This article focuses on hydronephrosis that is detected by means of antenatal ultrasonography. This method of surveillance detects a significant fetal anomaly in 1% of pregnancies, of which 20-30% of cases are genitourinary in origin, and 50% manifest as hydronephrosis. [1, 2, 3] If not for antenatal ultrasonographic detection, many of these urologic anomalies would manifest, as they did in the past, later in life as pyelonephritis, symptomatic flank or abdominal pain, renal calculi, hypertension, or even end-stage renal disease.
The degree and laterality of hydronephrosis may depend on the stage of pregnancy and the underlying etiology. Ultrasonography can detect the fetal bladder and kidney by 15 weeks' gestation and distinguish a central echo (renal sinus) by 18-20 weeks.  At 20 weeks' gestation, the fetus is larger, and an anomaly is easier to detect. Antenatal hydronephrosis has received significant attention since antenatal ultrasonography became a mainstream screening tool; however, management and treatment remain controversial in terms of patient outcome.
In addition, much of the controversy stems from diagnostic dilemmas and difficulties in ascertaining which lesions are obstructive and potentially harmful to the developing fetal kidney and other organ systems affected by renal function. In general, patients with obstructive uropathy that poses a significant risk of neonatal demise due to pulmonary hypoplasia may be considered candidates for antenatal treatment.
Once antenatal treatment is decided on, controversy remains regarding the efficacy of therapeutic intervention because of the limited knowledge of the underlying natural history and the difficulty of standardizing patient selection and determining appropriate outcome measures.  Furthermore, early diagnosis of hydronephrosis may cause significant parental anxieties during the rest of the pregnancy.
The ureteral bud arises from the mesonephric (wolffian) duct during the fifth week of gestation. It penetrates mesenchyme on the nephrogenic ridge, which is known as the metanephric blastema, and induces differentiation into renal parenchyma. Most nephrons are present by the middle of the second trimester, and differentiation is complete by 36 weeks' gestation.  The ureteral bud undergoes approximately 15 generations of division to complete the collecting system from collecting tubules proximally to the hemitrigone of the bladder distally.
Embryologically, the ureter begins development as a solid cord of tissue that lengthens and canalizes during development. Distal to the ureter, the urogenital sinus undergoes differentiation to form the bladder and urethra at 10 weeks' gestation and 12 weeks' gestation, respectively. Current technology does not allow renal imaging prior to completion of nephrogenesis.
The placenta, not the fetal kidney, functions as the fetal hemodialyzer maintaining salt and water homeostasis; however, the fetal kidney does begin producing hypotonic urine between weeks 5 and 9 of gestation and increases throughout gestation to reach rates as high as 50 mL/hr. 
Therefore, a deficiency at any point along the urinary tract can lead to transient or permanent partial or complete obstruction of urine flow, causing proximal dilation of the collecting system that manifests as antenatal hydronephrosis. This obstructive process may not be pathologic but, instead, the result of normal development; however, if significant obstruction is present and persistent, nephrogenic tissue can be affected, resulting in varying degrees of cystic dysplasia and renal impairment. 
Most anomalies of the urinary tract discovered in the prenatal period are characterized by hydronephrosis or dilatation of the upper urinary tract. Intuitively, these lesions may be considered obstructive in nature; however, antenatal hydronephrosis can be the result of nonobstructive processes, such as vesicoureteral reflux, nonrefluxing nonobstructed megaureter, and prune belly syndrome.
Obstructive lesions, particularly bilateral lesions, are more harmful to the developing kidneys, and the urine produced is a major component of amniotic fluid necessary for normal lung development and prevention of compression deformities. Therefore, differentiation of obstructive lesions and nonobstructive lesions is extremely important in determining the eventual outcome of the fetus. However, this may not be possible until the child is born.
