In this section, the sonographic markers that can be used in second- and third-trimester fetuses are reviewed.
Ventriculomegaly is a common finding that occurs in 5 to 25 per 10,000 deliveries (Figure 21-18). The normal measurement of the atrium should be less than 10 mm at any time during gestation. When it is between 10 and 15 mm (the gray zone), it is suspicious for aneuploidy; when it is greater than 15 mm, it is more likely to represent hydrocephalus. Approximately 15% of fetuses with ventriculomegaly will have an aneuploidy, only 2% if the hydrocephalus is isolated, but 17% if other findings are noted. Ventriculomegaly is not a predictor of a particular type of aneuploidy.
Mild ventriculomegaly may be sign of aneuploidy.
Choroid plexus cysts (CPCs) occur in 1% of the normal population (Figure 21-19). According to the literature, approximately 30% to 60% of fetuses with trisomy 18 have CPCs. Further, approximately 97% of fetuses that have trisomy 18 and CPCs have associated anomalies.
Choroid plexus cysts in a fetus with trisomy 18. Thecysts are indistinguishable from those of a normal fetus.
The issue of whether an amniocentesis should be performed for fetuses with CPC has been debated. The following example may decide the debate.29 Assume a population of 1 million fetuses. Because the incidence of CPC is 1% in midtrimester fetuses, among 1 million fetuses 10,000 will have CPC and be normal. Because the incidence of trisomy 18 is 3 per 10,000, 300 fetuses will have trisomy 18. Because the prevalence of CPC in trisomy 18 is 30%, the fetuses with trisomy 18 will be comprise 100 cases of CPCs. Because the sensitivity of ultrasound in detecting trisomy 18 is 75%, of those 100 fetuses that have trisomy 18 and CPCs, 25 would be missed by a normal ultrasound alone. Therefore, 25 fetuses with trisomy 18 and CPCs would be missed out of 10,075 fetuses that have CPC and correctly identified as either normal or having trisomy 18. This is roughly a miss of 1 in 400 fetuses with only CPC and trisomy 18, and this is the reason that performing amniocentesis in fetuses that have only CPC and no other finding is probably not worthwhile because, to detect that fetus with trisomy 18, one would have to perform amniocentesis in 400 normal fetuses. If one assumes that the risk of miscarriage after amniocentesis is 1 in 200 or even 1 in 400, amniocentesis would endanger the lives of 1 to 2 normal fetuses to detect 1 fetus with trisomy 18, whose outcome is dismal in any case.
Dysgenesis of the corpus callosum may be an isolated finding of little significance but may also be associated with trisomies 13 and 18, in which case it is rarely an isolated finding. Ultrasound findings include widening of the interhemispheric fissure, the teardrop-shaped lateral ventricles, and colpocephaly (Figure 21-20).
The presence of an enlarged cisterna magna or a Dandy–Walker malformation, either with a blocked fourth ventricle or a communicating fourth ventricle, may also be an indicator of aneuploidy. Isolated Dandy–Walker cysts do not indicate great risk; however, when associated anomalies are present, the risk increase to approximately 50%, predominantly for trisomies 13 and trisomy 18.
Dandy–Walker variant in trisomy 13.
Holoprosencephaly occurs in 1 per 10,000 deliveries. Fetuses with holoprosencephaly have a higher risk of aneuploidies than do those with simple ventriculomegaly. Approximately one-third will have an aneuploidy, if holoprosencephaly is isolated; if other anomalies are noted, almost 40% will have an aneuploidy. Thus, this is a very significant finding, and trisomies 13 and 18 will be the most likely aneuploidies.
Microcephaly occurs in 10 per 10,000 deliveries. This is a difficult diagnosis to make. The commonly used criteria include a biparietal diameter (BPD) below the first percentile or a head perimeter greater to femur length below the 2.5 percentile.30,31 The BPD is a difficult criterion to use the because it is not much affected by microcephaly. What is affected is the size of the cranial vault in relation to the size of the face, which is easier to determine in a sagittal view of the head. Twenty percent of fetuses with microcephaly have an aneuploidy.
