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    Using ultrasound to recognize fetal anomalies: Part 1

    First- and second-trimester ultrasounds are key prenatal tools for revealing structural anomalies that may point to genetic conditions.

     

    Face

    A profile view in the first and second trimester can identify a small mandible, or micrognathia (Figure 17). Median or bilateral cleft lip (Figures 18, 19) can also be suspected based on profile views. The lips and palate are best evaluated with coronal imaging and transverse views at the level of the palate. The most common clefts are unilateral, and generally will not be identified until the second trimester. Cleft-palate in the presence of a cleft-lip involves the bony palate, and it can generally be suspected. Isolated cleft palate typically affects the soft palate, and it is rarely detected. Real-time imaging of the entire face can identify masses, including teratomas and lymphangiomas.

    Neck

    The nuchal skin fold is visible in the same plane as the posterior fossa. A thickened nuchal skin fold is associated with chromosomal and cardiovascular abnormalities. The neck can also be evaluated and neck masses identified in the profile view. Persistent hyperextension of the neck can also indicate presence of an anterior neck mass.

    Chest

    In the second trimester, the heart should be visible in the middle of the chest with its axis pointed toward the fetal left, with lung surrounding the heart on both sides, left smaller than right. When the heart is not in the midline position, a mediastinal shift can indicate a unilateral lung mass or diaphragmatic hernia. Congenital pulmonary airway malformations are more echogenic or bright than normal lung tissue, and can appear solid or contain cysts. Diaphragmatic hernia should be suspected if abdominal contents are visible in the chest in the presence of a mediastinal shift (Figures 20, 21). In the absence of significant mediastinal shift, smaller right-sided lesions are less likely to be detected. The echotexture of herniated liver in the right thorax may appear similar to that of lung, while the stomach bubble or small intestine in the chest with left-sided lesions is usually obvious. When transverse imaging of the chest is suggestive of diaphragmatic hernia, sagittal and coronal images can directly identify the defect in the affected hemi-diaphragm in most cases.

    Heart and cardiac outflow tracts

    A 4-chamber view can identify defects of the ventricular septum (or VSDs) (Figure 22), and abnormalities of chamber size such as hypoplastic right or left ventricles. Small VSDs are commonly missed, however, and atrial septal defects (ASDs) are generally not diagnosed in utero due to the presence of the foramen ovale, a physiologic connection between the atria. Obstruction of the aorta, such as with coarctation or stenosis, will typically cause enlargement of the right ventricle, which supplies most of the aortic blood flow through the ductus arteriosus. Because of the fetal circulation, however, milder degrees of aortic coarctation may be missed.

    While the 4-chamber view of the heart is very useful, it alone will not detect several major abnormalities. Evaluation of the outflow tracts, or the aorta and pulmonary artery as they exit the left and right ventricles, is recommended if possible. Major conditions such as Tetralogy of Fallot (Figure 23), Transposition of the great vessels (Figure 24), and Truncus arteriosus will have a normal 4-chamber view of the heart in most cases, but will usually be apparent if the outflow tracts are included.

    Abdominal structures

    Structures that should be imaged include the stomach bubble, in the left upper quadrant, ventral wall, umbilical cord insertion, bowel, and gall bladder. Gastrointestinal obstruction, including intestinal atresia, is often not apparent before the third trimester. Esophageal atresia should be suspected if the stomach bubble is persistently small or absent. A cystic mass medial to the gall bladder connecting to the cystic duct is likely to represent a choledochal cyst. Anomalies that should not be missed in the first trimester should not be missed in the second trimester, such as omphalocele (Figure 25) and gastroschisis (Figure 26).

    Genitourinary structures

    The kidneys should be visible in the renal fossae. Unilateral renal agenesis or ectopic kidney (Figure 27) may be missed, as the adrenal gland or adjacent bowel can be mistaken for a kidney in the renal fossa. To avoid that, the renal cortex and pelvis should be identified before concluding that the kidney is present in the renal fossa. Disorders affecting both kidneys, such as renal agenesis, dysplastic kidneys (Figure 28), or autosomal-recessive polycystic kidney disease are associated with oligohydramnios (Figure 29), and are seldom missed.

