Scanning at 18 to 24 weeks should check the fetus, placenta, amniotic fluid and the cervix.
I usually perform this scan twice at 20 weeks and again at 24 weeks, only experts with high
resolution ultrasound machines are able to perform the anomaly scan once.
and can be applied easily in obstetric clinic setting, these are the items of the checklist;
details are listed in the relevant categories in the website. (categories are in the upper left part of this screen)
(2) and anteriorly a midline fluid filled cavity (larger than the third ventricle) is the cavum septum pellucidum
(3) lying between the anterior horns of the lateral ventricle (4). The BPD is taken at this level.
- Transcerebellar view (Cerebellum - Vermis - Posterior fossa <10mm - Nuchal fold < 6mm)
below the transthalamic view and is obtained when the cerebellum and cisterna magna are visualized (Oblique axial).
(should not exceed 6mm) are evaluated.
- Trans-ventricular view (Choroid plexus - Posterior horn of lateral ventricle < 10mm ) which is just
above the transthalamic view the lateral cerebral ventricles are visualized with the echogenic
choroid plexus seen filling the bodies of the lateral ventricles, as gestation progresses from
18 to 24 weeks the ventricles and choroid plexus appear less pronounced with the growth of the cerebral hemispheres.
All are axial (transverse) of the fetal head with the exception of the cerebellar view, which is axial oblique.
- You should examine fetal vertebrae by visualizing them in the saggital and parasaggital planes tilting the
transducer so you can identify three parallel lines (rossettes) that represent the three ossific centers
(2 lamina and vertebral body) they widen slightely at the lumbar region and converge at the sacral region.
- Examination should be from the cervical down to the sacral region (to avoid missing sacral agenesis -
Use the iliac rest to identify the converging sacral lamina).
- Also make sure no cysts or masses arising from the posterior aspect of the VC and overlying soft tissue.
- One of the important point of examination is verify movement of the fetal Upper and
lower limbs is an important sign of spinal cord integrity.
The standard views are 4, namely
"4 chamber view” then
"5 chamber view” then
"3 vessel view” then
but actually the upper 2 views are overlapping so I prefer to refer to them together as
"Great vessels view”. Each view is "just" above the other while sweeping the probe towards the fetal head.
Start by having a good look below at illustration 1 demonstrating anatomy of the fetal heart and
illustration 2 showing the three main views with relevant relation between the cardiac and vessel structures.
Illustration 1 Illustration 2
THE STANDARD VIEWS
THE 4 CHAMBER VIEW:
AT THE TRANSVERSE PLANE OF LOWER CHEST
Always start be identifying the right and left ventricles:
The chamber nearest to the spine is the left atrium.
The right ventricle is the one with large contact with the anterior chest wall
The left ventricle is only in contact with the anterior chest wall at its apex,
The right ventricle is smaller as it contains the moderator band and the
chorda is attached to the septum while in the left ventricle the chorda is attached to the free wall.
When you can identify the ventricles then the right and left atrium are
consequently identified. The ventricular and atrial septa are
also identified and the valve leaflets are seen. It is worth noting
that the tricuspid valve is slightly at a lower level than the mitral valve.
Scroll through 4 chamber view images and clips to identify the chambers.
REMEMBER: Always start be identifying the right and left ventricles;
Much more images and clips of 4 chamber view are under "4 chmaber view category".
In the 4 chamber view many anomalies can be diagnosed.
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It is indeed the most important view. The following should be checked in the 4 chamber view:
1- Position, axis and size of the heart:
Levopsition: the heart is anterior and to the left of the midline
occupying approximately one third of the chest (The largest heart you will see is with Ebstein Anomaly),
make sure you know where is the left side of the fetus before commenting on cardiac position.
Abnormal cardiac position is mainly seen in non-cardiac anomalies e.g. diaphragmatic hernia, lung lesions
Levocardia :The angle between the A-P chest diameter line and a line drawn through
the inter-ventricular septum will be 45 degrees + 20 degrees
Abnormal cardiac axis is mainly seen in cardiac anomalies
e.g. conotruncal anomalies and cardiosplenic syndromes.
