Antenatal Hydronephrosis (Urinary Tract Dilation) Workup

Updated: Jul 27, 2021
  • Author: Dennis B Liu, MD; Chief Editor: Marc Cendron, MD  more...
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Laboratory Studies

Theoretically, a fetus with oligohydramnios and good renal function can benefit from intervention in utero. Therefore, developing methods to evaluate fetal renal function by analyzing fetal urinary components may be helpful in determining which fetuses have the most potential benefit from such intervention. With the help of ultrasonographic (US) guidance, samples of fetal urine can be obtained and analyzed. This should be performed at a specialized center that has experience in high-risk obstetrics and fetal intervention.

A retrospective review from the University of California, San Francisco, helped initiate evaluation of urine constituents as a marker of renal function into mainstream practice. [25] A healthy fetus produces hypotonic urine that becomes isotonic in the presence of progressive renal damage that impairs proximal tubular function. Elevations in urinary sodium, chloride, calcium, alpha2-microglobulin, and osmolality indicate renal injury and potentially irreversible dysplasia. [26]

Urinary calcium is currently thought to be the most sensitive predictor of renal dysplasia. These measurements are inherently difficult to evaluate because of the dynamic nature of fetal renal function; however, evaluation of multiple urinary components on serial examinations can improve diagnostic accuracy. [27]

Individual values are believed to have variable accuracy, but the combination of urinary sodium less than 100 mg/dL, osmolality less than 200 mOsm/L, and total protein less than 20 mg/dL on the third or fourth bladder tap is generally associated with normal renal function (see Table 1 below). [28] Currently, evaluation of urinary components by means of vesicocentesis is an extremely valuable tool in determining which fetuses are candidates for in-utero intervention.

Table 1. Urinary Components and Their Usefulness in Predicting Renal Dysplasia (Open Table in a new window)






Predictive Value


Predictive Value


< 100 mg/dL






< 8 mg/dL






< 200 mOsm/L






< 4 mg/dL





Total protein

< 20 mg/dL







Antenatal US is the imaging modality that originally brought the patient with antenatal hydronephrosis (dilatation of the upper urinary tract) to medical attention. This is the main diagnostic imaging method for evaluating and monitoring the fetus. Antenatal detection of fetal genitourinary abnormalities by means of US was first reported in 1970. [29]  Since then, assessment of the genitourinary system has been routine, and it should be part of every fetal US examination.

If an abnormality is found on screening or dating US, a detailed study is performed to evaluate the collecting system architecture, renal architecture and size, parenchymal echogenicity, amniotic fluid volume, and bladder fullness and function. [28]  Antenatal US is helpful in formulating a differential diagnosis of the genitourinary lesion and other coexistent fetal anomalies that can affect the treatment and clinical outcome of the fetus.

The degree of hydronephrosis is significant and can herald an obstructive process. This finding is not synonymous with the diagnosis of obstruction; however, the likelihood of having a significant urinary tract abnormality that is obstructive in nature is directly proportional to the severity of hydronephrosis. [2, 30]

Renal anterior-posterior (AP) pelvic diameter (APD) is commonly used to evaluate the significance of dilation. In one series, surgery or at least long-term monitoring for a significant urinary tract lesion was required by 94% of fetuses with a renal APD exceeding 2 cm, 50% of those with an APD of 1-1.5 cm, and only 3% of those with an APD smaller than 1 cm. [31]  In a subsequent study, a renal APD of at least 4 mm before 33 weeks' gestation and at least 7 mm after 33 weeks' gestation was considered significant. [32]  Caliectasis correlates best with the presence of a significant dilation or an obstructive process.

Grading systems in current use include the Society for Fetal Urology (SFU) system [33]  and the Urinary Tract Dilation (UTD) system. [34]  A study by Dos Santos et al found that a grading system using a combination of APD and diffuse caliectasis was able to identify children who were more likely to require surgery, particularly in the context of moderate-grade hydronephrosis. [35]  Kiener et al found that the second-trimester APD is a useful parameter for predicting the risk for postnatal surgery and recommended that the SFU grade be assessed in every antenatal US examination. [36]

The presence of a distended bladder without functional emptying is important in developing a differential diagnosis. A filled bladder should be visualized, with functional emptying observed every 30-60 minutes. A system that does not function in this manner suggests the presence of posterior urethral valves, prune belly syndrome, or the often-fatal urethral atresia. [28]  In these cases, the bladder and (sometimes) the upper urinary tract are massively dilated.

