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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5786_Библиотеки_им_академика_М_И_Перельмана.pdf
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mm
LMP-based gestational age (weeks)
n = 29
10
30
50
70
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Correct measurement of the
crown−rump length (CRL)
9 weeks
7 weeks
A
B
C
10 weeks
MEASUREMENTS OF THE EMBRYO/EARLY FETUS
The crown–rump length (CRL) is measured as the greatest length in a straight
line from the cranial to the caudal end of the body in the straightest possible position of the embryo/fetus (Fig. 4.7). The CRL diagram presented by
Robinson in 1975 is still widely used for the evaluation and dating of the early
pregnancy.
The width of the head, also designated as the biparietal diameter (BPD),
is measured in the horizontal section perpendicular to the body axis. Due to
the development of the brain, the largest width alters its position in relation to
cerebral landmarks during the embryonic and early fetal period (Fig. 4.8). At
7 weeks, BPD is measured at the height of the rhombencephalon. In the early
fetal period the future cranium becomes more distinguished such that the BPD
can be obtained by placing the calipers at the outer border of the not yet ossified
11
Investigation of early pregnancy
Fig. 4.7 The CRL is measured as the greatest length in a straight line from the cranial to
the caudal end of the body in the straightest possible position of the embryo/fetus. At
7 weeks, the rhombencephalic cavity lies at the top of the head; later, the midbrain is at the
top. Below: Growth curve of CRL in 29 healthy embryos (reproduced from reference 9 with
permission).
63
7 weeks
Head
“BPD”
A
9 weeks
B
13 weeks
C
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Ultrasound in obstetrics and gynaecology
Fig. 4.8 Measurement of the head at 7, 9 and 13 weeks. The reference plane and landmarks
change during the first trimester. At 13 weeks, the third ventricle is visible. The cavum septi
pellucidi is not yet developed.
cranium in a horizontal section at the level of the thalamus. The anteroposterior diameter of the embryonic head, designated as the occipitofrontal diameter
(OFD), is measured in the same section perpendicular to the BPD. The embryonic head circumference (HC) measurement is usually calculated from the BPD
and the OFD, using the formula for an ellipse.
Measurements of the embryonic trunk (abdominal circumference, AC) have
64
been introduced as a possible parameter for the estimation of embryonic age
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mm
0
5
10
15
20
MAD
n = 29
BPD
0
5
10
15
20
25
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mm
LMP-based gestational age (weeks)
n = 29
11 weeks
Stomach
Spine
during first-trimester biometry.12 It is advantageous to use comparable parameters when describing embryonic and fetal biometry. Instead of measuring the
mean diameter of the abdomen and multiplying it with the constant 2π to obtain
the AC, one may use a simpler parameter such as mean abdominal diameter
(MAD)9 alone. This parameter is derived from two perpendicular measurements
taken in the horizontal plane through the upper embryonic abdomen below the
heart and above the umbilicus/midgut herniation (Fig. 4.9). Longitudinal exami-
nations of the BPD and MAD in 29 normal pregnancies showed that the growth
of the healthy embryo is constant (Figs. 4.7, 4.10).
9
In 1973, Robinson showed that the heart rate reached a maximum at 9 weeks
in the first trimester.13 The heart can easily be recognized by real-time ultrasound
Investigation of early pregnancy
Fig. 4.9 Measurement of the abdomen at 11 weeks. The reference plane is in the height
of the embryonic/fetal stomach (arrow). The MAD is calculated from two perpendicular
measurements. The abdominal circumference is also calculated from these diameters.
Fig. 4.10 Growth curves of the head width (BPD) and MAD in 29 pregnancies showing
that the growth of the healthy embryo is constant (reproduced from reference 9 with
permission).
65
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bpm
CRL-based gestational age (weeks)
0
50
100
150
200
567891011121314
Heart rate
n = 448
as a relatively large beating structure below the embryonic head. The heart rate
should be analysed electronically using the M-mode facility (Fig. 4.11). ‘Manual’
counting results in lower maximum heart rates, which again may result in incorrect counselling and poorer management of the patient. In normal pregnancies, the heart rate develops in a specific pattern, increasing from approximately
100 bpm at the end of 5 weeks to a peak mean of 175 bpm at 9 weeks, and slowly
decreasing to 150 bpm in the second trimester (Fig. 4.12).
