Intro
The fetal anatomy scan is the foundation for prenatal diagnosis and management. This scan is sometimes called a “Level II Scan,” although that nomenclature has been dropped by the AIUM.
The elements of the basic fetal anatomy include evaluation of the uterus, fetal biometry and fetal anatomy. The sequence of ultrasound pictures shows how I perform a basic anatomy scan. The central principle of the scan is the demonstration of normal anatomy. The presence of a fetal anomaly is often recognized by the absence of normal anatomy. Many different approaches are possible, but it is important that the end result is a thorough review of fetal anatomy. I evaluate the overall fetal presentation, lower segment, placenta, fluid, biometry and fetal anatomy, in that order. In evaluating fetal anatomy I scan from top down (head to rump) to evaluate the neural axis (intracranial structures) and then from bottom up (rump to head) to evalaute fetal organs. Scanning in an organized fashion (much like a CT scan), helps me to remember what structures need to be visualized and avoids missing something. Jumping from structure to structure (i.e., “there’s the face, there’s the stomach, there’s the cord insertion, there’s the heart, …) increases that chance that you will forget something (“oops, I saw everything but the …”).
Although not a standard part of the fetal anatomy scan, testing fetal well-being is an important part of ultrasound scanning. A section is included on the biophysical profile.
Overview
When you first scan the patient, determine the number of fetuses, fetal lie, presentation and presence of a fetal heart beat. It is very embarrassing, not to mention distrubing for the patient, to scan for several minutes, commenting on structures, before you notice that the fetal heart is not beating. This does not mean that you should immediately say, “The fetus is dead!” if you identify an intrautrine fetal demise. Breaking bad news to the patient is a difficult and delicate task. I can only give you my approach.
If this is a multiple pregnancy, the fetus closest to the cervix is designated “Fetus A”, next away from cervix is “B”, etc. It is important to determine zygosity if at all possible, as monozygotic twins have a higher risk of complications.
During this initial few minutes, fix the three-dimensional position of the fetus in your head. You should be able to remove the transducer and place it back on the abdomen at the location of any desired body part. (For example, place the probe over the fetal heart). This initial orientation will help you to work through the fetal biometry and anatomy.
Orientation is important in your pictures -- on a long axis (sagittal) scan, the maternal feet should be to the right of the image, head to the left.
On transverse scans, the usual orientation is maternal left on the right side of the screen. It is just as if the mother is laying with her feet coming towards the screen.
Lower Uterine Segment & Placenta
Establish fetal presentation.
Look at the cervix—although abdominal ultrasound is not reliable to measure cervical length, cervical shortening or funneling may be evident. Endovaginal scan for cervical length may be indicated if patient has a history of cervical incompetence or prior cervical surgery.
Note the relationship of the placental edge to the lower uterine segment and internal cervical os.
Orientation is important in your pictures—on a long axis (sagittal) scan, the maternal feet should be to the right of the image, head to the left.
Breech Considerations:
- Presentation
- Cervix
- Placenta previa
- Vasa previa
- Placenta accreta
Note placental location: posterior, anterior, fundal, left or right lateral
Note relationship of placenta to cervix (presence or absence of previa).
I do not recommend reporting placenta previa prior to 16 weeks gestation.
Placental grading evaluates the degree of calcification of the placenta. Although placental grading is reproducible between different examiners, it is not predictive of fetal outcomes. Therefore, I do not recommend performing nor reporting placental grade.
Biometry
A - Proper caliper placement for BPD (leading edge to leading edge, or "outer to inner")
B - Proper caliper placement for Head Circumference
C - Improper caliper placement for any measurement. Neither the edge of the calipers, nor the center are identically placed on each edge.
Biparietal Diameter (BPD)
Important landmarks for a good BPD:
Midline should be straight and equidistance between both sides of the cranium
Fetal skull should have a symmetric oval shape
Cavum septum pelludicem should be visible anteriorly
Thalamus should be visible
Cerebellum should not be visible
Measurement is made from the outer skull table to the inner skull table ("leading edge to leading edge") (see Biometry for details).
Good ultrasound image for measurement of BPD and head circumference
The cephalic index is the Biparietal Diameter divided by the Occipto-Frontal Diameter times 100. The normal range for cephalic index is 70–86.