The measurement of fetal sagittal bladder length (FSBL) in combination with the presence of pelviectasis has been suggested as a tool to determine the outcome of antenatal hydronephrosis. The presence of megacystis, determined as FSBL of greater than gestational age (GA) plus 12, in association with pyelectasis, suggests the presence of posterior urethral valve or vesicoureteral reflux. [37]

Renal architecture, renal size, and renal parenchymal echogenicity are important in developing a differential and determining the potential function of the kidney. Increased echogenicity may be associated with renal dysplasia.

Caliceal anatomy revealed by US is also important to help differentiate obstructive and nonobstructive antenatal hydronephrosis. The presence of the "eggshell sign," a crescent of increased echogenicity at the caliceal-parenchymal interface, may indicate increased intrarenal pressure; thus, it is more commonly associated with obstructive processes. [38]

Differentiation between hydronephrosis and polycystic kidneys can be difficult but helpful in determining the overall survival of the fetus. Hydronephrosis tends to be an orderly process with visible connections between the dilated calices and pelvis. Polycystic kidneys do not appear as orderly, and connections between cysts are not present. Furthermore, hyperechogenicity of the renal parenchyma is more often associated with polycystic kidneys rather than hydronephrosis.

Renal size and echogenicity are also important in determining renal function. Noteworthy findings include the presence of renal cortical cysts, echogenic parenchyma, and a discernible corticomedullary junction that can herald the presence of renal dysplasia or severe and irreversible damage to the renal unit. These findings are nearly 100% specific but accurately predict dysplasia in only 60% of fetuses. [39]

Amniotic fluid volume is the single most significant determinant of fetal well-being and survival. Amniotic fluid volume is maintained by fetal urine production by 16 weeks' gestation and remains constant throughout gestation. Oligohydramnios is associated with pulmonary hypoplasia and compression deformities of the head, thorax, and extremities (Potter syndrome).

Oligohydramnios has been demonstrated to be the key factor in the development of pulmonary hypoplasia, and restoration of this fluid prevents hypoplasia. The most vulnerable period for lung development is the second trimester; late-onset oligohydramnios has little impact on overall pulmonary function.

The final consideration for the imaging team is a global assessment of the fetus. Finding abnormalities in other systems is not uncommon when a urologic abnormality is detected. One series showed that 55% of fetuses with bilateral hydronephrosis and oligohydramnios had an associated structural or chromosomal abnormality. [24]  These anomalies are commonly of cardiovascular, neurologic, and orthopedic origin, but they can be found elsewhere and should be sought during the imaging evaluation.


Magnetic Resonance Imaging

Fetal magnetic resonance imaging (MRI) is increasingly being used in the management of antenatal hydronephrosis. A study by Chalmers et al (47 patients, 88 renal units; median gestational age, 22 weeks) evaluated fetal MRI and US for this purpose in relation to various grading methods (APD, SFU, and UTD). [40]  They found that fetal MRI improved the interrater reliability of the SFU grading system and enhanced the APD intraclass correlation. MRI tended to yield a higher SFU grade than US did, but it had no significant effect on the UTD grade.


Other Tests

Chromosomal analysis can be helpful. Abnormal chromosome complement or aneuploidy is a major factor in fetal demise. Prenatal screening for aneuploidy can be accomplished by several means, including imaging techniques and maternal serum biochemistry. More invasive fetal sampling following these screening techniques is a more definitive but morbid method of confirming the diagnosis.

Normal findings on US are associated with a twofold to threefold reduction in the risk of a chromosomal abnormality as compared with the presence of minor and major abnormalities on US. [41]

Maternal serum biochemistries, such as beta human chorionic gonadotropin (β-hCG), alpha fetoprotein (AFP), and others, are used to detect aneuploidy, with a reported accuracy of approximately 65%. The best individual risk estimation is based on maternal age, US examination, and maternal biochemistry. [41]

More definitive methods of determining the presence and the exact nature of the abnormality are chorionic villus sampling and amniocentesis. The risks should be weighed against the benefits of performing the procedure.

Several studies have focused on noninvasive techniques of determining the chromosomal defect. Researchers have isolated and analyzed nucleated fetal cells from maternal blood; however, limiting factors include the relative rarity of fetal cells in maternal blood and the need to establish their fetal origin. [42] Although promising, these techniques are not yet standardized or available to the population at large.



Procedures that may be considered in the workup include the following:

  • Aspiration of urine from the fetal bladder
  • Antenatal screening for aneuploidy - Chorionic villus sampling; amniocentesis; sampling maternal blood (not standardized)