Diagnostic ultrasound ( PDFDrive )

CHAPTER 30 The First Trimester 1057
have dropped to as low as 390 mIU/mL. 38 Extensive eforts have
been made to identify a discriminatory level for the serum β-hCG,
deined as the level of serum β-hCG above which sonographers
should always visualize an intrauterine sac on ultrasound to be
considered normal. Work by Barnhart indicated that this level,
using TVS, should be set at 1000 to 2000 mIU/mL. 38-40 However,
additional studies have shown that a single value of β-hCG
is of limited value. Mehta at el. 41 showed that although the absence
of a visible sac at a β-hCG level of 2000 mIU/mL is suggestive
of an abnormal outcome, it is not diagnostic, as 33% (17 of 51
in her series) of such early pregnancies went on to have normal
outcomes.
Discriminatory levels can be used to guide management but
cannot be used as absolute indicators that the absence of a
sonographically demonstrable gestation sac is abnormal. Serial
β-hCG measurements are usually more helpful than a single
measurement in identifying abnormal from normal pregnancy,
and ultrasound can document important diagnostic features
regardless of the exact β-hCG value. 5
Barriers to Use of Strict β-hCG Thresholds in
Determination of Pregnancy
Resolution of the ultrasound scanner
Patient body habitus
Position of the uterus
Type of hormonal assay
Experience of the sonographer
Masses such as ibroids
Multiple gestations
Yolk Sac
At 4 weeks’ menstrual age, the primary yolk sac begins to regress
and the secondary yolk sac develops. he secondary yolk sac is
the irst structure to be seen normally within the gestational sac.
Using TAS, it is oten seen when the MSD is 10 to 15 mm and
should typically be visualized by an MSD of 20 mm. 42 TVS allows
earlier and more detailed visualization of the yolk sac (Figs. 30.6
and 30.9), which is typically visualized by an MSD of 8 mm. 28
However, the visualization of a yolk sac without an embryonic
pole occurs only for a brief period of time. In patients being
scanned for pregnancy of unknown viability, the MSD of 25 is
used as the threshold, with need to visualize an embryo at this
time.
he demonstration of a yolk sac may be critical in diferentiating
an early intrauterine gestational sac from a pseudosac. 28
Although the double-decidual sign is not 100% speciic for
presence of an IUP, the identiication of a yolk sac within the
early gestational sac is diagnostic of IUP. he yolk sac plays an
important role in human embryonic development. 10 While the
placental circulation is developing, the yolk sac plays a role in
transfer of nutrients to the developing embryo in the third and
fourth weeks. Angiogenesis occurs in the wall of the yolk sac
in the ith week. he mesenchymal cells or angioblasts aggregate
to form “blood islands”; a cavity forms within these islands,
which fuse with others to form networks of endothelial channels.
Vessels extend into adjacent areas by endothelial budding and
fusion with other vessels. his vascular network in the wall of
the yolk sac eventually joins the embryonic circulation via the
paired vitelline arteries and veins through a stalk called the
vitelline duct. Hematopoiesis occurs irst in the well-vascularized
extraembryonic mesoderm covering the yolk sac wall in the ith
week, in the liver in the eighth week, and later in the spleen,
bone marrow, and lymph nodes. he dorsal part of the yolk sac
is incorporated into the embryo as primitive gut (foregut, midgut,
and hindgut) in the sixth week. he yolk sac remains connected
to the midgut by the vitelline duct (Fig. 30.10).
Lindsay et al. 43 reported that the yolk sac grows at a rate of
0.1 mm per millimeter of MSD growth when the MSD is less
than 15 mm, then slows to 0.03 mm. he upper limit of normal
for yolk sac diameter between 5 and 10 weeks of gestational age
is 5.6 mm.
he number of yolk sacs present can be helpful in determining
amnionicity of a multifetal pregnancy (Fig. 30.11). In general,
the number of yolk sacs and the number of amniotic sacs are
the same. In a monochorionic monoamniotic (MCMA) twin
gestation, there will be two embryos, one chorionic sac, one
amniotic sac, and one yolk sac. Levi et al. 44 examined four MCMA
twin pregnancies, all with a single yolk sac. One was a conjoined
twin and one a twin ectopic, with both pregnancies terminated.
he other two pregnancies delivered normally at 34 weeks. Of
the four cases, two had a larger-than-normal yolk sac (>5.6 mm),
and one yolk sac was irregular in contour. herefore in MCMA
twins, a single, large, or normal-sized yolk sac with two live
embryos can result in a normal twin delivery.
Embryo and Amnion
Visualization of the amnion in the absence of an embryo usually
occurs in intrauterine embryonic death as a result of resorption
of the embryo 45 (Fig. 30.12).
Amniotic luid is a colorless, fetal dermal transudate; as the
skin corniies and the kidneys begin to function, at about 11
weeks, it becomes pale yellow. he amnion becomes visible when
the embryo has a CRL of 2 mm at 6 weeks. he cavity becomes
almost spherical by about 7 weeks, likely a result of the more
rapid increase in luid volume relative to the growth of the sac
membrane to accommodate it. he actual rate of luid increase
is more rapid ater about 9 weeks (Fig. 30.13), when urine is
produced. Fluid accumulates at about 5 mL per day at 12 weeks.
he amniotic cavity expands to ill the chorionic cavity completely
by week 16. It is therefore normal to identify the amnion as a
separate membrane or sac within the chorionic cavity before 16
weeks (Fig. 30.14). Occasionally, the amnion and chorionic
membranes may fail to be juxtaposed at week 16 (so-called
unfused amnion), and separation of these membranes may persist
for a short time. 46
Iatrogenic or spontaneous rupture of the amniotic membrane
in the irst trimester is a rare occurrence and even more rarely
results in the amniotic band sequence. his rupture may result
in retraction of the amnion in part or in whole, up to the base
of the umbilical cord where the amnion and chorion are adherent.
More oten, the loating amniotic membranes do not adhere to
the fetus, and no fetal anomalies occur.