    The urinary bladder is visible inferior to the umbilical cord insertion, and the umbilical arteries can be seen laterally using color Doppler. Normal amniotic fluid with a persistently non-visualized bladder is suggestive of bladder exstrophy, a rare disorder.

    Recommended: Secondtrimester ultrasound of fetal anomalies in the abdomen, spine, genitourinary structures, and extremities

    The fetal genitalia can be evaluated starting early in the second trimester. Abnormalities in genitalia can include hypospadias and ambiguous genitalia. If the fetal genotype, based on karyotype or cell-free fetal DNA, is known, the appearance of the genitalia can be correlated with this information.

    Spine

    The cervical, thoracic, lumbar, and sacral spine should be imaged in sagittal and transverse planes. Real-time ultrasound can follow the spinal column from the base of the skull to the sacrum, evaluating each level. While spina bifida can be suspected based on sagittal imaging (Figure 30), smaller lesions may be apparent only with transverse imaging (Figure 31). Most cases will be associated with the Arnold-Chiari II malformation, with obvious abnormalities in the brain. Sagittal imaging can identify hemivertebra (Figure 32). The soft tissue superficial to the spine should be evaluated, and rare lesions such as sacrococcygeal teratoma can be identified.

    Extremities

    Measurement of the femur and humerus are done to confirm adequate growth of long bones. Assessment should also include documenting presence of other long bones in all extremities, including radius and ulna in the forearms and tibia and fibula in the legs. Abnormalities of skeletal structures can be quantitative or qualitative. Quantitative abnormalities involve abnormal growth of the long bones of the arms and legs, such as limb reduction defects (Figure 33). Qualitative abnormalities of bones include conditions leading to abnormal appearance, such as bowing, fractures, or hypomineralization (Figure 34). While severe skeletal dysplasias are usually apparent in the second trimester, mild skeletal dysplasias, such as achondoroplasia, are often not diagnosed until later in pregnancy.

    Assessment of hands should include attempts to rule out polydactyly and syndactyly (Figure 35), and to document presence of the thumb. Assessment of the feet should include their position relative to the tibia and fibula to rule out signs of clubfoot deformity (Figure 36). Real-time ultrasound assessment is important to document normal movement and tone in all extremities, including opening and closing of the hands. Abnormal tone, such as fixed extension of the legs or clenching of the fingers Figure 37), can indicate a neurologic, neuromuscular, or musculoskeletal abnormality.

    Skeletal dysplasias can involve abnormalities of structures other than the arms and legs, including the skull, spine, and ribcage (Figure 38). Evaluation of these structures is important when skeletal dysplasia is suspected based on abnormalities of the extremities.

    Conclusion

    Fetal anomalies can be diagnosed by ultrasound in early pregnancy, though second-trimester ultrasound can identify or exclude more conditions. There are some anomalies that are difficult to diagnose or that do not lend themselves to ultrasound diagnosis and will be missed.

    REFERENCES

    1. Abuhamad A. Technical aspects of nuchal translucency measurement. Semin Perinatol. 2005 Dec;29(6):376-9.

    2. Molina FS, Avgidou K, Kagan KO, Poggi S, Nicolaides KH. Cystic hygromas, nuchal edema, and nuchal translucency at 11-14 weeks of gestation. Obstet Gynecol. 2006 Mar;107(3):678-83.

    3. Sinkovskaya ES, Chaoui R, Karl K, Andreeva E, Zhuchenko l, Abuhamad AZ. Fetal cardiac axis and congenital heart defects in early gestation. Obstet Gynecol. 2015 Feb;125(2):453-60.

    4. Chasen ST, Kalish RB. Can early ultrasound reduce the gestational age at abortion for fetal anomalies? Contraception. 2013 Jan;87(1):63-6.

     

    Stephen T Chasen, MD
    Dr Chasen is Professor of Clinical Obstetrics and Gynecology in the Department of Obstetrics and Gynecology at Weill-Cornell Medical ...
    Daniel W Skupski, MD
    Dr Skupski is Professor of Clinical Obstetrics and Gynecology in the Department of Obstetrics and Gynecology at Weill-Cornell Medical ...

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