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are imbedded in Wharton’s jelly. The umbilical arter-
ies arise from the fetal internal iliac arteries and in the
newborn become the superior vesical arteries and the
medial umbilical ligaments. The umbilical vein carries
oxygenated blood from the placenta to the fetus. The
oxygenated blood is shunted through the ductus venosus
into the inferior vena cava and the heart. The single left
umbilical vein in the newborn becomes the ligamentum
teres, which attaches to the left branch of the portal vein.
The ductus venosus becomes the ligamentum venosum.
The allantois is associated with bladder development
and becomes the urachus and the median umbilical ligament. It extends into the proximal portion of the umbilical cord. The yolk stalk connects the primitive gut to
the yolk sac. The paired vitelline arteries and veins
accompany the stalk to provide blood supply to the yolk
sac. The arteries arise from the dorsal aorta to supply
initially the yolk sac, then the primitive gut. The arteries
remain as the celiac axis, superior and inferior mesenteric
arteries supplying the foregut, midgut, and hindgut,
respectively. The vitelline veins drain directly into the
sinus venosus of the heart. The right vein is later incorporated into the right hepatic vein. The portal vein is
also formed by an anastomotic network of vitelline veins.
The length of the umbilical cord has a close linear
relationship with gestational age in normal pregnancies.
Hill et al.
41
found they could reliably measure the cord
lengths in 53 fetuses at 6 to 11 weeks’ gestational age.
Also, the cord lengths in 60% of dead fetuses were more
than two standard deviations (2 SD) below the value for
that expected gestational age.
The width of the umbilical cord has also been measured sonographically in first-trimester pregnancies, and
Ghezzi et al.
42
found a steady increase from 8 to 15
weeks. There was a significant correlation between cord
diameter and gestational age (r = 0.78; p <0.001), CRL
(r = 0.75; p <0.001), and biparietal diameter (r = 0.81;
p <0.001), but no correlation with birth weight or
placental weight. The cord diameter was significantly
smaller by at least 2 SD in patients who developed preeclampsia or had a miscarriage.
Cysts and pseudocysts within the cord have been
described in the first trimester.
43
Cysts are usually seen
in the eighth week and disappear by the 12th week. They
are singular, closer to the fetus than the placenta, with a
mean size of 5.2 mm. Cysts may originate from remnants of the allantois or omphalomesenteric duct and
characteristically have an epithelial lining
44
and usually
resolve in utero. Although umbilical cord cysts have been
associated with chromosomal abnormalities if seen in the
second and third trimesters, those seen in the first trimester have been normal at delivery. It is hypothesized
that the cyst is an amnion inclusion cyst that occurs as
the amnion was enveloping the umbilical cord (Fig.
30-20). In a series of 1159 consecutive patients scanned
between 7 and 14 weeks, Ghezzi et al.
45
found 24 cord
cysts at a prevalence of 2.1%. Single cysts in the first
Chapter 30 ■ The First Trimester 1087
A
C
FIGURE 30-20. Umbilical cord cyst. A, Live embryo at
9 weeks’ menstrual age with a cyst on the cord (arrow) close to
the embryonic end. On subsequent examination (not shown) the
cyst was no longer seen. B, Color Doppler image of the cord and
cyst with flow in the vessels of the cord and no flow in the cyst.
C, Another example of a 9-week cord cyst (arrow) in the midpor-
tion of the cord, with good visualization of the whole cord,
embryo, and yolk sac.
B
trimester were associated with a normal outcome and
a healthy infant, whereas multiple or complex cysts
were associated with an increased risk of miscarriage or
aneuploidy.
ESTIMATION OF
GESTATIONAL AGE
During the first trimester, gestational age may be estimated sonographically with greater accuracy than at any
other stage of pregnancy. As pregnancy progresses, biologic variation results in wider variation around the
mean for all sonographic parameters at a given gestational age. In order of appearance, the following structures can be measured as indicators of gestational age:
sac, CRL, and biparietal diameter.
1088 PART IV ■ Obstetric Sonography
Gestational Sac Size
It is possible to estimate gestational age from weeks 5 to
10 on the basis of gestational sac size. Dating of the
gestational sac alone is important because it is the first
structure seen before visualization of the yolk sac and
then the embryo. The pregnancy should be followed,
however, until the embryo with cardiac activity is identified as a reliable indicator of embryonic life. Most practitioners actually “eyeball” the sac; if MSD is very small,
about 2 mm, gestational age is 4 to 4
), and MSD of about 5 mm is 5 weeks. At 5
30-11
1
weeks (see Fig.
