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There are two distinct patterns, illustrated by the images. A smooth elevation of thin skin with ultrasound shining through a simple fluid space (nuchal translucency, NT) and a "shaggy lion's mane" of multiple dilated lymphatics and lymph collections, which are 'cystic hygromas'. NL represents a tissue malformation and has a very high association with chromosome problems. NT involves a circulatory overload and has a lower association with chromosome problems and a higher association with heart problems and other syndromes that involve the heart. A slight degree of NT is normal 6 weeks after conception and is typically gone during the next week of development. NT is identified when the elevated fluid rim is obvious and stage is after 10 weeks GA. NT can be transient in normal fetuses, at other times it progresses to congestive heart failure with fluid in other body compartments, which is referred to as 'hydrops' when it is generalized.
The newest theory is that the genetic basis involvesfailure of the developing lumphatics to form as open channels. When the point of closure involves the thoracic duct, which is the place where all the lymphatics drain into the venous system,neck swelling is large and of the NT type, when there are multiple closures throughout the developing lymphatics, there are cystic hygromas. With lymphatic leakage into skin, there is a mixed pattern of findings.
In early pregnancy, neck finding need to be evaluated with endovaginal imaging and not limited to transabdominal viewing. For screening purposes, findings are best sought between 10 and 14 weeks GA, not afterwards. True neck findings are very important and need to be evaluated further, usually with amniocentesis as the first step.
Absence of NT or cystic hygromas does not exclude chromosome problems.
