Diagnostic ultrasound ( PDFDrive )

CHAPTER 39 The Fetal Urogenital Tract 1367
A
B
FIG. 39.41 First-Trimester Gender Identiication. (A) Sagittal scan of a 12-week male fetus shows the vertical and relatively cranially directed
phallus (arrow). (B) Sagittal scan of a 12-week female fetus shows the horizontal/caudally directed phallus (arrow).
rectum, when performed on an axial image of the pelvis, can be
used to distinguish between male and female internal genitalia. 245
However, further studies are required to reine the discriminatory
values.
In the late irst trimester, fetal sex determination is possible,
based on the direction of the genital tubercle in the midsagittal
plane. In males the genital tubercle (penis) points cranially or
vertically; in females the genital tubercle (clitoris) points caudally
or horizontally (Fig. 39.41). By measuring the angle of the genital
tubercle to a horizontal line through the lumbosacral skin surface
(genital angle), gender assignment was possible in 93% of fetuses
at 12 to 14 weeks of gestation (male when the angle was greater
than 30 degrees and female when the angle was less than 10
degrees). 246 At all ages, male gender was correctly assigned in
99% to 100%. However, the accuracy of female gender assignment
improved with gestational age, from 91.5% at 12 to 12 + 3 weeks,
to 100% at 13 to 13 + 6 weeks. he use of 3-D multiplanar
ultrasound allows the midsagittal plane to be obtained and the
genitalia to be visualized more easily. 247 As a result the genital
angle can be measured accurately, and this technique has been
found to be accurate and reproducible for fetal gender assignment
before 14 weeks of gestation. 248
Testicular descent into the scrotum occurs ater 25 weeks’
gestation. 249 Ater 32 weeks, both testes have descended in 97%
of the fetuses. Small hydroceles are common in third-trimester
male fetuses (15%) and are usually of no clinical importance
(Fig. 39.42). 250 However, large hydroceles, especially if they
increase in size over time, suggest an open communication
between the processus vaginalis and peritoneal cavity. In such
cases, postnatal evaluation for inguinal hernia should be
performed. 251
Abnormal Genitalia
An abnormality of the genitalia may be an isolated inding or
associated with other major malformations. In a male fetus,
abnormal ultrasound indings include micropenis, penile chordee
(abnormal curvature of penis), a shawl or biid scrotum, and
undescended testes (in third trimester). 252-254 Conirmation of
hypospadias can be achieved by demonstrating the origin of
the urine jet from either the midshat or the base of the penis.
In the female an enlarged clitoris is the most common inding.
It can be diicult to deine ambiguous genitalia accurately (Fig.
39.43). A male fetus with micropenis, biid scrotum, and
FIG. 39.42 Hydroceles. Scan of the scrotal sac in a 38-week male
fetus demonstrates testes and bilateral hydroceles.
undescended testes may not be distinguished from the virilized
female with clitoral enlargement (and labial fusion). If ambiguous
genitalia are suspected, a karyotype is obtained to determine the
genetic makeup of the fetus. Ambiguous genitalia, syndactyly,
and multiple other anomalies raise suspicion for Smith-Lemli-
Opitz syndrome, an autosomal recessive disorder due to mutations
in the gene encoding 7-dehydrocholesterol reductase
(DHCR7); thus elevation of 7-dehydrocholesterol in amniotic
luid is a reliable marker for prenatal diagnosis of Smith-Lemli-
Opitz syndrome. 255 his condition is also suggested by low
maternal serum estriol (uE3) in the second trimester. 256 DNA
analysis can also be performed. 257
When ultrasound suggests male genitalia and the fetus is
genetically female, congenital adrenal hyperplasia (CAH) is
the most common cause. he condition is autosomal recessive,
so there may be a previously afected sibling. More than 90% of
cases result from mutations of the CYP21A2 gene, leading to
21-hydroxylase deiciency. When a proband exists, prenatal
diagnosis is possible with chorionic villous sampling. 258 Recent
advances in the use of cell-free fetal DNA have shown potential
for the noninvasive prenatal diagnosis of CAH as well. 259 In the