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Fig. 5.29 Feet. Axial view shows normal paired feet (F).
to the next, adjusting probe position as needed to obtain the desired images.82
Scanning beyond the femur will outline the hypoechoic cartilages of the distal
femur and proximal tibia. Subsequently the tibia and fibula and orientation of
Normal fetal anatomy at 18–22 weeks
Fig. 5.30 View of a normal hand including the thumb. Note three bones (proximal phalanx,
middle phalanx and distal phalanx) of each finger.
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Ultrasound in obstetrics and gynaecology
104
Fig. 5.31 Hand, 3D view. Three-dimensional ultrasound with surface rendering better shows
the complex shape of the hand.
the leg to the foot can be shown. Measurement of foot length and documentation of digits are possible with appropriate fetal positioning and careful scanning.
Similar scanning through the upper extremities can detail the humerus, radius
and ulna, and hand.
The clavicle and scapula define the shoulder girdle. The scapula imaged in long
axis coronally has a characteristic shape resembling a ‘Y’ with the supraspinatus,
subscapularis and infraspinatus muscles in their respective fossae. The scapula
has a triangular shape when imaged posteriorly. The clavicles can be seen if not
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obscured by flexion of the fetal head. They grow at a linear rate of approximately 1 mm per week, reaching a length of 20 mm at 20 weeks and 40 mm at
40 weeks.
The humeral head epiphyseal cartilage lies between the ossified distal clavicle,
scapula and proximal humeral diaphysis. At the elbow, the non-ossified coronoid
fossa delineates the medial and lateral humeral epicondyles. The more proximal
extent of the ulna at the elbow distinguishes it from the radius. Demonstration of
the ulna and radius ending at the same level at the wrist effectively excludes many
radial ray defects. The non-ossified carpals produce a conglomerate zone of grey
echoes antenatally, but the ossified metacarpal and phalanges are readily visualized
if the fetus extends the hand. The foot length is similar to the ossified femoral shaft
throughout much of pregnancy.
83
CONCLUSION
A second-trimester scan has now become widely accepted throughout much of
the developed world. We believe it should be considered an essential part of
obstetric care, but only when performed by experienced sonographers or sonologists. Armed with a systematic approach, a good understanding of normal fetal
anatomy and familiarity with normal sonographic appearances and common variants, the sonographer/sonologist can offer accurate reassurance in the vast majority of normal pregnancies, and at the same time detect the majority of major
defects in anomalous fetuses. Finally, it will be of interest to see to what extent
new techniques like echo-planar magnetic resonance will complement or even
replace certain areas of fetal anatomic assessment.
90
Normal fetal anatomy at 18–22 weeks
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Amniotic fluid and placental localization
Juriy W Wladimiroff Sturla H Eik-Nes
ABSTRACT
Various pathways determine amniotic fluid production and absorption, which
include fetal swallowing, lung fluid and urine production and flow across the
chorionic plate. Measurements of the amount of amniotic fluid include the
1 or 2 cm pocket rule and the amniotic fluid index.
Oligohydramnios is associated with fetal renal pathology and fetal growth
restriction, whereas polyhydramnios is often associated with a wide range of fetal
congenital anomalies, maternal diabetes mellitus, multiple pregnancy and nonimmune hydrops.
Ultrasound is the method of choice to locate the placenta. The most common
indications for locating the placenta are in connection with first-trimester invasive
procedures, bleeding in the second and third trimesters, as part of the routine
second-trimester exam and prior to external version of the fetus in late pregnancy.
The placenta is located in the fundal area, on the left or right lateral side, the
posterior or the anterior side or a combination thereof. Clinically it is most useful
to distinguish the relation between the inner os of the cervical canal and the edge
of the placenta.
KEYWORDS
Amniotic fluid absorption, amniotic fluid production, amniotic fluid volume,
management of abnormal placental location, oligohydramnios, placenta
praevia, placental embryology, placental functional anatomy, placental location,
polyhydramnios.
AMNIOTIC FLUID
The amount of amniotic fluid surrounding the fetus provides us with information about the fetal condition. To appreciate abnormal changes in amniotic fluid
109
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volume, it is necessary not only to reliably measure amniotic fluid volume but
also to understand the various pathways which determine amniotic fluid production and absorption.
AMNIOTIC FLUID PHYSIOLOGY
It is during the period of embryonic development that the amnion is formed and
surrounds the embryo. In the beginning the amnion itself is surrounded by coelomic fluid which will disappear from 9–10 weeks of gestation and a rapid expansion of amniotic fluid volume will occur thereafter. Amniotic fluid is 98–99%
water and its chemical composition varies with gestational age.10 There are five
pathways playing a major role in the exchange of water and solutes between fetus
and amniotic fluid.14 Excretion from the fetus into the amniotic cavity consists of
fetal urine flow and lung flow. The onset of fetal micturition is associated with a
reduction of amniotic osmolarity which will continue with advancing gestational
age.10 Reabsorption of amniotic fluid takes place through fetal swallowing and
absorption into the fetal circulation across the fetal surface of the placenta1 and
Ultrasound in obstetrics and gynaecology
exchange across the fetal skin before completion of the keratinization process at
approximately 22 weeks of gestation. Finally, there appears to be excretion from
the fetal salivary glands into the amniotic fluid. A short resumé of a few of the
most important pathways will now follow.