The physiological midgut herniation is recorded by measuring the length of
the protruded bowel into the cordal coelom. It is usually detectable at 8 weeks,
has its maximal extension at the beginning of week 10, and can be seen until the
end of week 11.
8,14,15
Significant ossification of the long bones is not seen before 10 LMP weeks and
later.16 This was confirmed in a study on the development of the skeleton comparing longitudinal ultrasound imaging from living embryos/fetuses with radiographs
Ultrasound in obstetrics and gynaecology
Fig. 4.11 M-mode registration of embryonic heart activity (upper arrows) and maternal pulse
(lower arrows). The embryonic heart rate is measured as beats per minute, here 168 bpm.
66
Fig. 4.12 Embryonic and fetal heart rate in 448 examinations; the age of the embryos/
fetuses is based on CRL measurements.
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obtained from aborted silver nitrate-impregnated embryos and fetuses.10 At
10.5 weeks the ossified part of the femur was just measurable to 2.1 mm by ultrasound. In a 10-week-old silver nitrate-impregnated embryo, the femur length was
even shorter. Therefore, measurement of limbs does not have clinical significance
in the first trimester.
EXTRAEMBRYONIC STRUCTURES: THE THREE SACS
The gestational sac corresponds to the chorionic cavity. Its size has been used to
evaluate normal progress of the early pregnancy.
The amniotic membrane is easily depicted by transvaginal ultrasound at
7 weeks. For measurements, the calipers are placed on the thin membranes of
the chorionic and amniotic cavities. As with the chorionic sac, the amniotic sac is
measured by three perpendicular diameters and the arithmetical mean of these
diameters is calculated.
The yolk sac appears as a small ring with rather bright walls lying within the
chorionic cavity (extraembryonic coelom), and lying outside the amniotic cavity
after 7 weeks. Due to the loss of its physiological function, the yolk sac alters its
shape during weeks 9–11.9 The wall of the yolk sac is thinner than 0.3 mm, but
because of the transducer-dependent point spread function and the gain setting,
the echogenic wall of the yolk sac appears significantly thicker in the ultraound
image. Therefore, the calipers should be placed outside–inside or just on the middle of the yolk sac wall to avoid possible measurement bias. Thus, the measurements that most likely represent the true diameters are obtained by ‘outer–inner’
or ‘middle–middle’ placement of the calipers on the wall of the yolk sac.
The growth of the amniotic cavity and the yolk sac is uniform and constant in
healthy pregnancies9 (Figs. 4.13, 4.14).
Investigation of early pregnancy
Fig. 4.13 The three sacs: chorionic cavity and measurement of the amniotic cavity and the
yolk sac.
67
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LMP-based gestational age (weeks)
Yolk sac diameter
mm
0
1
2
3
4
5
6
7
8
n = 29
Amniotic cavity diameter
0
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20
30
40
50
60
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mm
n = 29
Ultrasound in obstetrics and gynaecology
Fig. 4.14 Growth curves of the mean amniotic cavity diameter and mean yolk sac diameter
in 29 healthy pregnancies showing that the growth of extraembryonic structures is constant.
(Arrow) At approximately 9 weeks the physiological function of the yolk sac ceases
(reproduced from reference 9 with permission).
MULTIPLE PREGNANCY: DETERMINATION OF CHORIONICITY AND AMNIONICITY
The chorionicity and amnionicity can be explained quite easily, considering the
developmental stage at which the twinning event occurs.16 Late twinning will be
incomplete and will result in conjoined twins. The ‘cleavage’ or twinning event
does not take place after 5 completed weeks.
68
The spectrum varies from dichorionic (DC) diamniotic (DA) to monochorionic (MC) monoamniotic (MA) twins, where conjoined twins represent the
extreme form of monoamniotic twins. MCDA twins have two yolk sacs; MCMA
twins usually have only one. The chorionicity and amnionicity can be diagnosed
at the end of week 5, when both embryo and yolk sac are detectable. MCDA
twins always have thin dividing amniotic membranes that become visible at
7 weeks, while MCMA twins have a common amniotic cavity without dividing
membranes (Fig. 4.15). DC pregnancies always have two thick trophoblastic tis-
sue layers and two amniotic membranes between the twins; on the ultrasound
image these layers and membranes appear as one thick wall.17 At the end of
the first trimester and beginning of the second, trophoblastic tissue in the angle
between two placentas and chorionic cavities constitutes the ‘lambda’ sign, which
is characteristic for DC pregnancies.