2
1
weeks, a yolk sac appears. At 6 weeks, an embryo first
appears adjacent to the yolk sac.
The MSD is measured using the sum of three orthogonal dimensions of the fluid–sac wall interface divided
by three. The measurements are most accurate when
obtained by a high-frequency transvaginal probe in the
sagittal and transverse planes at right angles to one
another. Normally, a yolk sac will be present when
the MSD is 8 mm or less, and an embryo will be seen
at 16 mm or less. Gestational sacs larger than 8 mm
without a yolk sac or larger than 16 mm without an
embryo should be watched carefully for impending early
pregnancy failure. Occasionally, a gestational sac up to
20 mm will be seen without an embryo, and the outcome
will be a normal pregnancy. Although this is uncommon,
the clinician must always give the pregnancy the benefit
of the doubt and use not only the gestational sac measurements but clinical presentation as well.
Crown-Rump Length
Using TVS, the embryo can be visualized from the fifth
week onward. Conventional CRL charts are available
beginning from 6 weeks, 2 days. A well-performed CRL
measurement in the first trimester of pregnancy is accurate to 5 to 7 days.
Biparietal Diameter
By the end of the first trimester, measurement of the
biparietal diameter (BPD) becomes more accurate than
the CRL, which by that time reflects errors associated
with fetal flexion and extension.
46
If the patient is to receive only one ultrasound exam-
ination during pregnancy, a first-trimester sonographic
examination solely to assess gestational age is not recommended.
47
Although this is the most accurate time for
estimation of gestational age, there is inadequate embryonic development to identify confidently anomalies that
could be seen in the second trimester. Currently, late first
trimester is considered the ideal time to screen for aneuploidy using a combination of maternal serum screening
and nuchal translucency (see Chapter 31).
EARLY PREGNANCY FAILURE
One of the most important roles of ultrasound in the
first trimester is to identify early pregnancies that have
already failed or that are more likely to fail. Many clinical
and sonographic terms are used to describe an early
pregnancy failure. This term is descriptive of a process
rather than of what one suspects based on the clinical or
sonographic changes. The pregnancy shows sonographic
evidence that the process of growth and development has
stopped, ideally with supportive clinical findings. A
2
large, empty gestational sac; a gestational sac and yolk
sac only; a smaller-than-normal or even an appropriatesized embryo with no cardiac activity; or only the remnants of a gestational sac all could be appropriately
described as “early pregnancy failure.” The clinical
descriptors of “threatened” or “missed” or “incomplete”
abortion or “blighted ovum” contribute little to the
understanding of the findings. Early pregnancy failure
indicates that whatever is in the endometrial cavity, it
will never produce a live baby.
Studies have demonstrated a 20% to 31% rate of
early pregnancy loss after implantation in normal healthy
volunteers.
will fail. About 15% of fertilized ova fail to divide, 15%
are lost before implantation, 30% during implantation,
13% to 16% after implantation and before the first
missed period,
period. Wilcox et al.
nized or preclinical pregnancy loss after implantation
was 22%. Many pregnancies aborted before the time
when a gestational sac would be demonstrable by TVS.
The higher numbers of preclinical losses reported more
recently likely reflect the use of more sensitive pregnancy
tests. Cytogenetic abnormalities have also been documented in 20% of ostensibly normal in vitro fertilization embryos.
the early pathologic studies of Hertig and Rock,
showed a high frequency of morphologic abnormalities
in preimplantation embryos. Loss rates are increased
with increased maternal age and use of tobacco and
alcohol.
Sorokin et al.
(CVS) in 795 first-trimester pregnancies and found that
35 had a nonviable pregnancy before the procedure; 19
of the 35 women had subsequent CVS, and all were
aneuploid. Ten cases had chromosomal abnormalities,
virtually always lethal in the embryonic period, and nine
had defects with moderate potential for fetal viability.
Gestations with low viability potential had a larger discrepancy (23.4 ± 8.3 days) in estimated minus observed
gestational age, which was significantly greater than that
of gestations with moderate viability potential (8.9 ± 4.3
days; p <0.001). The absence of an embryonic pole was
more common in the first group. This demonstrates that
the more severe the anomaly, the more likely that very
48,49
Overall, about 75% of all pregnancies
49
and 9% to 10% after the first missed
48
found that the rate of unrecog-
50
All these findings are consistent with
51
performed chorionic villous sampling
49
who
Chapter 30 ■ The First Trimester 1089
early embryonic demise or intrauterine growth restriction (IUGR) will occur.
