2006-07-24-11 First trimester examination © Manson www.thefetus.net/
First-trimester examination
Francois Manson, MD1, Kathleen Comalli Dillon, BA, RDMS2
1 Fecamp, France
2 Kaiser Permanente, Vallejo, California, USA
Introduction
In France, three scans are performed during a normal pregnancy. Usually, these scans are done at about 12, 22, and 32 weeks of pregnancy (gestational age).
The objective of this work is to review most of the elements that should be seen during the twelve-week scan. Obviously, all the structures described here cannot be seen in all fetuses during a screening exam. The challenge of the screening exam lies in the tiny size of the structures studied, the spatial resolution of which being sometimes impeded by the technical limits of the ultrasound system. With ever-increasing technical advances, smaller and smaller structures yield high-quality images at an earlier gestational age.
The two essential objectives of this first scan are:
Crown-rump length (CRL)
The measurement of the CRL is obtained in a perfectly sagittal section. The head must be in an intermediate position (this criterion limits the risk of under- or overestimation of the CRL and of fetal age).
The image below shows a perfect sagittal section permitting CRL measurement. Note head position and visibility of bladder and genitalia.
In the image below, note the good position of the calipers; note also good visualisation of the skin line.
The nuchal translucency measurement is important to assess the risk for aneuploidy. This risk increases as nuchal thickness increases. The criteria for the nuchal translucency measurement are well defined. Below are two images showing nuchal translucency.
Note the strict sagittal plane of these sections:
Skull and brain
The skull
The shape of the skull appears regular, and the as-yet weak mineralization allows the visualisation of both proximal and distal cerebral hemispheres. The coronal suture (CS) can be seen (below).
The brain
The study of the brain is quite limited at this age. However, some structures can be observed.
Some other structures can be imaged at the midline, such as:
Below: Coronal section of thalami. The choroid plexuses lie superior to them.
Midline brain structures:
On a sagittal section, the following midline brain cavities can be seen:
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diencephalon (1)
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mesencephalon (2)
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metencephalon (3)
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myelencephalon (4)
ACA = anterior cerebral artery; MCA = middle cerebral artery; CA ant = anterior communicating artery; CA post = posterior communicating artery; PCA = posterior cerebral artery.
Below: Coronal view of both middle cerebral arteries.
Face
The eyes
The orbits are readily observed (image 1), but their presence does not exclude anophtalmia (“empty orbits”). To confirm the existence of both ocular globes, it is necessary to demonstrate the two lenses (image 2).
The outer ears
The best way to see the outer ears is in an axial section of the fetal head. The ears appear as echogenic appendages located on either side of the skull directed outward and backward. In the second image, we see the eyes and ears in the same section.
The nose
The nose is visible on a strict sagittal section. On this type of section, the nasal bone can be seen and the normality of the profile (no frontal bossing or slanting, for instance) can be appreciated.
On the following magnified image, note the two echogenic parallel lines. The upper one corresponds to the cutaneous plane and the lower one corresponds to the nasal bone.
The following picture shows absence of the nasal bone in a fetus with trisomy 21.
The mouth and lips
These structures are quite difficult to see at this gestational age. The upper lip can be visualized, but the study of this structure will be much easier during the second-trimester scan.
Heart
The heart is nearly completely formed at this gestational age. The difficulties in studying this organ are its small size (comparable to a green pea) and its motion (due to the heartbeat). However, the global architecture is accessible.
Four-chamber view
The four-chamber view can be seen on both anteroposterior and transverse images.
Great vessels
The origin of the great vessels can be analyzed.
On sagittal sections of the fetus, the aorta and the bicaval view can be followed (here at 13 weeks).
On the coronal section below, we can see the aortic arch in blue. Rostrally, we see the two carotids. These vessels originate from the aortic arch, the left directly, and the right one by the intermediary of the right brachiocephalic trunk. Note that the bifurcation of the right carotid is seen on this image (arrows).
Thorax
Two structures can be identified during the 12-week scan.
Below: On an axial section, the lungs appear on either side of the heart.
Below: On a sagittal section, the difference between lungs and abdominal structures is seen clearly. Note the downward concavity of the diaphragm.
This separation between thorax and abdomen is also visible on a coronal section (note the intra-abdominal position of the stomach).
The thymus
To close this chapter, I would like to present an image about whose origin there may be no consensus. It is an axial section of the upper part of the chest.
This interesting part of this image is located superficial (ventral) to the two vessels (aorta and pulmonary artery) that appear in green.
It seems to me that there is an interface (yellow arrows) between the structure in question and the lungs. This analysis lets me say that the structure is the thymus, but .......? (Philippe J. is skeptical... Time is, of course, on Francois" side, since with improved image resolution for sure we will one day be able to see it... but for now...??)
Abdomen
At this stage of pregnancy, the study of the fetal abdomen does not yield a lot of information. The principal items to evaluate are the presence of the stomach and the absence of any abdominal wall defect. The largest intra-abdominal organ at this time is the liver, which performs a hemopoietic function. The other organs (spleen, bowel, gallbladder, etc.) cannot be evaluated.