Facial clefts occur in 14 per 10,000 deliveries, and they are associated with several aneuploidies such as trisomies 13 and 18, but others such as 4p syndrome may also have clefts.
Micrognathia is a nonspecific finding but may associated with trisomies 18 and 13 and triploidy. This finding is easier to determine in a sagittal view of the face. Figures 21-21, and 21-22 show two normal sagittal views of the profile; note the position of the chin in relation to the middle section, which indicates a fetus with trisomy 18, on both the ultrasound image and the postdelivery image.
Micrognathia. Note the small chin and compare it with the postnatal appearance (Figure 22–22) in this fetus with trisomy 18.
Micrognathia. Note the small chin and compare it with the ultrasound appearance (Figure 22-21) in this fetus with trisomy 18.
Macroglossia is the presence of a tongue that is too large and is a finding typical of trisomy 21 and Beckwith–Wiedeman syndrome. Normally, the tongue should not pass the alveolar ridge of the teeth, but in cases of macroglossia the tongue extends pass the tooth buds (Figures 21-23, and 21-24).
In macroglossia, the tongue extends past the tooth buds.
A nuchal fold is usually considered abnormal if it is greater than 6 mm between 15 and 22 weeks. Although the measurement can be obtained in a sagittal section of the hand and neck, it is more typically obtained in the axial section of the head through the cerebellum. Even an isolated nuchal fold is a significant finding, mainly as a predictor for trisomy 21.
Cystic hygroma is a very common finding, occurring in 0.5% of all spontaneous abortions. It is also associated with hydrops in 40% to 100% of cases, with congenital heart defect in 0% to 92% of cases, and with aneuploidy in 46 to 90% of cases. Cystic hygroma can be localized in the back of the neck or may extend farther down the back of the embryo or fetus (Figure 21-25).
This is a large cystic hygroma in a fetus with monosomy X, or Turner's syndrome. What appears to be amniotic fluid behind the neck of the fetus is in fact a large cystic hygroma. Note not only the large cysts but also the large septations that separate the cysts. These septations differentiate this condition from cephaloceles.
Ears that are too small or too round or have a malformed helix may be associated with trisomies 13, 18, and 21; monosomy X; and several other translocations. Therefore, in the proper context, the finding of an abnormal ear may suggest the need for a karyotype.
Microphthalmia is a condition in which the eyes that too small, and it is often associated with hypotelorism. Microphthalmia is associated with several aneuploidies. In Figure 21-26, the eyes are also a bit more echogenic than they would normally appear.
Microphthalmia is associated with several aneuploidies. Note that the eyes are also more echogenic than they would normally appear.
A cataract, which is an opacification of the lens of the eye, is a finding that may be associated with aneuploidy. It is associated with other conditions such as TORCH infections, but it may also be associated with aneuploidy. The way to observe a cataract is to make a section through the eye and observe echoes of low-level intensity inside the lens.
The finding of hypotelorism should suggest that the fetus has cyclopia and proboscis, which are typical of holoprosencephaly and thus are associated with trisomy 13. Hypotelorism is a decrease of the interocular distance, and fetuses will have an interocular distance less than one-third the binocular distance.
Wormian bones, described by Olaus Worm, a Danish anatomist living in the 16th century, are little bones that occur in the fontanels and sutures (Figure 21-27). They are also called Inca bones. They may occur in several disorders including trisomy 21, cleidocranial dysplasia, osteogenesis imperfecta, hypothyroidism, pycknodysostosis, and progeria.
Two small wormian bones appear in the posterior fontanel.
Hypoplasia of the nose is a typical finding in trisomy 18. A low nasal bridge is a common finding in many aneuploidies, but it is also common in other conditions such as skeletal dysplasias (Figure 21-28). The portion that is hypoplastic is the nasal spine; and when it is too small, it appears as if the nose base is too low. This is seen by ultrasound as a continuous dark echo between the eyes of the fetus.