- Normal heart occupies 1/3 to 1/2 the chest in the 4 chamber view.
The two atria are almost equal in size,
- Both ventricles are almost of the same thickness while the left ventricle is
slightly larger than the right ventricle due to presence of the moderator band in the right ventricle.
Rhythm and contractility of both ventricles.
Attachment of the papillary muscle (septal in the right ventricle and free wall in the left ventricle).
2- The septa and valves:
T he Atrial septum appears thin and shows the physiological defect of the foramen ovale which flaps into the left atrium .
The inter-ventricular septum is thick and continuous except for its upper
thin membranous part where the atrial and ventricular septa meet the mitral
and tricuspid valve to form the crux of the heart.
The mitral and tricuspid valves open and close separately
The Atrio-ventricular Septum: between the attachment of the tricuspid v alve is at a
lower level than the mitral (in the perfect 4 chamber view) and the attachment of the mitral valve.
T he 5 chamber and Great Vessels view:
After identifying the 4 chambers in the previous view (the 4 chambe view), with a
very gentle sweep of the probe towards fetal head you can see the aorta as a
fifth chamber in the middle of the heart.
Aorta arises from the left ventricle and is directed to the right, if you sweep
up you can not trace the aorta from this view to reach the aortic arch because
this will soon be replaced by the Main Pulmonary Artery that runs above the aortic
root and below the aortic arch and divides to the right and left branches.
The "3 vessel view”
Where superior vena cava and the aorta now appear in a cross-section view while
the full length of the main pulmonary artery appears, and slightly above this
view you will lose the MPA and start viewing the arch of the aorta with the
ductus arteriosus connecting the aorta to the left pulmonary artery.
Slightly above you will see both arches (Ductal and aortic) they are almost of the
same size and using color doppler the give the same color as blood is moving
in the same direction in both arches)
These views are very close and you will always find these views overlapping.
My advice is to concentrate on finding the three vessel view and trace the structures from there,
if the vessels are not clear try to tilt the probe and sweep to-and-fro till the vessels are clear.
The objective of the great vessels view is to check the outflow tracts and great vessels:
- - The aorta arises from the center of the heart and its wall is continuous with the ventricular septum.
- The Main pulmonary artery arises from the right ventricle and gives rise to the
ductus arterious and the pulmonary arteries.
- It is important to document the aorta and MPA crossing at their origin.
- Size is also important: MPA is larger than the aorta which is larger than Superior vena cava
REMEMBER to always start be identifying the right and left ventricles
After being able to comment on the standard views and identify the
different structures, I would recommend documenting the following simple points:
1- Heart not shifted to one side of the midline
2- 4 heart chambers are of normal size
3- Inter-ventricular septum looks intact - AV septum present - Foramen ovale seen.
4- Aorta and the MPA arise from the relevant ventricle and cross each other
4. Fetal Abdomen
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Transverse and longitudinal views are useful to evaluate the following:
- Abdominal circumference measure (AC) at level of umbilical vein as it curves to join the portal system -
only a short segment of the UV within the liver should be visualized
(a long anterior segment reflects an oblique view) at the correct axial
level the measure should correlate with gestational age and other fetal measures
- Try to see the following structures in order:
Diaphragm, stomach,kidneys, bowel and UC insertion, and finally the urinary bladder.
Diaphragm: seen as a line separating chest from abdomen in longitudinal view, defects in this separation may indicate diaphragmatic hernia.
Stomach appears as a fluid filled structure at the upper left part of the abdomen on the same side of the cardiac apex. Generally stomach bubble circumference is 15 to 25% of the abdominal circumference.