In some instances, the head is very elongated, making the BPD inaccurate. This is called dolicocephaly. Dolicocephaly is often associated with breech presentation or oligohydramnios (for example, in renal agenesis). In these circumstances, the HC is a more accurate predictor of EGA than the BPD.
Conversely, a very rounded head (relatively short OFD) is termed brachycephalic.
Example of dolicocephaly
Head Circumference (HC)
Important landmarks for a good HC are the same as for BPD:
Fetal skull should have a symmetric oval shape
Midline should be straight and equidistance between both sides of the cranium
Cavum septum pelludicem should be visible anteriorly
Thalamus should be visible
Cerebellum should not be visible
Measurement is made circumferentially around the outer table of fetal skull. Calipers should be placed at the outer edges of the skull in all four quadrants (see Biometry for details).
Head circumference: U/S image
Abdominal Circumference (AC)
Important landmarks for a good AC:
The abdomen should be as nearly circular as possible
Spinal processes should be visible, in a compact triangle (elongation of the processes suggests a skewed view)
Stomach should be visible
The hepatic portion of the umbilical vein as it curves into the hepatic vein should be visible (if the umbilical vein extends to the abdominal wall, the view is too low)
Measure circumferentially around the outer edge of the skin. Do not move the calipers in to the ribs, nor the spine. This is a measurement at the skin level.
Good image for abdominal circumference
Abdominal circumference - U/S image
Femur Length (FL)
Adjust the scanning plane to maximize the femur length. Among several measurements, the longest measurement is usually (although not always) the best.
Measure from the midway between the trochanters (femoral neck) to the midway between the epicondyles.
Be sure that your FL measurement does not include the iliac wing (sometimes a problem in mid-trimester scans)
Good image of femur
Femur length - U/S Image
Fetal Anatomy
Scan fetal structural anatomy in an organized fashion. I begin with CNS and scan from head to coccyx, then scan fetal organs from perineum back up to face. Think of the fetal scan like a CT scan—a series of cuts (usually transverse through the fetal body). By scanning in a sequential manner you are less likely to forget to image a specific structure
CNS scan encompasses:
- Ventricular atrium, midline, choroid plexus, cavum septum pellucidum
- Cerebellum, cisterna magna, nuchal fold thickness
- Spine: Both transverse and long axis views
Body anatomy scan encompasses:
- Genitalia
- Bladder
- Umbilical cord insertion, number of vessels
- Kidneys
- Stomach
- Heart
- Face
- Extremities
Amniotic Fluid
Amniotic fluid is usually assessed by visual (subjective) assessment up to 22–24 weeks.
After 24 weeks, amniotic fluid should be specifically measured by the four-quadrant AFI technique.
Vertical pocket in four quadrants measured and summed
Considerations:
- Polyhydramnios (especially note the presence, or absence, of stomach bubble)
- Excess production
- Diabetes
- Macrosomia
- Decreased swallowing
- CNS abnormality (aneuploidy, hydrocephalus, viral infection)
- Esophageal atresia
- T-E fistula
- Excess production
- Oligohydramnios
- Loss (PROM)
- Decreased Urination
- Decreased urine production
- IUGR
- Renal disease/anomaly
- Obstruction
- Kidney
- Ureter
- Bladder
- Absent kidneys (renal agenesis)
- Decreased urine production
Head
Evaluate cerebral ventricles, choroid plexus, midline, thalamus.
The presence of the cavum septum pellucidum should be noted.
In the mid-trimester the cerebral cortex is hypoechogenic. Be sure to identify the outline of the ventricle to avoid overestimating the ventricle (false diagnosis of hydrocephalus). Normal ventricle measurement is <= 1.0 cm.
From the ventricular view, angle down posteriorly to identify the cerebellum, cisternal magna and posterior nuchal fold. Normal cisternal magna measurement is 0.5 - 1.0 cm. Nuchal fold should be <=0.5 cm.
Considerations:
- Absent cranium
- Defect in cranium
- Absent cavum septum pellucidum
- Abnormal ventricles
- Abnormal cerebellum
- Other abnormalities
- porencephaly
Spine
Evaluate the spine in transverse view from cervical region to coccyx.
Transverse images of the spine are more sensitive for detection of neural tube defect. Note the normal triangular arrangement of the vertebral bodies and laminae.
