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Sonographic detection of major malformations
ОглавлениеThe genetic sonogram is a term that has been used to describe second‐trimester sonographic assessment of the fetus for signs of aneuploidy. The detection of certain major structural malformations that are known to be associated with aneuploidy should prompt an offer of genetic amniocentesis. Table 5.2 summarizes the major structural malformations that are associated with the most common trisomies. Given the increasing popularity of first‐trimester screening, many advanced obstetric ultrasound practitioners have attempted to bring the genetic sonogram forward in gestation so that an anomaly scan may also be performed toward the end of the first trimester. Relatively limited data are available to validate the accuracy of the genetic sonogram in the first trimester for general population screening, and therefore the optimal time remains at about 18–22 weeks of gestation.
Table 5.2 Sonographic findings associated with trisomies 21, 18, and 13
Trisomy 21 | Trisomy 18 | Trisomy 13 |
---|---|---|
Major structural malformations | ||
Cardiac defects: | Cardiac defects: | Holoprosencephaly |
• Atrioventricular (AV) canal defect | • Double outlet right ventricle | Orofacial clefting |
• Ventricular septal defect | • Ventricular septal defect | Cyclopia |
• Tetralogy of Fallot | • AV canal defect | Proboscis |
Duodenal atresia | Meningomyelocele | Omphalocele |
Cystic hygroma | Agenesis of the corpus callosum | Cardiac defects: |
Hydrops fetalis | Omphalocele | • Ventricular septal defect |
Diaphragmatic hernia | • Hypoplastic left heart | |
Esophageal atresia Clubbed or rocker‐bottom feet Renal abnormalities Orofacial clefting Cystic hygroma Hydrops fetalis | Polydactyly Clubbed or rocker‐bottom feet Echogenic kidneys Cystic hygroma Hydrops fetalis | |
Minor sonographic markers | ||
Nuchal thickening | Nuchal thickening | Nuchal thickening |
Mild ventriculomegaly | Mild ventriculomegaly | Mild ventriculomegaly |
Short humerus or femur | Short humerus or femur | Echogenic bowel |
Echogenic bowel | Echogenic bowel | Enlarged cisterna magna |
Renal pyelectasis | Enlarged cisterna magna | Echogenic intracardiac focus |
Echogenic intracardiac focus | Choroid plexus cysts | Single umbilical artery |
Hypoplastic nasal bones | Micrognathia | Overlapping fingers |
Brachycephaly | Strawberry‐shaped head | Growth restriction |
Clinodactyly | Clenched or overlapping fingers | |
Sandal gap toe | Single umbilical artery | |
Widened iliac angle | Growth restriction | |
Growth restriction |
When a major structural malformation is found, such as an atrioventricular canal defect or a double‐bubble suggestive of duodenal atresia, the risk of trisomy 21 in that pregnancy is increased by approximately 20–30‐fold. For many patients, such an increase in their background risk for aneuploidy will be sufficiently high to justify genetic amniocentesis.