The etiology of first-trimester pregnancy loss is still
not fully understood, with many known and suspected
causes. In a study of 232 first-trimester patients (normal,
healthy women with good nutrition, good medical/prenatal care, positive urinary pregnancy test, and no vaginal
bleeding) with TVS at the first visit, Goldstein
52
determined the incidence of subsequent pregnancy loss by
following all to delivery or spontaneous abortion. This
group had an overall pregnancy loss rate of 11.5% in the
embryonic period, (i.e., <70 days from last menstrual
period [LMP]). The loss rate diminished as the pregnancy progressed. The loss rate was 8.5% when a yolk
sac was seen, 7.2% with an embryo of CRL less than
5 mm, 3.3% with CRL 6 to 10 mm, and 0.5% with
CRL greater than 10 mm. The loss rate leveled off at 2%
from 14 to 20 weeks, the fetal period. Therefore, under
the best circumstances, the pregnancy loss rate will be
11.5% overall, from 5 weeks onward. Once the embryo
reaches a CRL of 10 mm, there is about a 98% chance
of a successful outcome.
Patients who present with bleeding have a much
higher incidence of pregnancy loss. In patients who
present with a closed cervical os and uterine bleeding in
the first trimester, 50% will eventually abort. Using
TVS, Falco et al.
53
studied 270 patients with first-trimester bleeding at 5 to 12 weeks’ gestation; 45% were diagnosed initially as a nonviable pregnancy or anembryonic
sac. Those with multiple gestations were excluded. Of
the 149 remaining with demonstrable fetal cardiac activity, 15% (23/149) subsequently aborted.
Table 30-1 summarizes the rate of spontaneous abor-
tion in a number of studies of women with and without
bleeding in early pregnancy.
52-55
Another cause of early pregnancy failure is luteal
phase defect, thought to be failure of the corpus luteum
to support the conceptus adequately once implantation
has occurred. This may result from a shortened luteal
phase in cases of ovulation induction and in vitro fertilization, or from luteal dysfunction, more frequently seen
in obese women or women over 37 years of age.
56
Luteal
phase defect has been defined as a delay of more than 2
days in histologic development of the endometrium relative to the day of the cycle. The underlying cause may
be decreased hormone production by the corpus luteum,
decreased levels of FSH or LH or abnormal patterns of
secretion, or a decreased response of the endometrium
to progesterone.
Angiogenesis of the corpus luteum may be needed
for the regulation of progesterone production. Kupesic
57
found that the resistive index (RI) in intraovarian
et al.
arteries in normal nongravid women dropped below
0.47 in the luteal phase compared to a group with luteal
phase defect who had a high resistance throughout the
menstrual cycle, with RI always above 0.50. They suggest
that Doppler sonography may predict functional capacity of the corpus luteum, at least in the nongravid state.
Blumenfeld and Ruach
56
were successful in treating
luteal phase defect in a group undergoing ovulation
induction and in patients with previous abortions, using
hCG administration twice weekly in the sixth and tenth
weeks. This reduced the rate of miscarriage from 49%
to 17.8% (p <0.01).
Currently, clinical management centers on whether or
not the embryo is present and alive. Menstrual history
may be unreliable, and sonographic diagnosis of embryonic demise based on the menstrual history may be
incorrect. It is more appropriate to predict outcome by
comparing sonographic findings to quantifiable parameters, including other sonographic measurements (MSD)
or quantitative serum β-hCG. Menstrual age can be used
if there is corroboration with a previous positive β-hCG
test result. For example, if a patient had a positive serum
β-hCG test result 5 weeks ago, the current gestational
age must be at least 8 weeks.
Sonographic Diagnosis of
Embryonic Demise
After the gestational sac becomes demonstrable on ultrasound, the diagnosis of early pregnancy failure can be
made reliably using sonographic criteria.
TABLE 30-1. RATE OF SPONTANEOUS ABORTION IN EARLY PREGNANCY*
STUDY AGE (WK) NUMBER INDICATION ABORTION RATE (%)
Goldstein
Pandya et al.
Stabile et al.
Falco et al.
Falco et al.
Pandya et al.
*In women with and without bleeding.
