Diagnostic ultrasound ( PDFDrive )

CHAPTER 39 The Fetal Urogenital Tract 1339
TABLE 39.1 Renal Lengths at 14-42
Weeks’ Gestation
FITTED CENTILES
Week N 3rd 10th 50th 90th 97th
14 3 7.5 8.0 9.3 10.8 11.6
15 3 8.8 9.5 11.0 12.8 13.7
16 2 10.2 11.0 12.7 14.8 15.8
17 12 11.6 12.5 14.5 16.8 18.1
18 10 13.1 14.1 16.3 18.9 20.3
19 15 14.6 15.6 18.2 21.1 22.6
20 15 16.1 17.2 20.0 23.2 24.9
21 15 17.5 18.8 21.8 25.4 27.2
22 14 19.0 20.4 23.6 27.4 29.4
23 16 20.4 21.9 25.4 29.5 31.6
24 17 21.8 23.4 27.1 31.5 33.8
25 18 23.1 24.8 28.8 33.4 35.8
26 20 24.4 26.2 30.4 35.3 37.8
27 24 25.6 27.5 31.9 37.1 39.7
28 18 26.8 28.7 33.4 38.7 41.5
29 19 27.9 29.9 34.7 40.3 43.2
30 19 28.9 31.0 36.0 41.8 44.8
31 23 29.9 32.1 37.2 43.2 46.3
32 23 30.8 33.0 38.3 44.5 47.7
33 22 31.6 33.9 39.4 45.7 49.0
34 19 32.4 34.7 40.3 46.8 50.2
35 20 33.1 35.4 41.1 47.8 51.2
36 23 33.7 36.1 41.9 48.7 52.2
37 14 34.2 36.7 42.6 49.4 53.0
38 17 34.7 37.2 43.2 50.1 53.8
39 13 35.1 37.6 43.7 50.7 54.4
40 14 35.4 38.0 44.1 51.2 54.9
41 26 35.7 38.3 44.5 51.6 55.4
42 17 36.0 38.6 44.8 52.0 55.7
N, Number of fetuses for each week of gestation.
With permission from Chitty LS, Altman DG. Charts of fetal size:
kidney and renal pelvis measurements. Prenat Diagn. 2003;23(11):
891-897. 6
charts of renal anteroposterior diameter, transverse diameter,
and volume. 6 Sometimes, it is diicult to deine the exact renal
borders, especially at the upper pole, because of shadowing from
ribs or poor distinction from the adrenal gland. Fetal breathing
can aid in renal visualization. It is also important to avoid using
an oblique section through the kidney for measurement. he
renal/abdominal circumference ratio remains constant at 0.27
to 0.30 throughout pregnancy. 7
he calyces are not normally visualized, but some luid is
typically seen in the renal pelvis. he highly characteristic renal
pelvic echo is oten the key to inding the kidneys in the second
trimester. Measurements of the renal pelvis are discussed in the
section on upper urinary tract (UT) dilation. he normal ureter
is 1 to 2 mm in diameter and is not normally visible.
By using transvaginal sonography, the bladder can be seen
as early as 11 weeks of gestation. 4 By 12 to 13 weeks, the bladder
is visualized in 98% of cases using both transabdominal and
transvaginal sonography. 5 he bladder is thin walled and situated
anteriorly in the pelvis. he umbilical (superior vesical) arteries
run lateral to the bladder as they course toward the umbilicus
(Fig. 39.3). he hourly fetal urine production increases with
advancing gestation, from a mean value of 4 to 5 mL/hr at 20
weeks to 52 to 56 mL/hr at 40 weeks. 8,9 hree-dimensional (3-D)
ultrasound measurements demonstrate reproducible urine
production rates based on bladder volumes, but tend to estimate
higher rates in the third trimester compared with the standard
two-dimensional (2-D) technique. 10 he maximum bladder
volume increases from a mean value of 1 mL at 20 weeks to
36 mL at 41 weeks. 8 he normal bladder ills and empties (either
partially or completely) approximately every 25 minutes (range,
7-43 min). herefore changes in bladder volume should be
observed during the course of the obstetric sonogram.
Amniotic Fluid Volume
Evaluation of amniotic luid volume (AFV) provides important
information about some maternal and fetal conditions, as well
as placental function. Evaluation of AFV is a key component of
fetal biophysical assessment. Ater 16 weeks, fetal urine production
becomes the major source of amniotic luid. 11 Fetal lung secretions,
fetal swallowing, and intramembranous resorption of water into
the fetal circulation via fetal vessels on the placental surface also
contribute to AFV homeostasis. 12
Several methods are used to assess AFV. Subjective assessment
can be combined with semiquantitative techniques, such as
measurement of the maximum vertical pocket (or single deepest
vertical pocket) and amniotic luid index (AFI). Signiicant
oligohydramnios results in compression of the fetus, marked
crowding of fetal parts, and poor visualization of fetal anatomy.
With polyhydramnios, one gets the impression of a “swimming”
fetus. he subjective assessment of AFV by experienced sonographers
has been shown to be reliable. 13 However, there are
conlicting data regarding the reliability of both the maximum
vertical pocket technique and the AFI. 13-15 he technique for
measuring the deepest amniotic luid pockets should follow these
guidelines:
• he maximum depth of the luid pocket should be measured
in a perpendicular direction.
• he pocket should have a width of at least 1 cm (avoid measuring
narrow, slitlike pockets).
• he pocket should be free of fetal small parts and umbilical
cord.
he following classiication has been proposed for the
maximum vertical pocket method: vertical depth of the pocket
less than 2 cm indicates moderate to severe oligohydramnios, 2
to 8 cm is normal, and greater than 8 cm indicates
polyhydramnios. 16
Amniotic Fluid Assessment a
Vertical Depth
<2 cm Oligohydramnios
2-8 cm Normal
>8 cm Polyhydramnios
a Maximum vertical pocket method.