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Second trimester scan

Markers of chromosomal defects

In the second trimester scan each chromosomal defect has its own syndromal pattern of detectable abnormalities.

When an abnormality is detected a thorough check should be made for the other features of the chromosomal defect known to be associated with that marker.

In the case of multiple abnormalities the risk for chromosomal defects is substantially increased and fetal karyotyping should be considered.

In the case of isolated abnormalities the decision of whether to carry out an invasive test depends on whether the abnormality is major or minor.

 

Trisomy 21: Nasal hypoplasia, increased prenasal and nuchal fold thickness, cardiac defects, intracardiac echogenic foci, duodenal atresia and echogenic bowel, mild hydronephrosis, shortening of the femur, sandal gap and clinodactyly or mid-phalanx hypoplasia of the fifth finger.

Trisomy 18: Strawberry-shaped head, choroid plexus cysts, absent corpus callosum, enlarged cisterna magna, facial cleft, micrognathia, nuchal edema, heart defects, diaphragmatic hernia, esophageal atresia, exomphalos, single umbilical artery, renal abnormalities, echogenic bowel, myelomeningocoele, growth restriction and shortening of the limbs, radial aplasia, overlapping fingers and talipes or rocker bottom feet.

Trisomy 13: Holoprosencephaly, microcephaly, facial abnormalities, cardiac abnormalities, enlarged and echogenic kidneys, exomphalos and post axial polydactyly.

Triploidy: When there is double paternal contribution there is a molar placenta and the pregnancy rarely persists beyond 20 weeks. When there is a double maternal contribution the placenta is thin but of normal consistency and the pregnancy may persist into the third trimester. The fetus demonstrates severe asymmetrical growth restriction, mild ventriculomegaly, micrognathia, cardiac abnormalities, myelomeningocoele, syndactyly, and 'hitch-hiker' toe deformity.

Turner syndrome: Large cystic hygromas, generalised edema, mild pleural effusions and ascites, cardiac abnormalities and horseshoe kidneys, which are suspected by the ultrasonographic appearance of bilateral mild hydronephrosis.