Low nasal bridge in a fetus with trisomy 21.
Cardiac anomalies are very common: 85 per 10,000 newborns will have a cardiac anomaly. However, among fetuses with an aneuploidy, the association is even more dramatic: 99% of fetuses with trisomy 18 have a cardiac anomaly, 90% trisomy 13, and 50% trisomy 21. Therefore, there is a great association between the finding of a cardiac anomaly and an aneuploidy. Overall, 29% of fetuses with a cardiac anomaly also have an aneuploidy, and 16% of fetuses that have an isolated cardiac anomaly have an aneuploidy; and when the cardiac anomaly is associated with other anomalies, 66% of these fetuses will have an aneuploidy. Overall, the finding of a cardiac anomaly is related more to trisomies 18 and 21 because they are so common and less to the other aneuploidies (Figure 21-29).
Frequency of cardiac anomalies in several aneuploidies.
Echogenic Focus in the Heart
Brown et al. were the first to describe the association between echogenic foci and trisomy 21.32 Subsequent studies have demonstrated that approximately 5% of midtrimester pregnancies have an echogenic foci in the heart. Different investigators have reported an association of 0% to 3% with trisomy 21.33,34,35,36,37,38, and 39 The issue of one or more echogenic foci or whether the foci are on the right or left side is more predictive of aneuploidy has not been settled at the current time. Figures 21-30, and 21-31 show an echogenic focus in a normal fetus and a fetus with trisomy 21, respectively; there are no criteria for distinguishing the echogenic foci of normal fetuses from those of fetuses with aneuploidies.
Echogenic focus in the left ventricle of a normal fetus.
Echogenic focus in the left ventricle of a fetus with trisomy 21.
The presence of a small layer of pericardial fluid is a common finding (Figure 21-32). Up to 2 mm is usually considered normal40; when the layer is greater than 2 mm, one can consider a pericardial effusion. Pericardial effusions are associated with some aneuploidies, in particular trisomy 21, and with TORCH infections.
The presence of a small amount of pericardial fluid is often a normal finding, but pericardial fluid may be an indicator of an aneuploidy, in particular trisomy 21.
Hydrops is a fairly common finding at delivery, 10 per 10,000, and 12% to 16% of nonimmune hydrops may be associated with aneuploidy.
Most of the other findings that are present in the chest are discussed in sections concerning the heart, gastrointestinal, and skeletal systems. Approximately 5% of fetuses with a pleural effusion will have an aneuploidy, usually trisomies 21 and 13 and monosomy X.
Between 0.2% and 1% of pregnancies present with a two-vessel cord (Figure 21-33). Among these, approximately 1% to 10% have an aneuploidy, including trisomies 18 and 13, triploidy, and monosomy X.41,42,43,44,45,46,47,48,49,50,51, and 52
Sometimes the easiest way to identify a two-vessel cord is to look at the aortic bifurcation rather than the cord in amniotic fluid. In this image, one iliac is larger than the other because the larger one transports blood to the umbilical artery and the smaller one transports blood to half of the pelvis and other leg.
Although most cord cysts are benign, they may be present in fetuses with aneuploidy, in particular trisomies 13 and 18 (Figure 21-34).
Fetus with trisomy 13: has a cyst at the base of the cord.
The typical appearance of a Swiss-cheese placenta is that of a placenta that contains innumerable, small, round vesicles that are quite different from bleeding or blood accumulated in the cotyledons (Figure 21-35). These are much smaller and much rounder. The Swiss-cheese placenta is very typical of triploidy, but it may also occur in trisomy 18.
Placental vessicles in triploidy.
