Malformations of the kidneys and urinary tract constitute the second most commonly diagnosed group of diseases in the prenatal period, with an incidence of 0.3–0.5%. Most cases of fetal urinary tract malformation are diagnosed during routine examination. However, the most important indication for researching urinary tract malformations is the finding of decreasing amniotic fluid or the observation of a small uterus for gestational age.

A proper evaluation of the urinary tract should include at least one cross-section of the fetal abdomen at the level immediately above the renal hilum, a longitudinal section paramedian through each kidney’s store, visualization of the fetal bladder, and evaluation of the amount of amniotic fluid.

In the cross-section the kidney’s stores should be identified at each side of the column. It is in this section that the anteroposterior diameter of the renal pelvis is measured, and in which the renal pelvis can be measured when it is visible. The standardization of this measurement is important, because it enables us to monitor the case followed, and it is the universal section proposed for the diagnosis of hydronephrosis.

As previously described, the renal pelvis varies according to gestational age, and to accompany its growth, there are appropriate tables. In practical terms, we consider a renal pelvis to be increased when it is above 5 mm at 24 weeks or above 10 mm at 32 weeks. It should be noted if the dilatation affects merely the renal pelvis, which is known as pyelectasis, or if there is involvement (swelling) of the renal calyces, which is called ureteropelvic dilation, and represents a more advanced stage of expansion.

In a longitudinal section, the lateromedial and longitudinal renal measurements can be obtained. By performing this last measurement, care should be taken not to include the adrenal gland, which appears as a slightly hypoechoic structure with the upper pole of the kidney. It is in this section that the measurement of renal thickness is obtained.

The calyx pyramidal system can be observed and the renal parenchyma may also be evaluated, usually on an enlarged image. The normal parenchyma has a slightly higher echogenicity in the cortex compared with marrow, called cortical differentiation, which is lost in cases of renal dysplasia, in which the parenchyma may appear globally hyperechogenic and often presents cortical cysts indicating terminal functional impairment.

The ureters are usually not displayed on the ultrasound because of its virtual light, not filled with liquid, but when they are dilated, they may be identified as cystic structures, usually in the form of “stacked coins” extending from the renal pelvis to the fetal bladder.

The fetal bladder can usually be identified in the fetal pelvis along the iliac bones. The filling and emptying of the normal fetal bladder is an easily demonstrable phenomenon in ultrasound. When the bladder becomes dilated (megacystis), it can occupy the whole fetal abdomen.

The sensitivity of ultrasound for detecting these abnormalities, especially the obstructive ones, increases with gestational age, reaching 80% at 28 weeks’ gestation. Although detection of such abnormalities during the prenatal period is frequent, often an accurate diagnosis of the lesion, in addition to its prognostic value, is difficult to assess.

The evaluation of the amount of amniotic fluid is critical in the evaluation of urinary tract malformations. Bilateral or infravesical urinary tract abnormalities can lead to a lack of production or of the disposal of urine into the amniotic fluid, causing it to diminish markedly.