The long axis views of the spine are easier for the patient to see, but generally give less information than a careful transverse evaluation.
Long axis view of cervical and thoracic spine
Long axis view of lumbar and sacral spine
Considerations:
- Neural tube defect (spina bifida)
- Mass
- sacrococcygeal teratoma
- Abnormal angulation
- limb-body wall complex
- Other abnormalities
- caudal regression
Gender
In most cases, identification of gender is mostly for the patient's information. Be sure to ask the patient if she wants to know the gender before revealing it. If the patient does not want to know gender, I do not look and do not record it unless there is a medical necessity.
Medical indications to identify gender include twins (where identifying different genders immediately confirms dizygosity), some endocrinopathies, drug exposures (virilizing durgs), post-amniocentesis (to confirm that gender phenotype matches genotype).
Males are recognized by positive identification of scrotum and penis. Females are not identified by absence of male genitalia, but by a positive identification of the labial folds, which have a trilaminar appearance (so-called "hamburger sign").
Considerations:
- If known gender (i.e., genetic amniocentesis):
check U/S gender against amnio report—consider laboratory error or sexual differentiation error (e.g., testicular feminization) - Ambiguous genitalia
Bladder
The bladder is usually readily recognized in the fetal pelvis. The presence of a normal appearing bladder indicates that at least one kidney is functioning.
An excessively large bladder may indicate a bladder outlet obstruction (e.g., posterior urethral valve).
Considerations:
- Bladder outlet obstruction
- Absent bladder—confirm expected bladder position by identifying umbilical arteries with color flow doppler
- renal agenesis
- infantile polycystic kidney disease
- Other
- ectopic ureterocele
Umbilical Cord
Umbilical cord insertion and number of vessels should be noted. The number of umbilical cord vessels can be determined by color flow doppler, if available, but identifying the two umbilical arteries on either side of the bladder. If using color flow doppler to identify umbilical arteries, be sure you are scanning in a true transverse plane and are not identifying the iliac arteries or the femoral arteries.
Alternatively, identify a clear cross section of the umbilical cord and look for the two arteries and vein ("Mickey Mouse appearance").
Color flow doppler demonstrates both umbilical arteries around the bladder
Cross section of umbilical cord demonstrates two umbilical arteries and one vein
Considerations:
- Abdominal wall defect
- omphalocele
- gastroschisis
- pentalogy of Cantrell (includes ectopia cordis)
- Two vessel cord—associated with increased risk of cardiac or renal anomalies
Kidneys
Kidneys may be identified in the transverse plane on either side of the spine (looks like "spectacles"). Normal kidneys will have some echogenicity of the renal calyces. The central collecting system (renal pelvis) should be <= 0.5 cm in AP diameter.
Considerations:
- Absence (renal agenesis)—bilateral or unilateral
- Obstruction
- uretero-pelvic junction (UPJ)
- ureteral
- ectopic ureterocele
- bladder outlet obstruction (posterior urethral valve in male)
- Pyelectasis (dilated renal pelvis)
- aneuploidy
- obstruction
- Abnormal morphology
- multicystic dysplastic kidney
- polycystic kidney disease
- Prune-belly syndrome
Stomach
Identify the stomach in the upper abdomen. Be careful to avoid confusing stomach with gall bladder. Note on which side the stomach is located. In a transverse view, the stomach and heart should not be adjacent (consider diaphragmatic hernia).
Sometimes the diaphragm can be imaged in long axis view
Considerations:
- Absent stomach (especially if associated with polyhydramnios)
- esophageal atresia or T-E fistula
- Excessively large stomach (“double-bubble”) (possibly associated with polyhydramnios)
- duodenal atresia
- Abnormal location
- diaphragmatic hernia
- situs inversus
- polysplenia/asplenia (laterality sequence)
Heart
A normal four-chamber view of the heart excludes approximately 50% of cardiac anomalies. To get a four-chamber view, start with an abdominal cross section (as if measuring AC) and rotate (angle) the transducer cephalad. Generally, you will come right to the four-chamber view. Note the axis of the heart, symmetry of chambers, and valve motion. A very small degree of pericardial fluid (1–2 mm maximum) may be normal, but anything more than this is abnormal pericardial effusion.
