Evaluation of Fetal Anatomy in the First Trimester (2024)

Normal Ultrasound Findings in the First Trimester

Head and Neck

Central Nervous System

In the embryonic period, the brain is hypoechoic and characterized in the sagittal plane by the presence of three echogenic structures: prosencephalon (forebrain), mesencephalon (midbrain), and rhombencephalon (hindbrain). The rhombencephalon can be visualized as early as 7 weeks and is the most prominent cavity owing to its relatively larger size compared to the other two cavities. These fluid-filled cavities are positioned in a triangular fashion, laid out as to predict their future site of structure and function. The base of the triangle is formed by the forebrain in the anterior position and the hindbrain in the posterior position with the midbrain forming the apex of the triangle in between ( Fig. 5-2 ).

Evaluation of Fetal Anatomy in the First Trimester (1)

Forebrain and Midbrain.

The cerebral hemispheres are only tiny buds at 7 weeks, but by 9 weeks Blaas and associates demonstrated the appearance of measureable hemispheres in 79% of embryos with visualization in all fetuses by 10 weeks. As the hemispheres grow, they conceal the diencephalon and meet in the midline at 11 to 12 weeks. At 12 weeks the mean measurements of the cerebral hemisphere in the sagittal view are length 16.4 mm (12.4-20.9 mm); width 6.1 mm (4.2-8.4 mm); and height 2.0 mm (1.4-2.5 mm). As the first trimester progresses from the 8th week onward, the mesencephalon gradually moves toward a more central location, and the third and fourth ventricles become more organized with a distinct isthmus visible between the cavities of the prosencephalon and mesencephalon by the 9th week.

Lateral Ventricles.

At 7 weeks, a single ventricle is visible, as the midline falx cerebri has not yet developed. Timor-Tritsch and colleagues demonstrated variability in the development of the falx. Although the falx was visible in the majority (75%) of embryos by 9 weeks, in a small proportion the division of the ventricles was not visible until the 10th week, when the falx was clearly visualized dividing the two lateral ventricles in all embryos. The choroid plexus can be visualized routinely within each of the lateral ventricles by the middle of the 9th week, and by the 12th week it fills most of the space of the lateral ventricles ( Fig. 5-3 ).

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At 12 weeks, the corpus callosum is not visible, but the trunk from which it will continue to develop may be seen. The cavum septi pellucidi (CSP), an important marker of normal CNS development, can be seen in 40% of pregnancies at 15 weeks and in 82% at 16 to 17 weeks.

Cerebellum and Posterior Fossa.

The rhombencephalon is the most obvious structure of the early embryonic CNS. As the cerebellar hemispheres grow, they become measurable by the middle of the 10th week and appear to meet in the midline by 11 to 12 weeks. The choroid plexuses of the fourth ventricle are visible by the middle of the 10th week as echogenic areas that traverse the roof of the fourth ventricle.

Spine.

The spine will be detectable as two parallel lines as early as the 8th week in a majority of embryos. The cervical, thoracic, and lumbar spine can be visualized in 80%, 81%, and 72% of fetuses between 11 and 12 weeks and 89%, 87%, and 72% of fetuses between 13 and 14 weeks. The sacral spine is more difficult to visualize in the first trimester, with visualization rates of 35% between 11 and 12 weeks and 48% between 13 and 14 weeks ( Fig. 5-4A and B ).

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Skeletal Structures/Face

Skull.

The bony cranium begins to ossify at 9 weeks and can be precisely measured after 9 weeks using high-frequency transvagin*l sonography. Using a combination of transvagin*l and transabdominal sonography, Souka and colleagues were able to visualize the fetal head, including the complete cranium and the presence of the falx in all cases between 11 and 14 weeks’ gestation ( Fig. 5-5A ).

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Face.