Kidneys: Size: renal circumference is less than 0.3% of the abdominal circumference,
an easier estimation of the size of normal kidney is that normal kidney length
spans 4-5 vertebral bodies. Renal structure is examined to show a renal
pelvis that may contain small amount of fluid (urine) surrounded by
renal medulla that appears hypoechoic than the outer more echoic renal cortex
(this excludes hydronephrosis, cystic kidneys, renal agenesis, solitary renal cyst..etc.)
Bowel: should not be markedly distended bowel and not hyperechogenic bowel
(considered a soft tissue marker of chromosomal anomalies)
Hyperechogenecity of bowel is estimated by comparison to adjacent bone echogenicity.
Umbilical cord insertion site is seen (no herniation, omphalocele or gastroschisis)
Urinary bladder; appears at the lower abdomen as a cystic structure.
Visualization of the umbilical arteries along the lateral wall of the bladder confirms it as the urinary bladder.
Other organs identified should be as well documented
(Adrenals, spleen, gall bladder, major vascular structures, bowel)
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Can be detected as early as 14 weeks, is best viewed with both
femurs seen in one view and search between thighs for external genital structures.
Do not mention the gender if you do not see labia in a female fetus
(do not diagnose female fetus by exclusion).
6.Examination of Fetal Limbs:
a. Assure presence of 4 limbs and presence of 3 bones in each limb.
b. Assure measures of long bones femur length and humeral
Length correlate with other fetal measures and gestational age,
short measure of long bones is another marker that may
signify chromosomal anomalies, osteochondroplasia ...etc.)
c. Assure feet are oriented properly (to exclude talipes) this is
very easy in early scans around 18 weeks onward and difficult as the fetus grows and gets "stuck” in the uterus.
d. Assure limb movement is seen especially lower limbs.
e. Always try to count the fingers
7. Fetal activity and behavior; Make note if you detect:
a. Limb movement
b. Trunk movement
c. Breathing movement
d. Fine and coordinated movement: finger sucking, closing fist, eye lid movement, swallowing .. etc.
e. Notice fetal reaction to sound (with fetal heart sounds for example)
and fetal movement in response to moving the probe on the maternal abdomen.
8- Placental site:
At early ultrasound scans the screen can accommodate the whole uterus,
with the mother’s urinary bladder full you can easily spot the uterine fundus;
just make sure placenta (part of it at least) is reaching the fundus of the uterus.
DO NOT make this comment on placental site with an empty urinary bladder.
But since the placenta at 18-22 weeks gestation is always near or even
may overly the internal os you may use this management protocol to identify the future at-risk patients*:
- A placenta that is 1cm or more away from the internal os is not placenta previa.
- Placenta less than 1 cm from the internal os may be previa at term and follow-up scan is needed
- Placenta that 2.5cm of its tissue is covering the internal os is mostly placenta
previa at term, and managed as such till delivery by CS after verification of placental site.
9- Amniotic fluid
Amniotic fluid is adequate if it giving the fetus a space to move and the physician the contrast to examine the fetus..
amniotic fluid assessment:
Amniotic fluid index:
Amniotic fluid index is the objective measure of the subjective assessment of AFV,
measure the vertical depth of the largest pocket in every quadrant of the 4 uterine quadrants.
The sum of the 4 would be considered:
i. Normal 10 cm – 20 cm
ii. Below average 6 cm – 9 cm
iii. Oligohydramnios < 5 cm
iv. Above average 20 cm -24 cm
v. Polyhydramnios > 24 cm
deepest (maximal) vertical pocket (DVP) depth is considered a
reliable method for assessing AFV on ultrasound. It is performed by assessing a
pocket of maximal depth of amniotic fluid which is free of UC and fetal parts.
The usually accepted values are:
<2 cm: indicative of oligohydramnious
2-8 cm: normal but should be taken in the context of subjective volume
>8 cm: indicative of polyhydramnious (although some centres use a cut off of >10 cm)
- current recommendations is to assess site of Umbilical cord should be assessed
nsertion in the placenta and fetal abdomen.
- cutoff value of 2.5 cm is indicative 10- Assessment of the cervical length by TVS
of positive prediction of preterm birth in singleton