Below: Axial section showing the stomach and the superior end of the umbilical vein before the beginning of the portal sinus.
Liver
Right parasagittal section showing the right lobe of the liver (Li). The bowel (B) is seen in the lower part of the abdomen, and the liver is separated from the lungs (Lu) by the diaphragm.
Left parasagittal section showing the lung (Lu) and the heart (H). The stomach (S) appears behind the left part of the liver; the bowel appears more echogenic than the liver.
Coronal section showing the liver between the lung and the bowel.
Insertion of the umbilical cord:
Like the thorax, the abdomen is rich in vessels.
Below: Sagittal section showing the umbilical cord insertion with the umbilical vein (UV),the ductus venosus (DV), and the origin of one of the two umbilical arteries (UA).
Spectral Doppler flow of the ductus venosus can be studied. Visualization of abnormal flow could increase the specificity of a finding of cardiac malformations or of thickened nuchal translucency (for aneuploidy).
Normal spectrum Abnormal spectrum with
reversed flow (Trisomy 21)
Oblique section showing the left umbilical artery.
The final image shows the hepatic vein before its junction with the inferior vena cava.
Urinary tract
Kidneys
Each kidney appears quite echogenic without any corticomedullary differentiation; the renal pelvis is clearly visible.
Transverse section Coronal section
Using energy Doppler on a coronal section, the renal arteries are seen originating from the abdominal aorta (left image). On the right image, we also see the aortic bifurcation.
Bladder
The bladder appears as an anechoic structure in the fetal pelvis.
As usual, the umbilical arteries are seen on either side of the bladder. A single umbilical artery can be ruled out with this image. When the bladder is not visible, the sonographer must wait for the bladder to fill.
Full bladder Empty Bladder
As in the second and third trimesters, the ureters cannot be seen.
Genitalia
The fetal sex can usually be determined during the 12-week scan. The determination is made by studying the angle between the genital “bump“ and the direction of the lower portion of the spine. The following images are quite typical, but as intermediate forms exist, establishing gender can sometimes be difficult.
Female gender
In female fetuses, the genital tubercle protrudes in the same direction as the lower portion of the spine with an angle of less than 30° relative to the spine. These are pictures showing female gender with and without visible bladder.
Male gender
In male fetuses, the genital tubercle usually creates an angle of greater than 30° with the lower part of the spine. The following images show male sex with and without visible bladder.
Extremities
The extremities can be studied during the first trimester scan. The anatomy can be imaged, but it is also important to demonstrate the mobility of each segment in relation to the others.
Note entire upper extremity with arm, forearm and hand.
The upper arm: The major structure of the upper arm is the humerus.
The forearm: At the forearm level, we can already see the ulna and the radius.
The hand: The hand can also be analyzed when it is open. The thumb and four fingers can be seen. The phalanges can, in favorable circumstances, be counted.
The vessels of the extremities are visible only in their proximal aspect.
This is the humeral artery:
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Lower extremity: As the upper extremity, the lower extremity can be evaluated. Note that on the three following images we can clearly assess the position of the feet in relation to the legs, which will exclude a diagnosis of clubfoot.
Extension
Half flexion
Flexion
The thigh: Both femurs, transverse section and longitudinal section.
The leg: With the tibia and fibula.
The foot: Note that the five toes are shown.
As is true of the upper extremities, the lower-extremity vessels are seen only in their proximal aspect.
The femoral artery:
Axial skeleton
The spine
The spine is studied in three anatomical planes.
Deep plane (at vertebral bodies):
Dorsal level Lumbar level
Superficial plane (at transverse processes):
Lumbar level Thoracic level
Rib cage
Three planes of section can be used to evaluate the rib cage:
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tangential section
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coronal section
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axial section.
Tangential section
Coronal section Axial section
The clavicles
The pelvis
In the the lower trunk, we see the iliac wings as echogenic lines on either side of the spine.
Fetal membranes
The placenta
At this stage of pregnancy, the placenta is well-defined. It appears as an echogenic, homogeneous structure adherent to the uterine wall. Its inferior margin is usually close to the internal os, but sometimes, as in the second image, the placenta (Pl) covers the internal os (IO) of the cervix (Ce).
In the case of twin pregnancy, careful study of placentation is necessary. In this image, the lambda sign (“twin peak sign”) establishes a dichorionic twin pregnancy.
The yolk sac
At about 12 weeks’ gestational age, the yolk sac (UV) can be seen. It appears as a regular echogenic ring. Due to the growth of the amniotic cavity and the near-disappearance of the extraembryonic coelom (EC), its location is usually peripheral. The yolk sac communicates with the embryo via the vitelline duct. The amnion appears as a thin echogenic line (arrows).
Section showing the umbilical vesicle, the amnion, and the extraembryonic coelom.
Images showing the intra-amniotic location of the fetus.
The umbilical cord
Placental insertion of the cord.
Umbilical insertion of the cord.
Visualization of the two umbilical arteries with full bladder. Also, the cord coiling can be appreciated.
To finish the chapter, here are a few images of a fetus playing with its cord (what a beautiful scarf !!!).
Parasagittal sections:
