Beware the umbilical cord cyst.

http://onlinelibrary.wiley.com/doi/10.1002/uog.80/pdf

Art

There is indeed a weak correlation. However, in trisomy 18 you should see plenty of other major anomalies such as IUGR, clefts, cardiac defects, diaphragmatic hernia, oesophagial atresia, bowel anomalies, urogenital anomalies, talipes or rocker bottom feet and overlapping fingers. In the absence of any of these signs, an umbilical cord cyst should not trigger invasive testing. 

I suggest to use your finding with additional findings for trisomy 18.