52
54
55
53
53
54
5-10 232 Routine 11.5
10-13 17,870 Routine 2.8
5-16 624 Bleeding 45
5-12 270 Bleeding 51.5
5-12 149
10-13 17,870 Bleeding 15.6
Bleeding + live fetus
15
1090 PART IV ■ Obstetric Sonography
TABLE 30-2. PRESENCE OR ABSENCE OF
CARDIAC ACTIVITY IN NORMAL
EMBRYOS*
Cardiac Activity at TVS
CRL (MM) PRESENT ABSENT
0-0.9 0 0
1-1.9 0 3
2-2.9 12 0
3-3.9 11 2
4-4.9 12 0
total 35 5
From Levi CS, Lyons EA, Zheng XH, et al. Endovaginal ultrasound:
demonstration of cardiac activity in embryos of less than 5.0 mm in crown-rump
length. Radiology 1990;176:71-74.
*Based on crown-rump length (CRL) (N = 40); TVS, transvaginal sonography.
TABLE 30-3. PRESENCE OR ABSENCE
OF CARDIAC ACTIVITY IN EMBRYOS
THAT SUBSEQUENTLY ABORTED IN
FIRST TRIMESTER*
Cardiac Activity at TVS
CRL (MM) PRESENT ABSENT
0-0.9 0 0
1-1.9 1 0
2-2.9 1 8
3-3.9 6 6
4-4.9 3 6
total 11 20
From Levi CS, Lyons EA, Zheng XH, et al. Endovaginal ultrasound:
demonstration of cardiac activity in embryos of less than 5.0 mm in crown-rump
length. Radiology 1990;176:71-74.
*Based on crown-rump length (CRL) (N = 31); TVS, transvaginal sonography.
Embryonic Cardiac Activity
The most important feature for the confirmation of
embryonic and fetal life is the identification of cardiac
activity. The presence of cardiac activity indicates that
the embryo is alive. The absence of cardiac activity does
not necessarily indicate embryonic demise, however,
because TVS can identify a normal early embryo without
cardiac activity.
We reviewed a series of 96 patients with CRL of less
than 5 mm to assess the predictive value of the presence
or absence of cardiac activity using TVS3 (Tables 30-2,
30-3, and 30-4). Of the 71 patients available for follow-
up, 46 embryos had cardiac activity, 35 progressed to at
least the late second trimester, and 11 ended as firsttrimester demise. Of the 25 embryos without demonstrable cardiac activity, 5 (20%) were normal and 20
(80%) ended as first-trimester embryonic deaths. Of the
TABLE 30-4. PERCENTAGE OF EMBRYOS
THAT ABORTED AFTER SONOGRAPHIC
DEMONSTRATION OF CARDIAC
ACTIVITY*
CRL (MM)
0-0.9 0/0 0%
1-1.9 1/1 100%
2-2.9 1/1 38%
3-3.9 6/17 35%
4-4.9 3/15 20%
total 11/46 24%
From Levi CS, Lyons EA, Zheng XH, et al. Endovaginal ultrasound:
demonstration of cardiac activity in embryos of less than 5.0 mm in crown-rump
length. Radiology 1990;176:71-74.
*At specific crown-rump length (CRL).
SPONTANEOUS
ABORTIONS/TOTAL
PERCENTAGE
ABORTED
five normal embryos without demonstrable cardiac activity on initial TVS, three had initial CRL of less than
1.9 mm. Standard embryology texts indicate that the
embryonic heart begins to beat at the beginning of the
sixth week, when the CRL is 1.5 to 3 mm. Thus, it is
not surprising that we were unable to identify cardiac
activity in normal embryos with less than 2 mm CRL.
Initial TVS assessment failed to identify cardiac activity
in 2 of 25 normal embryos with CRL of 2 to 4 mm.
TVS enabled correct identification of cardiac activity in
100% of normal embryos with CRL of 4 to 4.9 mm.
Pennell et al.58 found that 16 of 18 embryos with CRL
less than 5 mm had no cardiac activity on initial transvaginal assessment but demonstrated cardiac activity on
follow-up TVS. Cardiac motion was seen on transvaginal scan in all pregnancies with CRL greater than 5 mm.
As a result, in our practice, follow-up sonography is
performed in patients with embryos of less than 5-mm
CRL with no cardiac activity, unless the yolk sac is
absent. We allow a few millimeters of leeway and follow
up otherwise normal-appearing embryos with no cardiac
activity on initial examination. The history and other
sonographic findings must be considered before making
the diagnosis of embryonic demise.
The combination of vaginal bleeding and absent
cardiac activity in embryos of CRL less than 5 mm on
TVS is associated with a very poor prognosis. Aziz et al.59
reviewed outcomes in embryos of CRL 5 mm or less
with absent cardiac activity on TVS, in women presenting with vaginal bleeding; all resulted in pregnancy
failure.
