 Introduction/Background Antenatal hydronephrosis is the most frequent urinary tract anomaly detected on prenatal screening by ultrasonography (US). Antenatal hydronephrosis occurs approximately twice as often in males as in females [1-4]. With an estimated 4 million births per year in the United States, up to 80,000 fetal studies may detect antenatal hydronephrosis. Most antenatal hydronephrosis is transient with little long-term significance [5-8]. Few children with antenatal hydronephrosis, including some with ureteropelvic junction obstruction (UPJO), vesicoureteral reflux (VUR), and primary megaureter will have significant obstruction, develop symptoms or complications, which requires surgical intervention. Some male children will be diagnosed with more serious conditions, such as posterior urethral valves (PUV), which may require intervention. Early detection and treatment of obstructive uropathy is necessary to mitigate the potential morbidity from loss of renal function. During prenatal evaluation, most antenatal hydronephrosis is mild, and the cause cannot always be determined with certainty. As a result, postnatal evaluation of these children is frequently performed. In an effort to standardize grading of antenatal hydronephrosis, the Society for Fetal Urology (SFU) introduced a five point grading system in 1988, based on degree of pelvic and calyceal dilation and the thickness of the parenchyma overlying the calices [9]. Since its introduction, the SFU grading system has become the most widely used method to grade pediatric hydronephrosis [10]. Subsequently, several alternative classification schemes have also been devised [3,4,11,12].
Antenatal Hydronephrosis Infant.  Introduction/Background Antenatal hydronephrosis is the most frequent urinary tract anomaly detected on prenatal screening by ultrasonography (US). Antenatal hydronephrosis occurs approximately twice as often in males as in females [1-4]. With an estimated 4 million births per year in the United States, up to 80,000 fetal studies may detect antenatal hydronephrosis. Most antenatal hydronephrosis is transient with little long-term significance [5-8]. Few children with antenatal hydronephrosis, including some with ureteropelvic junction obstruction (UPJO), vesicoureteral reflux (VUR), and primary megaureter will have significant obstruction, develop symptoms or complications, which requires surgical intervention. Some male children will be diagnosed with more serious conditions, such as posterior urethral valves (PUV), which may require intervention. Early detection and treatment of obstructive uropathy is necessary to mitigate the potential morbidity from loss of renal function. During prenatal evaluation, most antenatal hydronephrosis is mild, and the cause cannot always be determined with certainty. As a result, postnatal evaluation of these children is frequently performed. In an effort to standardize grading of antenatal hydronephrosis, the Society for Fetal Urology (SFU) introduced a five point grading system in 1988, based on degree of pelvic and calyceal dilation and the thickness of the parenchyma overlying the calices [9]. Since its introduction, the SFU grading system has become the most widely used method to grade pediatric hydronephrosis [10]. Subsequently, several alternative classification schemes have also been devised [3,4,11,12].