Duodenal atresia occurs in 1 per 10,000 deliveries (Figure 21-36). It is due to a failure of the recanalization of the gut distal to the ampulla of Vater. Because of the obstruction, the proximal portion the duodenum distends, which creates, with the distended stomach, the appearance of a double bubble. Another consequence of the obstruction is that content does not pass through the proximal jejunum and thus the jejunum thins down; this condition is called a disused jejunum.
Image shows a double bubble in a fetus with duodenal atresia and triploidy. The distended stomach and large bubble that compose the proximal portion of the diodenum are clearly visible. Interestingly, this fetus does not have polyhydramnios, although polyhydramnios is expected in fetuses with duodenal atresia because they cannot swallow fluid. This fetus also had a renal malformation; thus, it could not swallow fluid properly or produce an adequate amount of fluid.
Forty percent of fetuses that have an isolated duodenal atresia will have an aneuploidy; however, if other associated anomalies are present, the percentage rises to 66. Duodenal atresia is mostly a marker for trisomy 21.
Esophageal Atresia or Tracheoesophageal Fistulas
Esophageal atresia or tracheoesophageal fistulas occur in 2 per 10,000 deliveries. The ultrasound diagnosis is fairly straightforward: the stomach too small for its gestational age and there is an increase in amniotic fluid in the third trimester. A stomach that is too small is not much larger than the gallbladder. However, there are many conditions that present with the same findings. Esophageal atresia or tracheoesophageal fistulas are classified into five groups (Figure 21-37). A small stomach will be present only in cases of types B and C, and more than 95% of fetuses have an associated fistula. Therefore, the absence of a stomach is rare on ultrasound, and polyhydramnios develops only in the third trimester.
Type A, esophageal atresia, which is the most common type, occurs in 85% to 93% of cases. In type A, the proximal esopahgus terminates in a blind pouch. However, the distal esophagus is connected to the tracheobronchial tree, so some fluid will pass into the stomach. Type B is second most common type and occurs in 3% to 10% of cases. The proximal esophagus terminates in a blind pouch. However, the distal esophagus is not connected to the tracheobronchial tree, so not fluid will pass into the stomach, and the stomach will appear too small. In type C, there is a connection of the distal part of the proximal portion of the esophagus, but there is no connection to the distal portion; therefore, no fluid can pass into the stomach, and the stomach will not be full. Type D occurs in 1% to 1.5% of cases. Both the proximal and distal portions of the esophagus are connected to the tracheobronchial tree; thus, the ultrasound appearance of the stomach is essentially normal. Type E occurs in 1.8% to 4% of cases. The esophagus is continuous but has an H connection to the tracheobronchial tree; thus, the stomach will appear normal.
A recent study has addressed the association of tracheoesophageal fistula and aneuploidy.53 In the presence of esophageal atresia combined with tracheoesophageal fistula, 7% of these fetuses will have an aneuploidy. If there is only a tracheoesophageal fistula, the number differs between 3% and 10%. However, in cases of pure esophageal atresia without associated tracheoesophageal fistula, 20% to 25% of these fetuses will have an aneuploidy and the most likely one is trisomy 18 (Figure 21-38).
This fetus has a Klinefelter and trisomy 18 mosaic and presents with a tracheoesophageal fistula and a small stomach.
Omphaloceles occur in 1 per 10,000 deliveries, and 20 to 60% of these fetuses will have an aneuploidy (Figure 21-39). The risk of aneuploidy increases if the fetus has either oligohydramnios or polyhydramnios. Nyberg et al. demonstrated that the risk of aneuploidy is 16% if the omphalocele contains the liver and bowel and increases to 66% if only bowel is present.54 Thirteen percent of fetuses that have just an omphalocele have an aneuploidy, but with an associated anomaly the risk increases to 46%. Trisomies 18 and 13 are the most likely aneuploidies.
Large omphalocele in a fetus with trisomy 18.
