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Definitions
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.
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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
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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.
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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
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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
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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....