 
 there was no correlation between bilirubin levels and renal length and renal pelvis ap diameters ( p>0.05).conclusionin this study we were not able to demonstrate any relationship between serum bilirubin levels and renal pelvic diameters and pelvicaliceal dilatation in hyperbilirubinemic neonates . 
 so , it is thought that hyperbilirubinemia might not have a direct effect on outcome of the pelvicaliceal dilatation .