Diaphragmatic hernia occurs in 3 to 4 per 10,000 deliveries. This is a fairly easy diagnosis by ultrasound because the position of the stomach alongside the heart is characteristic (Figure 21-40). Although an isolated diaphragmatic hernia is seldom associated with aneuploidy, the presence of a diaphragmatic hernia and associated anomalies is associated with aneuploidy in approximately 30% of cases, and the most likely aneuploidy is trisomy 18.
Typical appearance of diaphragmatic hernia, with the stomach alongside the heart, the shift of the heart, and polyhydramnios.
Hyperechoic bowel is a common finding and occurs in approximately 0.5% of pregnancies during the second trimester. To be considered hyperechoic, the bowel has to be whiter than the other abdominal structures and it is usually as white as bone but without the shadowing. The reason for this occurrence is unknown; it may be a normal variant, it may represent swallowed blood that persists undigested in the gut of the fetus, later in gestation it may represent either meconium ileus or meconium peritonitis (this is rarely the case early in the second trimester), it may represent varicella infection, or it may be a marker for trisomy 21.
The difficulty with the echogenic bowel is that this is a poorly defined criteria (Figure 21-41). In addition, there is marked a variability between ultrasound manufacturers and between transducers. Some scanners can demonstrate an echogenic bowel, whereas others cannot.
This is a series of four images taken a few seconds apart. The first image (upper left) was obtained at 7 MHz, and the bowel is hyperechogenic. The second image (upper right) was obtained at 5 MHz; the echogeneity is still clearly noticeable but not as brilliant. The third image (lower left) was obtained at 3.5 MHz, and the bowel is not as hyperechogenic. Compare the echogeneity of the bowel with that of the placenta, which is adjacent to the fetus, and observe the difference between in the images taken at 7 and 3.5 MHz. The fourth image (lower right) was obtained at 2.5 MHz, and the hyperechogeneity has completely disappeared. Because of the variability, basing a clinical decision on an echogenic bowel is not recommended.
To Karyotype or not to Karyotype?
We do not consider a hyperechoic bowel with no associated findings a justification for amniocentesis. The indication, when it exists, is usually related to other findings, and the hyperechoic bowel rarely affects the decision one way or another. Some investigators disagree and recommend amniocentesis on the sole finding of hyperechoic bowel,55,56,57,58,59, and 60 whereas others do not recommend karyotyping.61,62, and 63
Malrotation of the bowel is another nonspecific finding that may be part of many aneuploidies.64,65,66,67,68,69,70, and 71
Although bowel obstructions are seldom associated with aneuploidies, approximately 1% of fetuses with trisomy 21 will present with bowel obstruction.72,73
Urinary tract anomalies are common and occurred in 20 to 30 per 10,000 deliveries. The presence of mild hydronephrosis (greater than 4 mm) is suggestive in some cases of trisomy 21 (Figure 21-42). When the hydronephrosis is moderate to severe, it may be associated with trisomy 13 or 18. Multicystic dysplasia is also associated with trisomies 13 and 18, as is renal agenesis.
Fetus with trisomy 21 and mild bilateral pyelectasis.
The risk of aneuploidy seldom depends on the type of urinary tract anomaly, by whether the anomaly is unilateral or bilateral, or by the level of amniotic fluid. When associated anomalies are present, only 25% of these fetuses will have an aneuploidy.
Ambiguous genitalia or clitoromegaly (Figure 21-43) is common in triploidy and in many other sex chromosomal anomalies such as XXY, XXYY, XYYY. They may also be present in trisomy 18, 4p deletion, and many other conditions.
Clitoromegaly in a fetus with trisomy 18.
In this section, the many skeletal findings that could be markers for aneuploidies are reviewed. These include shortening of the limbs, clinodactyly, syndactyly, simian crease, radial ray aplasia, clubfoot, rockerbottom foot, and sandal gap. In addition, other skeletal findings such as 11 pairs of ribs, hypoplasia of the clavicle, double ossification center of the manubrium, and wide angle of the iliac wings are reviewed.
