Renal agenesis: congenital absence of the kidneys.
Potter's syndrome: constellation of fetal and neonatal complications, such as lung hypoplasia and facial and skeletal abnormalites, as a result of severe oligohydramnios, classically due to renal agenesis.
Hydronephrosis: dilation of the renal collecting system usually due to partial urinary tract obstruction.
The fetal kidneys begin developing within the pelvis at approximately 7 weeks of gestation from the metanephric mesoderm and the ureteric bud. The metanephric tissue develops into the nephrons of the kidney, and the ureteric bud differentiates into the collecting tubules, calyces, pelvis, and ureter. Between 7 and 11 weeks, as the fetal body grows in length, the kidneys ascend to their permanent position in the flank due to disproportionate growth of more caudal structures.1
Initially, the kidney is made up of several loosely connected lobes, each with a thin cortex. During the second trimester the lobes fuse, becoming less distinct, and the cortex thickens, leaving the kidney with a lobular contour that persists for several years after birth.1,2 The kidneys begin to excrete urine at approximately 10 to 11 weeks of gestation, and from this age onward urine production increases progressively.
By the second trimester the kidneys become the major contributor to the amniotic fluid volume.3,4 Adequate amniotic fluid volume is necessary for normal fetal pulmonary and skeletal development because it provides space for fetal growth and movement. Therefore, a functioning urinary tract must be present for the lungs and skeleton to develop normally.5, 6, and 7
Using transvaginal ultrasound, normal fetal kidneys can first be seen as early as 9 weeks of gestation and should always be visible by the late first or early second trimester. With transabdominal scanning, the kidneys may first be visible at 13 to 14 weeks of gestation and are seen in most patients by 16 to 18 weeks (Figure 18-1). Delayed visibility may occur when factors such as maternal obesity or large uterine fibroids limit the fetal survey. The fetal bladder may be seen as early as 11 weeks with transvaginal scanning and should be visible transabdominally by 16 weeks in virtually all patients.8,9
Normal kidneys. A: Longitudinal sonogram of a normal kidney (arrows) demonstrating its reniform shape and hypoechoic pyramids. B: Transverse view of both kidneys (arrows), one on either side of the spine (S), which casts a shadow between them. Each kidney has a small amount of fluid in the renal pelvis.
The incidence of genitourinary anomalies at birth is 0.2% to 0.6%.10, 11, and 12 Genitourinary anomalies comprise approximately one-fourth of all congenital structural anomalies.12 Anomalies of the genitourinary tract result from arrested development early in organogenesis, failure of normal ascent, obstruction of the collecting system or bladder outlet, and abnormal formation of renal tubules. These anomalies are most often isolated but may also occur in association with, or may cause, other fetal structural abnormalities. Associations between genitourinary and other organ system anomalies occur in a broad variety of inherited or sporadic syndromes, including chromosomal abnormalities.4,5,13,14 In addition, urinary tract abnormalities that decrease urine production cause oligohydramnios, which may secondarily cause deformities involving other parts of the fetus. In particular, when severe oligohydramnios is present before 20 weeks of gestation, pulmonary hypoplasia; facial abnormalities, including flattened nose and low-set ears; and club feet or other limb positional abnormalities may result.15
Prenatal sonography can identify and characterize many anomalies of the genitourinary system. Ultrasound performed in the second and third trimesters can detect at least 80% to 85% of genitourinary anomalies.11,16 Prenatal diagnosis permits antenatal counseling and prompt postnatal evaluation and treatment, thus avoiding the therapeutic delays that, before the availability of ultrasound, often led to impairment or loss of renal function.17