The structure of the face changes dramatically throughout the first trimester and therefore is not reliably imaged until the 11th or 12th week. Souka and colleagues were able to visualize the fetal face, including the orbits, lenses, and profile 99.7% of the time using a combination of transabdominal and transvagin*l sonography from 11 to 14 weeks. In addition, there does not seem to be a significant difference in the ability to detect the structures of the face, including the lenses, profile, nose, and lips between 11 and 12 weeks compared to 13 and 14 weeks. In the embryonic and early fetal periods, the forehead is the dominant feature of the developing human face. After 11 weeks, the fetal profile takes on the expected contour with balance between the mandible, the maxilla, and the forehead. At 12 weeks a coronal section of the face can reliably be obtained and allows assessment for symmetry through imaging of the orbits, mandible, and maxilla ( Fig. 5-6 ). In the coronal section of the face, the fetal lenses can be visualized as an echogenic ring with a sonolucent center in 91% of fetuses at 11 to 12 weeks’ gestation.

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By 11 weeks, imaging of the palate and lips can be reliably performed ( Fig. 5-7 ). The nose also becomes fully formed by 11 weeks, and the presence or absence of the nasal bone can be evaluated at this time. The frontomaxillary facial angle measured using three-dimensional (3D) volumes of the fetal face has been investigated as a screening tool for trisomy 21, which is associated with significant flattening of the face. The frontomaxillary angle decreases with increasing crown-rump length. Nomograms for chromosomally normal fetuses have been reported.

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The international 3D focus group has published recommendations regarding imaging of the fetal face in the first trimester. These recommendations include imaging specifications that are unique to the developing first trimester face and are particularly useful for correct visualization of the nasal bone and excluding face malformations.

Neck

Evaluation of the nuchal anatomy in the first trimester has been a stepping stone that has led to the development of fetal anatomic assessment in the first trimester. The measurement of the nuchal translucency (NT) is now a widely accepted tool used for first trimester aneuploidy screening. The nuchal translucency is gestational age dependent and can be reliably measured from 11 to 14 weeks’ gestation ( Fig. 5-8 ).

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Chest

The fetal chest, imaged between 11 and 14 weeks’ gestation, demonstrates improved visualization of the pulmonary and cardiac structures occurring over the course of time. Using a combination of transvagin*l and transabdominal sonography, Timor-Tritsch and colleagues demonstrated a significant difference in the ability to visualize the fetal lungs between 13 and 14 weeks compared with 11 to 12 weeks (77% vs. 64%, P = 0.02). The fetal diaphragm was visualized by the same group in 87% of fetuses from 11 to 12 weeks ( Fig. 5-9 ).

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Cardiac Features

Using high-frequency transvagin*l sonography, the fetal heart motion is first visualized from 5 weeks and 4 days to 5 weeks and 6 days, with an average rate at 6 weeks of 104 bpm. In a longitudinal study of cardiac development in the first trimester, Blaas and colleagues demonstrated that the fetal heart rate follows an inverted- U curve from 7 weeks to 12 weeks. The fetal heart rate was noted to be at 138 bpm at 7 weeks, peaks at 175 bpm at 9 weeks, and then decreases to 166 bpm at 12 weeks.

Although it has been demonstrated that the major structures of the heart can be visualized as early as 10 weeks, the success rate of visualizing these structures has been shown by several authors to improve with each advancing week. In addition, Gembruch and colleagues have demonstrated better visualization using transvagin*l sonography compared to transabdominal sonography from 10 to 13 weeks, equivalent visualization of the two approaches at 14 weeks, and after 14 weeks superiority of the abdominal approach.

Situs can be confirmed by documenting appropriate orientation with the aorta to the right of the spine, inferior vena cava anterior and right of the spine, and the heart lying on the left side of the chest with the appropriate axis. Compared to the 45 ± 10 degree cardiac axis in the mid-second and third trimesters, the cardiac axis in the first trimester begins much more midline and gradually rotates to the 45-degree position by about the 12th week. From the 8th through the 9th weeks the heart sits at approximately 25 ± 12 degrees, from the 10th through the 11th week the axis rotates to 40 ± 9 degrees, and between the 12th and 15th weeks it reaches the final cardiac axis at 49 ± 7 degrees ( Fig. 5-10 ).