Angling the transducer further cephalad will identify first the aortic outflow tract and then the pulmonary outflow tract. The sequence of the outflow tracts is important (moving from caudal to cephalad: four-chamber view aortic outflow pulmonary outflow). Note that the outflows cross at approximately 60 degree angles; the outflow tracts should not run parallel to each other.
Animated image of 4 chambers and outflows:
Animated image of 4 chambers and outflows without helps
4 chamber heart and outflows
Considerations:
- Abnormal position
- pleural effusion
- pulmonary mass effect
- diaphragmatic hernia
- cystic adenomatoid malformation (CAM)
- eventration
- ectopia cordis (also found in pentalogy of Cantrell)
- Abnormal axis
- dextrocardia, situs inversus
- Abnormal 4 chamber view
- Abnormal outflow tracts
- Abnormal rhythm
- premature atrial contractions
- tachycardia
- bradycardia
- Pericardial effusion (component of hydrops)
- isoimmunization
- viral infection
Face
After imaging the heart, continuing to rotate (angle) the transducer cephalad will usually bring the face into view. Attempt to identify upper lip and nostrils to exclude cleft lip. Cleft palate is more difficult to visualize and cannot be reliably excluded. Evaluation of the fetal profile is important in some cases of skeletal dysplasia. This is also one of the most striking images to print for the patient.
Lip
Profile
Considerations:
- Cleft lip
- Hypertelorism
- Hypotelorism
Extremities
Identify both arms and hands and both legs and feet. Specific identification of digits is not always possible, depending on gestational age, patient body habitus, and fetal position. In some cases, careful evaluation for polydactyly may be important.
Leg
Hand
Humerus length is an important marker in aneuploidy screening (especially for risk of Down Syndrome, as is Femur length).
Considerations:
- Skeletal dysplasia
- Screening for aneuploidy
- Polydactyly (rarely seen unless specific evaluation is undertaken)
- Syndactyly
- Clubfoot
Incidental Anatomy
Some anatomic features may be seen on occasion, but are not a part of a “routine” fetal anatomy survey. In some cases, these findings may be mistaken for abnormalities or confused with other fetal structures. For example, the fetal adrenals may be mistaken for fetal kidneys. These features are also included here because they do not fit well in any of the previous categories.
Some examples include:
- Fetal adrenals
- Fetal ear
Screening for Aneuploidy
It is common for ultrasound to be requested to screen for the risk of aneuploidy. In some cases a patient at increased risk of aneuploidy (e.g., age >= 35 y/o or an abnormal trisomy screen) wants to avoid amniocentesis, but expects that an ultrasound examination will provide additional information about her risk. It is important that the patient be aware that ultrasound cannot positively identify aneuploidy, and ultrasound cannot absolutely exclude the risk of aneuploidy.
The absence of identifiable anomalies may slightly decrease the baseline risk (that is, the age related risk) of aneuploidy, but it does not guarantee normal chromosomes. The presence of two or more anomalies, or a cardiac anomaly, or a number of specific single anomalies discussed elsewhere in this series, should alert the physician to the possibility of chromosome aneuploidy.
Some sonologists refer to a “genetic sonogram” that specifically looks for indicators of aneuploidy. These markers of chromosome abnormalities are not all actual anomalies, but are findings that point to an increased risk of aneuploidy. Conversely, if all markers are absent (or are within normal range) the risk of aneuploidy is reduced.
Some markers of aneuploidy include:
- Nuchal thickening
- Choroid plexus cysts
- Cardiac anomalies
- Echogenic intracardiac focus
- Femur length
- Humerus length
- Echogenic bowel
- Renal pyelectasis
- Clinodactyly
Cervix: Endovaginal and Perineal Evaluation
Endovaginal and perineal ultrasound are usually performed with an empty bladder. Visualize the cervix in a sagittal plan. It is usually easiest to first identify the internal cervical os, then adjust the probe until the entire cervical canal is visualized. The external os may be difficult to see, but it can be identified by following the curve of the posterior lip of the cervix to its intrsection with the cervical canal. Gradually relax pressure on the transducer until the image becomes slightly blurred. Reapply gentle pressure sufficient to establish a sharp cervical image. The length of the closed cervical canal is measured. Funneling of the upper cervical canal, if present is not included in the measurement of cervical length. Funneling may indicate incompetent cervix (depending on gestational age, obstetric history, and clinical circumstances).