Using TVS, the embryo and embryonic cardiac activity can be reliably and consistently identified earlier than
with TAS. Before making a diagnosis of embryonic
demise, it is critical to ensure that the examination is of
high quality, performed with modern equipment and an
appropriate transducer frequency, and that the entire
embryo is visualized. A high frame rate must be used,
Chapter 30 ■ The First Trimester 1091
and the frame-averaging mode must be turned off. If
there is any doubt in the diagnosis, follow-up examination should be performed.
In patients with a sonographically demonstrable
embryo, absent cardiac activity is clearly the most important factor in predicting the pregnancy outcome (Video
30-2). It is also important to know the predictive value
of the presence of cardiac activity in an embryo with
respect to its ultimate viability. After 7 weeks’ gestational
age, the pregnancy loss rate is 2% to 2.3%,
16 weeks, the rate is only 1%.
62
In our series of predomi-
60,61
and after
nantly symptomatic patients with embryos of less than
5-mm CRL, identification of cardiac activity with TVS
was associated with a 24% risk of spontaneous abortion.
Falco et al.
53
found a 15% abortion rate in pregnancies
from 5 to 12 weeks with TVS-demonstrated cardiac
activity.
Other secondary findings may also be helpful in predicting the outcome of a pregnancy. In our series, the
combination of absent cardiac activity and vaginal bleeding was associated with 100% embryonic mortality.
Subchorionic hemorrhages and absent cardiac activity
were associated with 88% embryonic mortality.
Gestational Sac Features
In many patients the embryo is not visualized on the
initial sonogram, and the diagnosis of pregnancy failure
cannot be made on the basis of abnormal cardiac activity.
In these patients the diagnosis of pregnancy failure may
be made based on gestational sac characteristics. The
most reliable indicator of abnormal outcome based on
gestational sac features is abnormal size.
using TAS, Bernard and Cooperberg
2,33
63
In 1985
observed that a
gestational sac with MSD greater than 20 mm and no
embryo had a poor outcome. In 1986, also using TAS,
Nyberg et al.
33
refined the definition of an abnormal
gestational sac as MSD of 25 mm or more without an
embryo, or MSD of 20 mm or more without a yolk sac.
These criteria were reevaluated for TVS. MSD of
8 mm or more without a demonstrable yolk sac, or
16 mm with no demonstrable embryo, is abnormal and
indicates pregnancy failure.35 Most authors allow a few
millimeters of leeway in MSD measurements as a margin
of error, and many do not use the absent yolk sac as a
sign of pregnancy failure. Furthermore, these parameters
only apply to high-resolution TVS and cannot be used
for examinations performed with a 5-MHz transvaginal
probe. Rowling et al.
64
studied early pregnancies with
lower-frequency transvaginal probes (5 MHz) as well as
higher-frequency probes (9-5 MHz broadband). The
gestational sac was first seen at 6.4 mm in size with the
lower frequency but at 4.6 mm with higher frequencies.
A yolk sac was always seen in normal pregnancies with
a gestational sac greater than 5 mm, and an embryo was
always seen with a sac of 13 mm, using frequencies above
5 MHz.
Our practice is to use the 8-mm and 16-mm sac size
values and to repeat a suspicious or indeterminate study
in 1 week. Normal gestational sac growth is 1.1 mm/day.
Nyberg et al.65 found that patients with early pregnancy
failure had MSD growth rates of less than 0.7 mm/day.
This growth rate is useful information in assessing the
normal development in serial examinations. With an
expected growth rate of 1.1 mm/day, one should see an
appropriate increase in sac size and, if normal, the appearance of a yolk sac or an embryo. If the growth is less than
expected, it gives one confidence in the diagnosis of early
pregnancy failure. It is also important to view the pregnancy in light of the clinical condition. A patient who is
3
in the process of a spontaneous abortion will often present
with brownish spotting, a decrease in the symptoms of
pregnancy (breast tenderness, nausea), and on examination, a uterus smaller than expected. The latter sign is
subjective and not reliable in early gestation (Fig. 30-21).
In their study of early sac size from 4 to 6 weeks’ MA,
15
Oh et al.
3
to predict an abnormal outcome with a sensitivity of
found that MSD less than 6.5 mm was able
89.3%, a specificity of 63.2%, and a negative and positive predictive value of 80%. In practical terms, this
value is useful only if one is absolutely sure of the date
of the LMP.
