Diagnostic ultrasound ( PDFDrive )

1340 PART IV Obstetric and Fetal Sonography
12 w
20 w
B
A
B
FIG. 39.3 Normal Urinary Bladder. (A) Sagittal image of a 12-week fetus. Note normal urinary bladder (arrow). (B) Power Doppler image of
the umbilical arteries (arrows) at 20 weeks’ gestation helps in the identiication of any questionable luid-illed structure in the pelvis as the urinary
bladder (B).
he AFI is obtained by measuring the deepest amniotic luid
pocket in each of the four quadrants of the uterus, and the sum
of the four measurements is the index. 14,17 AFI varies with
gestational age (Table 39.2). 18 In the late second trimester and
in the third trimester, an AFI of 5 cm to 24 cm has been considered
as normal. 19 It is recommended that when AFI is less
than 10 cm, three measurements should be averaged. 14 he
semiquantitative methods are useful for following AFV on serial
examinations, particularly by multiple examiners of varying
experience.
Oligohydramnios is typically deined as either a single
maximum vertical pocket of amniotic luid of less than 2 cm
or alternatively, an AFI of less than 5 cm. 20 Recently, a metaanalysis
of four randomized controlled trials comparing the
use of AFI (<5 cm) versus the single deepest vertical pocket
(<2 cm) during antepartum fetal surveillance did not show any
evidence that one method is superior to the other in predicting
poor perinatal outcomes. 21 However, the use of AFI increased
the rate of diagnosis of oligohydramnios and subsequent induction
of labor. It was concluded that the single deepest vertical
pocket measurement is the method of choice for amniotic luid
assessment during fetal surveillance. 21 his approach is supported
by the American College of Obstetricians and Gynecologists
(ACOG), as well as the indings of the recently completed
SAFE trial. 20,22
In multiple pregnancies, the maximum vertical pocket
technique is commonly used for assessment of amniotic luid,
with similar cutofs for abnormal as for singletons. 12,19,23 A systematic
review of studies on amniotic luid assessment and adverse
outcomes in twin pregnancies has cautioned that sonographic
evaluation of AFV has high speciicity but poor sensitivity for
the detection of abnormal AFV, and more rigorous studies in
this area are required. 24
URINARY TRACT ABNORMALITIES
he prevalence of UT malformations varies among studies, likely
because of diferences in study population and methods of surveillance.
In a recent analysis of 709,030 births in 12 European
countries, the prevalence of congenital malformations of the UT
was 1.6 per 1000 births. 25 he overall prenatal detection rate was
high: 82% and 88.5% in two studies. 1,25 However, it varied from
36% to 100% in diferent centers. 25 Many factors could account
for the variation of prenatal detection rates, including the study
population (high risk vs. unselected), timing of the ultrasound
scan, expertise of the operator, quality of the ultrasound equipment,
extent of follow-up, and ascertainment of congenital
anomalies. For major UT anomalies, 57% were detected before
24 weeks. 1 Lethal UT anomalies account for 10% of pregnancy
terminations. 26
Prenatal Diagnosis of Urinary Tract
Abnormalities
Assessment of amniotic luid volume
Localization and characterization of urinary tract
abnormalities
Search for associated abnormalities
A systematic approach to the prenatal diagnosis of UT
abnormalities includes assessment of AFV, localization and
characterization of UT abnormalities, assessment of fetal gender,
and search for associated abnormalities.
Normal AFV in the second half of pregnancy implies at least
one functioning kidney and a patent urinary conduit to the
amniotic cavity. If oligohydramnios is present without a history