Diagnostic ultrasound ( PDFDrive )

CHAPTER 30 The First Trimester 1077
A B C
FIG. 30.37 Normal First-Trimester Anatomy. (A) Embryo (calipers) at 7 weeks 6 days demonstrates a cystic appearance in the cranial region
(arrow) representing the normal rhombencephalon. See also Video 30.9. (B) Transverse scan of a 12-week fetus shows normal lateral ventricles
with choroid plexus illing most of the lateral ventricles. (C) Ten-week embryo has the typical echogenic bowel herniated into the base of the
umbilical cord (arrows) due to physiologic midgut herniation.
a follow-up sonographic examination may be indicated for
evaluation of fetal morphologic characteristics in the second
trimester.
hree major points should be considered: (1) normal
embryologic/fetal development may mimic pathology, (2)
abnormal embryos/fetuses may appear normal early in pregnancy,
and (3) discrepancies between dates and embryo size may be
the only visible manifestation of pathology in some irst-trimester
examinations.
Normal Embryologic Findings
Mimicking Pathology
Normal embryologic development in the irst trimester may
mimic pathologic changes more oten seen in the second and
third trimesters.
Rhombencephalon
During the sixth week, three primary brain vesicles form: the
prosencephalon (forebrain), the mesencephalon (midbrain),
and the rhombencephalon (hindbrain). 10 Small cystic structures
can be seen normally in the posterior aspect of the embryonic
head. he earliest cystic structure seen at 6 to 8 weeks’ gestation
represents the normal embryonic cystic rhombencephalon,
which later forms the normal fourth ventricle and should not
be mistaken for a posterior fossa cyst of pathologic importance 131
(Fig. 30.37A, Video 30.9). he prosencephalon divides into an
anterior portion known as the telencephalon and a posterior
diencephalon. he telencephalic vesicles later form the lateral
ventricles, and the diencephalon (and to a lesser degree the
telencephalon) forms the third ventricle. Ater approximately 9
weeks, the lateral ventricles can be identiied sonographically as
two small cystic spaces in the embryonic head at 11 weeks and
are more evident with the large choroid plexus almost illing
them at 13 weeks (Fig. 30.37B). By 12 weeks the lateral ventricles
extend almost to the inner table of the skull, and on sonography,
only a small rim of cerebral cortex can be demonstrated to
surround them. he choroid plexus is echogenic and ills the
lateral ventricles completely except for the frontal horns.
Physiologic Anterior Abdominal Wall Herniation
During embryogenesis, the midgut normally herniates into the
umbilical cord at the beginning of the eighth week of gestation.
he midgut rotates 90 degrees counterclockwise and then returns
to the abdomen during the 12th week. As the midgut returns
to the abdomen, further rotation occurs, completing the normal
rotation of the midgut.
Schmidt et al. 132 describe the normal physiologic appearance
of the anterior abdominal wall during this period. he herniated
bowel appears as a small, echogenic mass (6-9 mm) protruding
into the cord at approximately 8 weeks (CRL, 17-20 mm). he
echogenic mass decreases to 5 to 6 mm at 9 weeks (CRL,
23-26 mm). he size of the mass of herniated bowel varies.
Follow-up examinations reveal reduction of the hernia between
10 and 12 weeks. In up to 20% of normal pregnancies, the herniated
bowel may still be found outside the fetal abdomen at 12
weeks (Fig. 30.37C).
Abnormal Embryos
Many embryonic abnormalities may not be sonographically visible
in the irst trimester. In the irst trimester, abnormal embryos
may appear normal. Ultrasound in the irst trimester is best at
detecting abnormalities of the neurologic system, such as
anencephaly; abdominal wall defects, such as omphalocele; and
severe abnormalities of the urologic system, such as megacystis.
hese are illustrated in the chapter on chromosome abnormalities
(Chapter 31, see Fig. 31.6) where irst trimester screening for
chromosome abnormalities is described. Individual irst-trimester
anomalies are discussed in the chapters regarding their speciic
organ systems.
GESTATIONAL TROPHOBLASTIC
DISEASE
Gestational trophoblastic disease is a spectrum of cellular proliferations
stemming from the placental villous trophoblast
encompassing four main types: hydatidiform mole (complete