It has long been known that children with Down's syndrome have shorter limbs than do normal children. In their study population, Benacceraf et al. observed a cutoff limit of 91% of the expected size of the femur in relation to the BPD, a sensitivity of 68% and a specificity of 98% in detecting fetuses with trisomy 21.74 These rates were not confirmed in other studies,75,76 although the trend clearly exists and has been extended to the finding of a short humerus.77
Brachymesophalangia of the fifth digit results from a too-small middle phalanx of the fifth digit, which occurs in 68% of cases with trisomy 21 (Figure 21-44). Benacceraf et al. studied the predictive value of this finding and found that, if the middle phalanx of the fifth digit is less than 70% of the middle phalanx of the fourth digit, there is a 70% chance of trisomy 21.78,79 Although few would make major clinical decisions based on this finding, a hypoplastic middle phalanx is an important observation. Another anomaly commonly associated with brachymesophalangia is clinodactyly, in which the distal end of the fifth finger is bent toward the hand because of the deformed and hypoplastic middle phalanx.
In this image, the middle phalanx of the fifth digit is too short.
A clenched fist is a typical finding of triploidy and trisomy 18 (Figure 21-45). The overlapping of the finger gives the impression of a clenched fist that does not change during the examination. The typical finding is the overlapping of the third digit by the second digit and of the fourth digit by the fifth digit.
Hand in a fetus with trisomy 18 shows the typical clenched fist. Note the overlapping of third digit by the second digit and of the fourth digit by the fifth digit.
There are two normal transverse palmar creases: the proximal palmar crease and the distal palmar crease. In fetuses that do not open and close their hands often enough during early gestation, only one crease, called the simian crease, develops (Figure 21-46).80 Simian creases are present in 4% of the normal population in one hand and in 1% of the normal population in both hands. These creases are interesting because they are present in 45% of the fetuses with trisomy 21.3,81 They also occur in cases of trisomies 18 and 13 and in many conditions associated with decreased flexion of the hands such as skeletal dysplasias. Although searching for a simian crease prenatally may seem very difficult, this is in fact fairly easy to do under the right conditions (amniotic fluid and semi-open hand).
Fetus with a clearly visible simian crease.
What is the risk of trisomy 21 when a simian crease is found? To answer that question, assume a population of 10,000 fetuses. Because 4% of the normal population has a simian crease, there will be 400 cases. Further, because trisomy 21 occurs at a rate of 13 per 10,000 and because 45% of fetuses with trisomy 21 have a simian crease, 6 of those 13 fetuses will have a simian crease. Therefore, the number of simian creases will be 406 (400 plus 6), and the likelihood of trisomy 21 when a simian crease is found is 6 of 406, which is roughly 1.5%.
Radial ray aplasia is a typical finding in trisomy 18, with very few differential diagnoses such as Holt–Oram syndrome and thrombocytopenia aplasia of the radius syndrome. The spectrum of radial ray aplasia includes absence of the radius, absence of the radius and thumb, or absence of the radius and the whole hand (Figure 21-47).
Radial ray aplasia with clubhand in a 19-week fetus with trisomy 18.
Another finding of trisomy 21 is elevation of the first toe. This can be seen by ultrasound because the first toe is no longer in the same plane as the other toes (Figure 21-48). In a sagittal section, one can see the angle between the big toe and the rest of the foot.82
Elevation of the first toe in a fetus with trisomy 21.
A rockerbottom foot is a foot that has a convex rather than a concave sole because of malpositioning of the calcaneus and talus. It is typically associated with trisomy 18.
A sandal gap is a slight interspace between the first and second toes, and this is a risk factor for trisomy 21 (Figure 21-49).83
Sandal gap in a 30-week fetus with trisomy 21.
