|Monosomy X (Turner Syndrome; 45X)|
Monosomy X, commonly known as Turner syndrome, is a chromosomal anomaly in which the fetus has an X chromosome as its only sex chromosome instead of the usual two sex chromosomes (XX in females and XY in males). Fetuses with Turner syndrome often have nuchal cystic hygromas, and may have other abnormal fluid accumulations, including diffuse subcutaneous edema (lymphangiectasia), pleural effusions, and ascites. They also have an increased incidence of aortic coarctation and horseshoe kidneys.|
Fetuses with Turner syndrome who have severe lymphangiectasia often die in the first or early second trimester. Newborns with this syndrome are phenotypically female. They typically have webbed necks (likely a result of regression of prior cystic hygroma) and grow to a short stature. Most have ovarian dysgenesis, resulting in infertility in adult life.
Turner syndrome is often detectable sonographically during the mid to late first trimester. The findings at this stage range from thickening of the nuchal translucency to a frank cystic hygroma (thickening with identifiable cystic spaces) to generalized subcutaneous edema (lymphangiectasia). Fetuses surviving into the second trimester will often be hydropic, with findings that include pleural effusions, pericardial effusion, ascites, and subcutaneous edema. When these findings are detected, the parents should be advised about the likelihood of Turner syndrome and offered karyotype testing via amniocentesis or chorionic villus sampling.
The cardiac and renal anomalies associated with Turner syndrome can sometimes be diagnosed by prenatal sonography. The finding of discrepant size of the cardiac ventricles, with the right ventricle being larger than the left ventricle, is suggestive of coarctation of the aorta. Most cases of coarctation, however, are not identified until after birth. Horseshoe kidney is more commonly diagnosed prenatally.
First trimester fetus with Turner syndrome. Sagittal view through the fetus demonstrates skin thickening (arrowheads) around the entire fetus, including a septated fluid collection (arrows) in the posterior neck.
First trimester fetus with Turner syndrome. Transverse view through the fetal neck demonstrates the septated fluid collection (arrows) posteriorly.
First trimester fetus with Turner syndrome. Transverse view through the fetal abdomen demonstrates circumferential skin thickening (arrowheads).
Second trimester fetus with Turner syndrome. Axial view through the fetal head demonstrates nuchal edema with a thickened nuchal fold (arrowheads).
Second trimester fetus with Turner syndrome. Transverse view of the chest demonstrates a pericardial effusion (arrow).
Second trimester fetus with Turner syndrome. A calf (arrow) and foot (arrowheads) are edematous.