Other gestational sac criteria are less reliable alone, but
together or with an abnormally large sac, these features
provide additional support for the diagnosis of early
pregnancy failure: distorted gestational sac shape (Fig.
30-22), thin trophoblastic reaction (<
2 mm), weakly
echogenic trophoblast, and abnormally low position
of the gestational sac within the endometrial cavity
(Fig. 30-23).
A gestational sac greater than 16 mm without an
embryo is a strong sign of early pregnancy failure.
However, a sac of 16 mm or less without an embryo and
with bleeding does not guarantee a positive outcome. In
a prospective study of 50 patients with MSD of 16 mm
or less, no embryo, and first-trimester bleeding, Falco
66
found that 64% eventually miscarried; 13/18
et al.
(72%) of those who continued to delivery had a yolk sac
seen; and 13/32 (40%) went on to fail even though a
yolk sac was present. Advanced maternal age (>35) and
low serum β-hCG (<
1200 mIU/mL IRP) were associ-
ated with increased risk of pregnancy failure. The finding
of a smaller-than-expected MSD (<
1.34 SD) carried a
risk of miscarriage of 93%.
The intact gestational or chorionic sac and embryo are
not usually seen after abortion. Figure 30-24 demonstrates sonographic and pathologic correlation in a
7-week, 3-day embryo within an intact sac immediately
after a spontaneous abortion.
Amnion and Yolk Sac Criteria
Visualization of the amnion in the absence of a sonographically demonstrable embryo after 7 weeks’ MA is
33
1092 PART IV ■ Obstetric Sonography
A B
FIGURE 30-21. Early pregnancy failure with large, empty sac. A, Transvaginal coronal, and B, transvaginal sagittal,
images of an empty gestational sac. Mean sac diameter (calipers)
is 18 mm. No yolk sac is identified.
A
FIGURE 30-22. Early pregnancy failure with irregular sac. A, Transvaginal sagittal and transverse views of an irregular
empty gestational sac in a 40-year-old woman with spotting at 11 weeks. Mean sac diameter (calipers)
is present, the sac is irregular, and the trophoblast is thin. B, Power Doppler ultrasound with a small area of vascularity at the implantation site (arrow).
abnormal and diagnostic of a nonviable pregnancy. The
amnion is usually visualized after the embryo, so it
should not be visualized in the absence of an embryo.
B
is 25 mm. No yolk sac or embryo
sac calcification. In general, however, other signs of
embryonic demise are present when these findings are
positive.
The clinician may see two sacs within the gestational sac.
Although it may be a monochorionic diamniotic pregnancy, it may also be a failed pregnancy with an empty
amnion and a yolk sac. Other findings that may be useful
in the diagnosis of embryonic demise include a collapsing, irregularly marginated amnion (Fig. 30-25) and yolk
Sonographic Predictors
of Abnormal Outcome
Sonographic findings may be used to predict abnormal
outcome in the presence of a live embryo, or before
A
B
FIGURE 30-23. Aborting sac. A 23-year-old pregnant
woman at 8 weeks’ gestation presented with cramps and spotting.
A, Transvaginal sagittal scan shows a gestational sac in the lower
uterine segment extending into the cervix. B, Sagittal scan of the
sac within the upper cervix. Note the small yolk sac and the adjacent small embryo. No cardiac activity was detected.
visualization of the embryo. These findings can be used
to identify a high-risk subgroup of embryos at risk
for embryonic demise or subsequent diagnosis of fetal
anomaly that require close follow-up.
Embryonic Bradycardia
Although embryonic cardiac activity indicates that the
embryo is alive at examination, an abnormally slow heart
rate may predict impending demise. Doubilet and
67
Benson
found that a heart rate less than 80 beats/min
in embryos with a CRL less than 5 mm was universally
associated with subsequent embryonic demise (Fig.
30-26). A rate of 80 to 90 beats/min was associated with
a 64% risk of demise, 90 to 99 beats/min with a 32%
risk, and 100 beats/min with an 11% risk. Heart rates
above 100 beats/min are considered normal in embryos
of CRL less than 5 mm. In embryos of CRL 5 to 9 mm,
a heart rate less than 100 beats/min was always associated
with abnormal outcome, with the normal rate 120 beats/
min or more. In embryos of CRL 10 to 15 mm, a heart
rate less than 110 beats/min appears to be associated
with a very poor prognosis.