Chronic partial unilateral ureteral obstruction has been experimentally demonstrated to result in significant renal maldevelopment, and early relief of the obstructive process is followed by significant hemodynamic recovery. [7, 8] Obstruction induces the renin-angiotensin-aldosterone system, causing vasoconstriction and subsequent interstitial fibrosis and ischemic atrophy, as well as induction of apoptosis in the obstructed kidney.
Josephson reported that obstruction is associated with decreased renal blood flow, glomerular filtration, and potassium excretion, and only a small percentage of this damage is benefited by early intervention. 
Numerous pathologic entities can cause antenatal hydronephrosis.
Antenatal hydronephrosis without associated urinary tract anomaly is the etiology in the vast majority of infants with hydronephrosis (79-84%) and has been termed isolated antenatal hydronephrosis (IAHN). IAHN is believed to be caused by a physiologic dilatation of the developing ureter.
As noted (see Pathophysiology), the ureter begins normal development as a solid cord of tissue that canalizes to allow unobstructed passage of urine. Metanephric urine production begins at approximately 8 weeks' gestation, potentially before completion of ureteral canalization. This results in transient obstruction with hydronephrosis. Once canalization is complete, this obstruction is relieved, and hydronephrosis should resolve.
The goal of evaluation is to differentiate benign physiologic dilation from significant obstructive disease or reflux.
United States statistics
Hydronephrosis is the most common pathologic finding in the urinary tract on antenatal screening by ultrasonography, accounting for 50% of all abnormal findings. The incidence varies among series because of criteria for dilation and timing of ultrasonography; however, the incidence of a significant uropathy in association with hydronephrosis is 0.2%. 
International studies have supported a similar incidence, with an incidence of 0.25% in Sweden and 0.92% in Great Britain.
Age-, sex-, and race-related demographics
Studies have uniformly shown that timing of hydronephrosis is important. Early onset of hydronephrosis in fetal development is directly related to prognosis. The incidence of antenatal hydronephrosis related to sex has not been reported. No known studies report the incidence of antenatal hydronephrosis related to race.
Most neonates with antenatal hydronephrosis have an excellent prognosis. Determination of mortality is difficult with antenatal hydronephrosis because of the significant incidence of stillbirths, terminated pregnancies, and missed diagnoses, all of which lead to underestimation of the true mortality. However, most research suggests that morbidity and mortality are directly related to the underlying etiology of hydronephrosis and the effect that the lesion has on the laterality, degree, and timing of hydronephrosis and resultant oligohydramnios.
Knowledge of the natural history of each disease entity that manifests as antenatal hydronephrosis provides a better understanding and estimation of morbidity and mortality than general surveys; however, a few statements can be made.
Obstructive lesions and lesions that affect both kidneys are uniformly more threatening than nonobstructive and unilateral lesions. The survival rate with unilateral renal obstruction approaches 100%, with only 15-25% of patients requiring surgery at 4 years' follow-up. [11, 12]
In the presence of a bilateral obstructive process, oligohydramnios is the best predictor of an adverse outcome. [13, 14] Fetal urine is a significant component of the amniotic fluid volume, and maintenance of adequate volumes is essential for normal lung development. If oligohydramnios is present, pulmonary hypoplasia and compression deformities of the skeletal system can result and significantly influence quality of life and survival (Potter syndrome).
The timing of oligohydramnios has a substantial effect on outcome. The earlier a lesion develops, the more likely it is to affect the fetal kidney, lungs, and overall outcome. In a series of 113 cases detected in the third trimester, mortality was 13%.  Detection in the second trimester was almost uniformly fatal, with an 83-100% mortality.  The most vulnerable period for pulmonary development is the second trimester; late-onset oligohydramnios exerts no adverse pulmonary effects. [17, 18] In general, the major determinant of survival is pulmonary development.
Many fetuses have an excellent prognosis, and this should be communicated to the parents. If a fetus has ultrasonographic findings suggestive of an adverse outcome, discussing the implications and providing the parents with information regarding further evaluation and management of their pregnancy is important.
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