The commonest chromosomal defects are trisomies 21, 18 or 13, sex chromosomal defects (45,X, 47,XXX, 47,XXY, 47,XYY) and triploidy.
In the first trimester, a common feature of many chromosomal defects is increased nuchal translucency thickness. In later pregnancy, each chromosomal defect has its own syndromal pattern of abnormalities.
- 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.
When there is double paternal contribution (diandric) there is a molar placenta and the pregnancy rarely persists beyond 20 weeks. When there is a double maternal contribution (digynic) 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.
- 47,XXX, 47,XXY or 47,XYY:
These are not associated with an increased prevalence of sonographically detectable defects.