Arrhythmia is also an indicator of first-trimester
68
In a group of 950 patients, Vaccaro et al.68 found
loss.
four arrhythmias, with three having ventricular brady-
Chapter 30 ■ The First Trimester 1093
cardia, all of which were dead on follow-up scan within
2 weeks.
Mean Sac Diameter and
Crown-Rump Length
Bromley et al.69 found that in 16 patients at 5
1
2
to 9
weeks’ gestational age with MSD less than 5 mm greater
than the CRL (i.e., MSD − CRL = <5 mm), sometimes
termed early oligohydramnios, 15 had spontaneous
first-trimester abortion despite a normal heart rate for
age (Figs. 30-27 and 30-28).
Yolk Sac Size and Shape
Perhaps the most important consideration is that yolk
sac abnormalities may predict abnormal outcome in
pregnancies that appear otherwise completely normal by
all other ultrasound criteria.
70
Rat embryo experiments
demonstrate defects in the yolk sac structure and ultrastructure in response to hyperglycemia. Human data
indicate that yolk sac malformations occur in embryos
of diabetic mothers in the first trimester of pregnancy
before 9 weeks.
71
Studies have attempted to characterize the normal
sonographic appearance of the yolk sac and to identify
abnormal morphologic features may predict poor fetal
outcome. Green and Hobbins
72
found that in patients
at 8 and 12 weeks’ gestation, yolk sacs 2 mm or less in
size were associated with a poor outcome. A solid, echodense yolk sac was associated with fetal death or an
anomalous fetus. In our experience, an echogenic yolk
sac is not always associated with anomalies or impending
demise and may revert to a more normal appearance.
Lindsay et al.
35
compared yolk sac internal diameter
to gestational age, CRL, and MSD (Fig. 30-29). A yolk
sac diameter outside the 95% confidence limits for these
other parameters is a relative indicator of increased risk
of embryonic demise or fetal abnormality. However, the
sensitivity of yolk sac size as a predictor of outcome is
only 15.6%, because 50% of abnormal pregnancies have
a sonographically normal yolk sac. Although the 5% and
95% confidence limits can be used to predict increased
risk, a yolk sac diameter greater than 5.6 mm between 5
and 10 weeks is always associated with an abnormal
outcome in singleton pregnancies (Fig. 30-30). A yolk
sac greater than 5.6 mm may be seen normally in
MCMA twins.36 Furthermore, a thick, symmetrical yolk
sac has a predictive value of 93.3% for normal outcome.
35
A thin yolk sac has a predictive value of 53.8% for
abnormal outcome.
Yolk sac shape is not as predictive of outcome as size.
Kucuk et al.
70
found that 10 of 219 normal pregnancies
(4.5%) and 9 of 31 early pregnancy failures (29%) had
an abnormal yolk sac shape. Shape alone had a sensitivity
of 29% and specificity of 95% in predicting an abnormal
outcome.
1094 PART IV ■ Obstetric Sonography
A
C
B
D
E
FIGURE 30-24. Aborted gestation at 7 weeks, 3 days. A recently aborted but intact sac about 2.8 cm in diameter with an
embryo. The sac was scanned in a water bath so that the frondlike chorionic villi can be seen around the sac floating freely. A and
B, Embryo with 12-mm crown-rump length is attached to the wall by a short umbilical cord. No yolk sac was seen; it likely regressed.
C, 3-D view. D, Sac is floating in a water bath so that the white chorionic villi are seen extending outward. The villi only cover a portion
of the sac. The villi normally degenerate over the area of the sac not at the implantation site. E, Magnified view of the villi, and F, a vessel
within the sac (arrow).
F
FIGURE 30-25. Collapsed amnion. Transvaginal power
Doppler ultrasound scan of a gestational sac in a 39-year-old
woman who presented with spotting at
is small with a crown-rump length (calipers)
with 7 weeks. No cardiac activity is seen. The amniotic membrane
(arrow) is collapsed adjacent to the embryo.
1
9
weeks. The embryo
2
of 7 mm, consistent
Chapter 30 ■ The First Trimester 1095
A
FIGURE 30-26. Fetal bradycardia. A small embryo in a
10-week gestation with a heart rate of 69 beats/min. This embryo
died, and the pregnancy aborted within 1 week. The embryo is
seen within a round amniotic sac on the left and lies beside a large
yolk sac on the right.
An abnormally large yolk sac is often the first sonographic indicator of pathology and is invariably associated with subsequent embryonic demise (Fig. 30-30).