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At 12 weeks the four-chamber view can be obtained reliably in 90% of fetuses using transvagin*l sonography. Two equally sized ventricles and atria with an intact ventricular septum up to the cardiac crux are seen in the normal four-chamber view ( Fig. 5-11A and C ). The atrial appendages are more pronounced in the first trimester fetal heart compared to later in gestation, and the ventricles are in a spiral arrangement; however, the resolution of 2D sonography limits our ability to appreciate these differences, which have been demonstrated using 4D high-resolution transvagin*l sonography (4D HREVS). The right and left ventricles can be difficult to distinguish because of their spiral orientation, but they can be distinguished by identification of the offset of the atrioventricular valves with the tricuspid valve on the right side positioned slightly lower than the mitral valve on the left. The atrioventricular septum grows relatively late in the first trimester, and as such, the offset of the mitral and tricuspid valves may not be apparent until after 12 weeks. Doppler sonography can be used to help identify the morphologic features of the right and left ventricles. Lombardi and colleagues described the “banana-shaped” right ventricle and “ballerina shoe–shaped” left ventricle with its less severe curvature. In addition, a combination of color Doppler and power Doppler is also recommended to help define the septal aortic continuity, evaluate normal valvular flow, and show pulmonary vein flow into the left atrium. The short axis view of the great vessels, which demonstrates the wrapping of the pulmonary artery from the right ventricle continuous with the branching of the ductus arteriosus, also demonstrates the tricuspid and pulmonary valves and is therefore useful for measuring flow across these valves. It should be noted that in the first trimester it is particularly important to apply the ALARA principle and keep the thermal index below 1, which can become an issue when Doppler is used.

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Demonstration of the outflow tracts can be achieved in many fetuses by the 12th week, with improved visualization in the 13th week. The long-axis view of the aorta with the aorta arising from the left ventricle adequately demonstrates the left ventricular outflow tract (LVOT) in 68% of fetuses in the 12th week, increasing to 92% in the 13th week ( Fig. 5-11B ). In the long-axis view of the aorta, the continuity of the ventricular septum with the anterior wall of the aorta and the anterior mitral leaflet with the posterior wall of the aorta can be visualized. By angling the transducer toward the fetal right side, if the aorta and pulmonary artery are oriented in their normal crossover configuration, the long-axis view of the pulmonary artery can be obtained, which demonstrates the right ventricular outflow tract as the pulmonary artery branches off the right ventricle cephalad toward the fetal left side.

The aorta is seen arising centrally from the left ventricle and the pulmonary trunk from the anterior right ventricle with the pulmonary trunk crossing over the ascending aorta to the left side of the fetus ( Fig. 5-12 ). The ductal arch is anterior to the transverse section of the aortic arch and heads almost directly posterior toward the spine where it converges with the descending aorta to the left of the spine ( Fig. 5-13 ). The three-vessel view with the pulmonary trunk originating from the right ventricle, the aorta, and the superior vena cava is useful for identifying and comparing the sizes of the great vessels. The use of color Doppler can confirm forward flow through the outflow tracts. Using power Doppler, the normal orientation of the great vessels can be demonstrated using the X sign, which identifies the normal crossover of the pulmonary trunk over the aorta, and the b sign, which demonstrates a transverse plane at the level of the sweeping arch of the aorta and near linear path of the pulmonary artery.

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Nomograms for cardiac biometry between the 10 and 15 weeks have been published by Smrcek and coworkers.

Abdomen

Examination of the abdomen in the first trimester begins with evaluation of the ventral wall ( Fig. 5-14 ). The physiologic herniation of the midgut can be seen as early as 7 weeks and becomes most identifiable from 9 to 10 weeks with retraction back into the abdominal cavity from the middle of the 10th week through the 11th week ( Fig. 5-15 ). By 12 weeks the thickening of the umbilical cord associated with midgut herniation has resolved, and the umbilical cord insertion into the abdominal cavity is able to be evaluated.