110
Fetal urinary production
Fetal urine flow constitutes an important source of amniotic fluid, hence the development of severe oligohydramnios in bilateral renal agenesis or urethral obstruction. Diagnostic ultrasound has provided data on hourly fetal urinary production
rates, with values from 2–3 mL/h at 20 weeks to 30–35 mL/h at term.17 This would
result in a urinary production rate of 700–800 mL/24 h, which is approximately
25% of fetal body weight/day. Over the years different urine production rates
have been reported as a result of different measuring techniques. It seems that the
above figures are probably more or less correct. Fetal urine contributes not only to
amniotic fluid volume but also to its composition, since osmolarity is about onehalf and chloride and sodium concentration are about one-third of plasma values.
Lung fluid
The production of lung fluid is 250–300 mL/24 h at term, which is approximately
10% of fetal body weight.9 A very small percentage of this fluid remains in the
lungs for expansion with growth. Nearly 99% of the fluid leaves the lungs through
the trachea. Beyond the trachea about 50% is swallowed and the remaining 50%
will appear in the amniotic fluid.
Flow across the chorionic plate
It is likely that the exchange from the amniotic cavity to the fetal blood compartment of water and solutes is quite considerable, with figures of 200–250 mL/day
of water in normal fetal development near term.
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AMNIOTIC FLUID VOLUME
Both amniotic fluid volume and its composition reflect the status of mother and fetus.
Methods of assessment
Several methods of amniotic fluid assessment have been employed to detect adverse
fetal conditions. Determination of total intrauterine volume reflecting the sum of
the volume of all intrauterine contents (fetus, placenta, amniotic fluid) is achieved
from longitudinal, transverse and anteroposterior uterine dimensions. This method
was particularly applied for the early detection of fetal growth restriction, but has
a low accuracy. A quantitative approach of assessing amniotic fluid volume is the
1 cm or, even better, the 2 cm pocket rule. Using ultrasound, the largest cord-free
pocket of amniotic fluid is detected and the vertical and transverse diameter of
this pocket is measured with the transducer always at right angles to the maternal abdominal wall. Amniotic fluid volume was considered normal if the pocket
measured 1 cm or more in its largest vertical diameter and reduced if the diameter was less than 1 cm.8 However, this approach leads to a pick-up rate of fetal
growth restriction of only 4%. Later it was demonstrated that amniotic fluid pockets greater than 1 cm but less than 2 cm should also undergo further investigation
for fetal underdevelopment.2 Here, the single deepest pocket was identified.
A semi-quantitative method of assessing amniotic fluid volume is the amniotic fluid index technique.13 Using the umbilicus as a reference point, the uterus
is divided into an upper and a lower half. The linea nigra is subsequently used to
divide the uterus into a right and a left half, resulting in four uterine quadrants. The
ultrasound transducer is placed in each quadrant with the transducer head always
at right angles to the floor. A more oblique positioning of the transducer head will
result in an inaccurate amniotic fluid volume measurement. Moreover, the investigation should extend to the lateral margins of the uterus since often substantial
amniotic fluid may be situated in the flanks of the pregnant woman when in the
supine position. In each quadrant the largest pocket of fluid is sought according
to the above technique. The vertical diameter of each of the pockets is then measured (Fig. 6.1). The numbers obtained from each quadrant are added up. The
resulting figure in centimetres represents the amniotic fluid index for that particular pregnant woman. A normal amniotic fluid index ranges between 5 and 25 cm.
The amniotic fluid index and single deepest pocket measurement appear to
perform best for the identification of normal amniotic fluid volumes, whereas
the identification of oligo- and polyhydramnios is of limited value.7 In another
study, the amniotic fluid index was not significantly correlated with perinatal outcome.11 The amniotic fluid index seems to offer no advantage in detecting adverse
outcomes compared with the single deepest pocket when performed with the
biophysical profile.
Three-dimensional ultrasound has been used in measuring amniotic fluid or
gestational sac volume at 11–14 weeks of gestation,4 whereas later in pregnancy
magnetic resonance imaging was found to be comparable with ultrasound evaluation for the prediction of oligohydramnios.
7
18
Amniotic fluid and placental localization
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4.7 cm 5.8 cm
4.9 cm 6.2 cm
Ultrasound in obstetrics and gynaecology
112
Fig. 6.1 Measurement of the vertical diameter in each of the four quadrants of the uterus.
The numbers are added up to calculate the amniotic fluid index.
Normal amniotic fluid volume values
Amniotic fluid volume increases from approximately 70 mL at 11 weeks of gestation to 800 mL at 28 weeks followed by a slower increase to about 1000 mL at
34 weeks. A decline in volume takes place during the last 6 weeks of gestation to
about 800 mL at 40 weeks. Gestational sac volume appears to be a poor predictor
of major chromosomal defects.
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Abnormal amniotic fluid volumes
Abnormalities in amniotic fluid volume are associated with increased perinatal
mortality and morbidity.
Oligohydramnios
Oligohydramnios is defined as a deepest fluid pocket of less than 2 cm or an
amniotic fluid index of 5 cm or less. It develops in 0.5–4.0% of all pregnancies
and can be associated with fetal growth restriction as a result of reduced renal
perfusion and urinary output. Severe oligohydramnios (deepest fluid pocket
smaller than 1 cm) or even anhydramnios may develop in the presence of bilateral renal agenesis or urethral obstruction/stenosis. Pregnancies beyond 40 weeks
may be complicated by reduced amounts of amniotic fluid with volumes down