Even if the pregnancy survives the first trimester,
however, the fetus may still be abnormal. In our experience, although the number of cases is small, large yolk
sacs have been associated with fetal pathologic states,
including chromosomal abnormalities (trisomy 21,
partial molar pregnancy
73
) and omphalocele. Yolk sac
abnormalities should be used as a predictor of abnormal
B
FIGURE 30-27. Twins: one normal, one with small
sac. A, Transverse transvaginal scan at 8 weeks shows two sacs
(A, B), with the left larger than the right sac. B, At 9 weeks the
normal-sized embryo on the maternal right is of appropriate size,
19.9 mm (calipers), with a normal-sized gestational sac. The other
twin did not grow normally.
FIGURE 30-28. Small gestational sac and embryo.
Sagittal transvaginal scan of a 21-year-old woman at 9 weeks’
gestational age with spotting. There is a small gestational sac that
is no larger than the embryo (arrow). The crown-rump length and
mean sac diameter are about equal. No heartbeat was seen.
1096 PART IV ■ Obstetric Sonography
A
B
FIGURE 30-29. Normal obstetric data. A, Yolk sac
versus mean sac diameter. B, Crown-rump length versus yolk sac.
Yolk sac diameter and shape at endovaginal ultrasound are predictors of pregnancy outcome in the first trimester. (From Lindsay
DJ, Lovett IS, Lyons EA, et al. Yolk sac diameter and shape at endovaginal ultrasound: predictors of pregnancy outcome in the first trimester. Radiology 1992;183:115-118.)
outcome, and patients with abnormal yolk sac size or
shape should be followed closely. If the fetus survives the
first trimester, follow-up examination should be performed at 18 to 20 weeks’ MA to evaluate the fetus for
anomalies. Genetic counseling should also be offered.
A calcified yolk sac appears as a shadowing echogenic
mass in the absence of any other identifiable yolk sac. It
has not been reported to be associated with a live embryo
before 12 weeks’ MA. In fact, a calcified yolk sac will
only be seen with a dead embryo and may calcify within
36 hours after demise (Fig. 30-31; Video 30-3).
The yolk sac can be filled with echogenic material
and is not the same as one that is calcified. This can be
seen in live pregnancies (Fig. 30-32). Szabo et al.
74
followed such cases alone and in conjunction with nuchal
lucency in 3620 first-trimester pregnancies. They found
39 cases (1.0%) of echogenic yolk sacs 1.8 to 4.0 mm in
diameter in pregnancies at 9 to 11 weeks’ MA; 19 of the
39 (49%) had both a nuchal lucency greater than 3 mm
and an echogenic yolk sac, with all 19 chromosomally
abnormal, and the other 20 (51%) had an echogenic
yolk sac as the only unusual finding, with all delivered
normally.
In our experience, in pregnancies less than 10 weeks’
gestation, an embryo without a visible yolk sac is abnormal and associated with an abnormal outcome (assuming
that careful TVS has been performed to look for the yolk
sac). If the embryo is alive and a yolk sac is not visible,
the index of suspicion for abnormal outcome should be
increased and a follow-up examination performed.
FIGURE 30-30. Large yolk sac. Transvaginal scan at 9
weeks shows gestational sac with a small embryo with bradycardia
(not shown) and a large yolk sac (calipers) with mean internal
diameter of 5.9 mm. On follow-up examination 7 days later
(not shown), no cardiac activity was identified, indicating embryonic demise and the yolk sac had become smaller and more
echogenic.
Low Human Chorionic Gonadotropin
Nyberg et al.20 found that 65% of abnormal pregnancies
had a disproportionately low serum β-hCG for gestational sac size. This had a positive predictive value and
specificity of 100%.
Subchorionic Hemorrhage
Subchorionic hemorrhage, or a hematoma resulting
from abruption of the placental margin or marginal sinus
rupture,
This is an uncommon finding late in the first trimester
and may be associated with vaginal bleeding. Ball et al.
75
causes elevation of the chorionic membrane.
76
found an overall incidence of 1.3%. The chorionic membrane is stripped from the endometrium (decidua vera)
and elevated by the hematoma (Figs. 30-33 and 30-34).
These hemorrhages are contiguous with the placental
edge, but the predominant accumulation of blood products is often remote from the placenta.
77
Acute hemor-
rhage is usually hyperechoic or isoechoic relative to the
placenta. The hemorrhage gradually becomes sonolucent
in 1 to 2 weeks. Often the cause of membrane elevation
is obvious because fluid is present deep to the membrane
and is more echoic than the amniotic fluid, indicating a
subchorionic hemorrhage.
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