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Gastrointestinal Tract

Stomach.

As early as 8 weeks, the stomach can be visualized as a left-sided hypoechoic cavity in the upper abdomen. By 11 to 14 weeks the stomach is visible in 99% of fetuses ( Fig. 5-16 ).

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Bowel.

After 12 weeks the entire bowel is intra-abdominal, with visualization more successful between 13 and 14 weeks compared to 11 to 12 weeks (88% vs. 77%, P = 0.02) ( Fig. 5-17 ).

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Genitourinary Tract

Kidneys.

The fetal kidneys appear as hyperechoic structures with hypoechoic centers lateral to the spine. Using a combination of transvagin*l and transabdominal sonography between 11 and 14 weeks, the visualization rate for the kidneys in the first trimester ranges from 80% to 95%. Timor-Tritsch and associates demonstrated improved visualization rates for the kidneys later in the first trimester with 80% visualization between 11 and 12 weeks compared to 91% visualization between 13 and 14 weeks ( P = .02) ( Fig. 5-18 ).

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Bladder.

The normal first trimester fetal bladder is identified as a hypoechoic structure in the fetal pelvis visualized in 93% to 100% of fetuses between 11 and 14 weeks. Using color Doppler, the umbilical arteries can be seen coursing from the umbilical cord insertion around the bladder in 84% of fetuses from 11 to 12 weeks ( Fig. 5-19 ). Normal measurements of the fetal kidney from 12 to 14 weeks have been published by Bronshtein and coworkers.

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Genitalia.

Early accurate determination of fetal gender has been reported as early 11 weeks with an accuracy rate ranging from 46% to 70%. The male and female genitalia are best distinguished in the first trimester by the direction of the genital tubercle (future phallus or cl*tor*s) seen in the sagittal plane. The male genital tubercle points in the cephalad direction, whereas the female genital tubercle is directed caudally or on the horizontal. Subjective assessment of the first trimester gender has been described and quantified for accuracy using a combination of the sagittal view and the transverse view. In the transverse view identification of a round structure at the base of the genital tubercle representing the scrotal swelling, or a midline echo at the base of the genital tubercle, representing the median penile raphe, aid in identifying the male fetus. Two, or four, parallel lines, representing the labia minora and majora, help to identify the female fetus. When the subjective two-view assessment is used 67% of male fetuses are correctly identified at 11 weeks with improvement in identification of male fetuses at 12 weeks to 89% without further significant improvement beyond 12 weeks. This is in contrast to the female fetus in which correct gender identification occurs 83% of the time but does not improve until 14 weeks when it is correct 95% of the time. Using an objective cutoff of 30 degrees from the horizontal for the direction of the genital tubercle in the sagittal view appears to have improved accuracy over the subjective two-view method, although there is no study directly comparing the two methods. Using a cutoff of 30 degrees, the gender is accurately determined in 70% of fetuses at 11 weeks, 99% at 12 weeks, and 100% at 13 weeks. Male fetuses are incorrectly assigned as female 56% of the time at 11 weeks, but only 3% of the time at 12 weeks, and 0% at 13 weeks. In contrast, female fetuses will be incorrectly assigned only 5% of the time at 11 weeks and 0% at 12 and 13 weeks.

Extremities

Using high-frequency transvagin*l sonography, the upper and lower limb buds can be imaged from 7 weeks onward. At this early gestational age, the tail of the embryo is longer than the lower limb buds and will be prominent. By the 10th week, the full length of the limbs are able to be fully imaged. Using a combination of high-frequency transvagin*l and transabdominal sonography, full examination of the extremities including biometry of the short and long bones, the fingers, toes, and the movement and posture of the joints is possible in 100% of fetuses by 11 weeks ( Fig. 5-20 ).