Diagnostic ultrasound ( PDFDrive )

1338 PART IV Obstetric and Fetal Sonography
collecting tubules. hrough interaction with the metanephric
mesoderm, the ureteric bud induces the formation of nephrons.
In early embryonic life, the kidneys are located in the pelvis, but
they “ascend” to their adult position by the 11th menstrual week.
At this gestation, the kidneys start to produce urine.
By the ninth menstrual week, the cloaca (caudal part of
hindgut) is divided by the urorectal septum into the rectum
posteriorly and the urogenital sinus anteriorly (see Fig. 39.1).
he urinary bladder, the female urethra, and most of the male
urethra develop from the urogenital sinus and the surrounding
splanchnic mesenchyme. Initially, the bladder is continuous with
the allantois, but this structure soon constricts and becomes a
ibrous cord, the urachus, which extends from the apex of the
bladder to the umbilicus.
Sonographic Appearance
In the irst trimester the fetal kidneys are best examined by
transvaginal sonography. he kidneys are seen as oval, hyperechoic
structures in the paravertebral regions, with a small, central
sonolucent area caused by luid in the renal pelvis 4 (Fig. 39.2A).
By 12 to 13 weeks of gestation, the kidneys can be visualized in
99% of cases with combined transabdominal and transvaginal
sonography. 5 In the second trimester the kidneys oten appear
as isoechoic structures adjacent to the fetal spine on transabdominal
sonography (Fig. 39.2B). As the fetus matures, corticomedullary
diferentiation becomes more obvious, especially
in the third trimester (Figs. 39.2C-D). he renal pyramids orient
in anterior and posterior rows and are hypoechoic relative to
the renal cortex. In the third trimester the renal cortex is isoechoic
or slightly hyperechoic to liver and spleen. With fat deposition
in the perinephric region, an echogenic border develops, and
the kidney becomes better delineated. Normal fetal lobations
are oten visible and give the kidneys an undulating contour.
he kidneys grow throughout pregnancy. Table 39.1 provides
a nomogram of renal lengths at 14 to 42 weeks of gestation. 6
he oten-quoted rule of thumb that “renal length in millimeters
approximates gestational age in weeks” applies only to a narrow
gestational age range of 18 to 21 weeks. here are also published
A
B
C
D
FIG. 39.2 Normal Sonographic Appearance of Kidneys at Different Gestational Ages. (A) Transvaginal scan at 13 weeks of gestation in
the coronal plane shows normal kidneys (calipers), which appear hyperechoic, with small central sonolucent area caused by luid in the renal pelvis.
(B) Transabdominal scan at 19 weeks in the transverse plane shows the kidneys (arrows) as paired isoechoic structures adjacent to the fetal spine,
with a small amount of luid in the renal pelvis. (C) and (D) Longitudinal and transverse scans at 33 weeks show the kidney well outlined by perinephric
fat, with normal corticomedullary differentiation. The pyramids (arrowheads) are hypoechoic. There is a small amount of luid in the renal pelvis
(arrow).