18
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A
B
Fig. 4.15 Twin pregnancies. (A) Dichorionic diamniotic twins at 7 weeks; notice the thick
trophoblastic tissue between the gestational sacs. (B) Monochorionic diamniotic twin
pregnancy at 8 weeks; the amniotic membrane (arrow) may be difficult to identify.
Investigation of early pregnancy
EVALUATION OF EARLY PREGNANCY FAILURE
EARLY PREGNANCY LOSS
According to the literature on embryology and sonoembryology, the size and
morphology of both embryonic and extraembryonic structures show little variation in pregnancies of the same age.
the yolk sac and the amniotic sac show a growth pattern that is closely related to
embryonic development. Measurement of the gestational sac has been used for
pregnancy evaluation, but one must be aware of the rather large variation of its
size in normal pregnancies. This knowledge can be used as the basis for the evaluation of the early pregnancy, when significant departures from normal development and measurements are found.
A threatened abortion is defined as a painless vaginal bleeding occurring before
24 weeks of pregnancy. A spontaneous abortion may be incomplete or complete.
In first-trimester bleeding, neither statistical prediction models based on signs
and symptoms nor clinical judgement are valid replacements for ultrasonographic
assessment in establishing a diagnosis.19 No single ultrasound measurement of different anatomical features in the first trimester has been shown to have a high
predictive value for determining early pregnancy outcome.20 Therefore a systematic evaluation of the early conceptus using combined biometric parameters is
recommended.
2
6–9,16
Of extraembryonic structures, especially
69
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Gestational sac (chorionic cavity) and amniotic cavity
The size of the gestational sac (chorionic cavity) has been used to evaluate
normal progress of the early pregnancy. An abnormal size of the chorionic
cavity, compared with the size of the embryo, has traditionally been associated with impending early pregnancy loss
21,22
but its significance has not been
extensively documented.23 This is probably due to the large variability of its
size in normal pregnancies.9 Abnormality is probable when the mean size of
the chorionic cavity is >10 mm without a yolk sac or >20 mm without an
embryo.
Another method of evaluating development is to compare the size of the
amniotic cavity with the CRL.23 Embryologists have shown the close relationship between the amniotic cavity volume and fetal size. This has been confirmed
by ultrasound studies showing a remarkable similarity in the absolute values
of CRL and the mean diameter of the amniotic cavity in normal pregnancies
between 7 and 11 weeks.9 A significant discrepancy between these two parameters is a possible sign for abnormality. A mean amniotic cavity diameter that is
significantly less or larger than the actual CRL or an amniotic sac that is smaller
Ultrasound in obstetrics and gynaecology
than the yolk sac, or even absent after 7 weeks, are suspicious signs of abnormal
development.
If the size of the gestational sac or the embryo is smaller than the expected
age, the possibility of incorrect age should always be considered and a repeat scan
should be performed after 1 week. Normal growth and appearance of additional
anatomical details may then rule out an abnormal early development.
70
Yolk sac
The yolk sac plays an important role in the early nutrition of the embryo, and is
the source of early haematopoiesis.16 Thus, abnormal embryonic development
may be reflected in an abnormal appearance of the yolk sac. However, many
pregnancies that end in abortion show normal appearance of the yolk sac at an
initial early scan; conversely, changes of shape and echogenicity have been found
in uncomplicated pregnancies.23 In general, the finding of a yolk sac which is
<3.0 mm between 6 and 10 weeks, >7 mm before 9 weeks, absent or clearly irregular in shape indicates a possible abnormal early pregnancy.