Perineal scanning is performed with a standard curvilinear probe, covered with a glove and applied to the perineum using a large amount of gel. Place the probe gently between the labia and adjust to get a clear image of the cervix. Perineal scanning is more difficult to learn than endovaginal scanning. The vagina curves, and there is frequently shadowing from the pubic bone and from stool in the rectum. Moving the probe slightly to the side or up or down may improve the visualization.
Technique of endovaginal scan to evaluate cervix and lower uterine segment. Use the least amount of pressure possible to get a good image.
Longitudinal image of normal cervix
Cervix measurements:
I use a slightly different nomenclature when measuring cervix after a cerclage.
Longitudinal image of a shorter cervix
Examples of funneling of the internal os (click on images to enlarge). "U" shaped funneling is generally more worrisome. The image to the right is (in my opinion) diagnostic of incompetent cervix if seen before 24 weeks gestation.
Perineal scan
Endovaginal scan
Perineal scan of same patient
Cervical Incompetence and Cerclage
In my opinion, funneling of the upper cervical canal is the most important diagnositic criterion for cervical incompetence. Funneling necessarily results in a shorter cervical length (since the cervical length measurement includes only the closed portion of the cervix). In some cases, the cervix may be very short, due to loss of the external portion of the cervix, yet the internal os remains competent. Evaluation of patients at risk of incompetent cervix may require serial scanning to identify developing cervical incompetence. Incompetent cervix can develop rapidly, as is demonstrated by the first two scans (even though these are abdominal). Obstetric history is extremely important. In some patients, I believe cerclage is indicated even with a normal cervical ultrasound if the obstetric history is strongly suggestive of cervical incompetence.
Cervical ultrasound measurement after cerclage is predictive of outcome. The cervical canal is measured above and below the cerclage (I usually designate these measurements as “Upper segment length” and “Cerclage to external os length”). The upper segment length is most predictive of outcome.
Scan at approximately 20 weeks gestation.
Same patient, scanned 24 hours later.
This endovaginal scan is (in my opinion) diagnostic of incompetent cervix if seen before 24 weeks gestation.
Cervix measurements after cerclage.
Biophysical Profile Testing fetal well-being
A Biophysical Profile (BPP) is a test of fetal well-being. It addresses the question, “Is this fetus healthy, or is it possibly showing signs of stress or distress that necessitate delivery?” A BPP has 5 components that are each scored 0 or 2. A BPP score of 8 or 10 is considered reassuring. A BPP score of 6 or less is not reassuring; further intervention will depend on clinical factors such as existing complications of the pregnancy, gestational age, etc.
The components of a BPP are:
If the finding (component) is positive score 2 points, otherwise score zero.
- NST: At least 2 accelerations (15 bpm x 15 seconds) in 20 minute window with normal baseline variability
- Fetal movement: At least 3 gross body movements in a 30-minute window
- Fetal tone: At least 1 flexion / extension movement in a 30-minute window
- Fetal breathing: At least 30 seconds of sustained fetal breathing in a 30-minute window
- Amniotic fluid: At least one pocket of fluid measuirng >2 x 2 cm, OR an AFI > 5 cm
Some comments about BPP:
- Although a full BPP includes an NST, sometimes we do an “ultrasound only” BPP, scoring 8/8 if all four ultrasound components are positive
- BPP scores are always even numbers (0, 2, 4, 6, 8, 10)—there are no odd-numbered BPP scores
- Oligohydramnios predicts an increased risk of adverse fetal outcome. Thus a BPP of 8/10 may be abnormal if the 2 points are taken off for amniotic fluid
Reactive NST
Gross body movements are generally rolling or shrugging movements.
Note that tone implies a flexion/extension movement (like the movement of the hand and fingers here). Tone is NOT indicated simply by observing the fetus to be in a flexed position.
Breathing is best seen by observing the diaphragm, or chest wall, or even kidneys for rhythmic (up/down or in/out) movement. A view that includes the heart may confuse transmitted movement from the heart with true breathing movement.
Amniotic fluid assessment. Add up the vertical fluid pockets in four quadrants.
Final AFI value. Normal AFI is 5–24 cm.