Findings Related to Anomalies of the Skull and Skull Shape
Brachycephaly is a deformity of the head in which the skull appears too round, and this is manifested by a short occipitofrontal distance (Figure 21-50). This is measured by the cephalic index, which is the ratio of the biparietal diameter to the occipital frontal diameter, and normal values should be between 75% and 85%. If the cephalic index is greater than 85%, the fetus has brachycephaly (Figure 21-51). If the head is too flat from side to side, the condition is referred to as scaphocephaly, a finding in fetuses that have in utero crowding or premature rupture of the membranes, for instance. This finding is not related to aneuploidy. Brachycephaly is a reliable indicator in children with trisomy 21 but a less reliable criterion in fetuses.
The left-most skull is too flat from side to side, a condition known as scaphocephaly, which is not associated with aneuploidies. The middle skull is normal. The right-most skull demonstrates brachycephaly.
Example of obvious brachycephaly: the cephalic index does not need to be measured in such obvious cases.
A strawberry head is a deformed head with narrowing of the frontal region and flattening of the occipital region. This is a marker for trisomy 18, and it has not been found without other associated anomalies (Figure 21-52).
Typical appearance of strawberry head in a fetus with trisomy 18.
The presence of 11 pairs of ribs may be an indicator of aneuploidy. Five percent of the normal population has 11 pairs of ribs. This finding is interesting because it occurs in one-third of fetuses with trisomy 21 and in fetuses with trisomy 18. It is also present in some skeletal dysplasias such as camptomelic dysplasia and cleidocranial dysplasia.83,84, and 85 Counting 11 pairs of ribs used to be very tedious: one used to have to take a single section that would include all pairs of ribs and not omit any. With the advent of the cineloop, it has become easier to take a sweep through the rib cage and then to go back and forth through the ribs in each frame. With the introduction of three-dimensional ultrasound, counting ribs has become even easier because a single reconstruction demonstrates all the ribs (Figure 21-53).
Three-dimensional reconstruction of the spine and rib cage. By spinning the array, it is easier to identify all the ribs.
Hypoplasia of the clavicle is a common finding in trisomy 18 (Figure 21-54). The length of the clavicle, expressed in millimeters, should be approximately equal to the gestational age, expressed in weeks. Therefore, a 22-week fetus should have a clavicle that measures approximately 22 mm.
Clavicle in a fetus with trisomy 18. It is much shorter than expected for its gestational age.
Borg et al. recently demonstrated that the increase in the angle between the iliac wings, which has been a staple of the radiologic diagnosis of trisomy 21 for many years, can also be recognized prenatally by ultrasound (Figure 21-55).86 In their nomogram, in an axial view of the pelvis, the normal angle between the iliac wings should be smaller than 70° ±15. In their studies, fetuses with trisomy 21 had iliac wing angles larger than 100° ±10.
Widened iliac wing in a fetus with trisomy 21.
Adouble ossification center of the manubrium is another marker for aneuploidy (Figure 21-56). One would look for two ossification centers in a craniocaudal alignment, which distinguishes the ossification center of the manubrium from the ossification centers of the sternebrae, which are side by side (Figure 21-57). Ossification of the manubrium starts in the fifth month of pregnancy. The manubrium first appears as a hypoechoic center and then it becomes hyperechoic as it ossifies. A double ossification center of the manubrium is present in 10% of the normal population, but it is also present in 33% to 80% of cases with trisomy 21 and in 25% of cases with monosomy X.
Double ossification center of the manubrium presents as two ossification centers in a craniocaudal alignment, which distinguishes the ossification center of the manubrium from those of the sternebrae, which are side by side (normal is shown at left, abnormal at right).
Sagittal section of a fetus with trisomy 21 in the third trimester (head to the left) shows the double ossification center of the manubrium (arrows) and the ossification centers of several sternebrae.
Fewer than 1% of fetuses that suffer from intrauterine growth restriction (IUGR) have an aneuploidy. Growth restriction is most typical of triploidy and trisomies 13 and 18. The association between IUGR and aneuploidy is more likely if the IUGR is detected in the second trimester. A very typical finding of triploidy is a very large disproportion between the head and the abdomen, in which the size abdomen is much smaller than the size of the head.