Haematoma
Intrauterine haematomas are blood accumulations that are subchorionic, retroplacental or both (Fig. 4.16). The results from numerous studies of the intrauterine haematoma are not unequivocal. Today, the importance of intrauterine
haematoma for early pregnancy loss is played down.23 A study from 2001 even
concludes that intrauterine haematomas do not have a deleterious effect on pregnancy outcome in a population with recurrent miscarriage.24 But it seems reasonable to assume that if the haematoma lies under the placenta and cord insertion,
it has the potential to lead to placental separation and abortion; and that also very
large subchorionic haematomas may cause uterine contractions with subsequent
pregnancy loss.
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Chorionic cavity
Uterus
Haematoma
Embryo
Trophoblastic
tissue
Umbilical
cord
Fig. 4.16 8/4 weeks pregnancy showing an intact normal embryo and its umbilical cord.
There is a large haematoma on the outer side of the gestational sac. The amniotic membrane
is not visible on this image.
Investigation of early pregnancy
Heart rate
There is a good correlation between the heart rate and embryonic size and age.
Alterations of the embryonic heart rate such as arrhythmia and/or bradycardia
may be associated with maldevelopment.
22,25
Embryonic heart rate measurements in early pregnancy may be useful in the prediction of first-trimester spontaneous abortion after ultrasound-proven viability, but a heart rate below the 95%
confidence interval of normal does not necessarily indicate a poor outcome.26
A general rule is that if the embryo has a CRL of 6 mm or more, the lack of heart
activity is highly suspicious for intrauterine embryonic/fetal death. A significant
relationship to abortion has been found when the heart rate is less than 1.2 SD
from the mean.
21
TROPHOBLASTIC DISEASE
Gestational trophoblastic diseases are complete, partial and invasive moles,
placental site trophoblastic tumours and choriocarcinomas. An invasive hydatidiform mole is defined by penetration of molar villi into the myometrium or
vasculature of the uterus. Both complete and partial moles can become invasive. Ultrasound has replaced all other techniques for early diagnosis and management of these conditions.27 However, a routine pre-evacuation ultrasound
examination identifies less than 50% of hydatidiform moles, the majority sonographically appearing as missed or incomplete miscarriage.28 Testing β-hCG
levels of maternal serum is still of major importance for the evaluation and
follow-up of treatment.
71
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Complete hydatidiform mole
Complete hydatidiform mole develops when the diploid chromosomal set of the
conceptus is entirely derived from paternal chromosomes. Patients with complete
hydatidiform mole present a large uterus, vaginal bleeding and abnormally high
β-hCG levels. The latter causes hyperstimulation of the ovaries, resulting in enlargement through theca lutein cysts in 50% of cases. The ultrasound examination reveals
a uterine cavity filled with multiple cysts and echogenic areas of variable size and
shape (‘snow-storm’ appearance) in the absence of an embryo or fetus (Fig. 4.17).
One must be aware that certain rare uterine tumours may resemble moles. Using
ultrasound, approximately 79% of complete hydatidiform moles are detected.
Partial hydatidiform mole
In partial hydatidiform mole, a fetus is found in association with molar degeneration of
the placenta. Partial moles are usually of triploid or diandric origin, having two sets of
chromosomes of paternal origin and one of maternal origin (69,XXX or 69,XXY).27
Partial mole presents on ultrasound examination as an enlarged placenta; it is thicker
than 4 cm at the level of the cord insertion at the second-trimester routine scan and
Ultrasound in obstetrics and gynaecology
contains many cystic areas (‘Swiss cheese’ appearance). The diagnosis of partial mole is
more difficult than that of a complete mole; only 29% were detected in a large study by
Fowler et al.28 Doppler investigation plays a limited role in diagnosis or management.27
The fetus is usually growth retarded and shows variable congenital anomalies.
28
72
Invasive hydatidiform mole
An invasive mole usually appears clinically with bleeding after surgical evacuation of a molar pregnancy. Sonographically, nodular areas of increased echogenicity are found in the uterine wall. The lesions may contain fluid-filled cavities.27
Doppler may be used to evaluate the effectiveness of medical therapy.
Choriocarcinoma
Choriocarcinoma is highly malignant, developing from trophoblastic tissue and
metastasizing into lungs, liver or brain. Women with metastases may present
Fig. 4.17 Complete hydatidiform mole: uterine cavity filled with multiple cysts and
echogenic areas of variable size and shape.