Hello, and I hope I can help you today. Unfortunately because of the way that cells and chromosomes divide to make twins, it is possible that a mismatch can occur genetically and that one fetus could be abnormal with the other being OK. Also the nucchal translucency is not 100% accurate, so you may find out that that fetus is completely normal. These tests results are given as a risk ratio: meaning you have a 1 on 532 chance on one baby being born with a genetic abnormality, compared to a 1 in 18 risk in the other. But that still means that you have the greatest likelihood of having nothing wrong with the fetus at all. The best way to proceed from here is for your OB provider to consult with a genetic counselor or a high-risk obstetrician on the best course of action. Only an amniocentesis or CVS (chorionic villus sampling) can give you a 100% answer of the genetics of the fetus. There is a risk of complications of both of these procedures, but if you are able to get care at a university medical center they have the best high risk OB departments and the best trained people to help you. I hope this information was helpful and everything turns out to be OK. Best wishes and good luck with the rest of the pregnancy, Dr. Brown