Renal agenesis: congenital absence of the kidneys.
Potter syndrome: constellation of fetal and neonatal complications, including lung hypoplasia and facial and skeletal abnormalities, as a result of severe prolonged oligohydramnios, most often from renal disease, classically due to renal agenesis.
Renal ectopia: abnormal position of one or both kidneys, in a location other than within the renal fossa.
Urinary tract dilation: expansion of the renal collecting system with fluid, often due to urinary tract obstruction; sometimes called hydronephrosis.
Renal dysplasia: abnormal renal parenchyma causing impaired renal function, often the result of severe or complete urinary tract obstruction in utero.
Polycystic kidneys: hereditary syndrome, usually due to an autosomal recessive genetic disorder, causing abnormal development of the kidneys, with multiple parenchymal cysts and impaired renal function.
Lower urinary tract dilation: dilation of the bladder and sometimes the urethra, typically due to obstruction of the bladder outlet, most often the result of posterior urethral valves but sometimes due to distal urethral stenosis or atresia or nonobstructive causes.
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.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.
From the second trimester onward, the kidneys are the major contributor to the amniotic fluid volume.3,4,5 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.5-8
Using transvaginal ultrasound, normal fetal kidneys can first be seen as early as 9 weeks of gestation and are generally visible by the end of the first trimester.9 With transabdominal scanning, the kidneys may first be visible at 12 to 14 weeks of gestation and are seen in most patients by 16 to 18 weeks (Figure 22-1).10 Delayed visualization may occur when factors such as maternal obesity or large uterine fibroids limit the sonographic assessment of fetal anatomy. The fetal bladder may be seen as early as 10 weeks of gestation with transvaginal scanning and should be visible transabdominally by 14 weeks in virtually all patients.10-12