SUA & Trisomy 21: Understanding the Connection & Next Steps

Question
Hi,

I have a friend who is 31 years old and 18 weeks pregnant.  During the first ultra sound, the doctor told her that she had SUA (single umbilical Artery) and they took a much longer time with the ultra sound to ensure everything looked ok otherwise (as this is a marker for trisomy21).  They did not recommend an amniocentesis, but did take a blood sample.  Upon reviewing the ultra sound pictures it seems as though the big toe has a large separation from the rest of the toes; another indicator for trisomy 21 (down's syndrome) but the doctor did not mention it, we were the ones who noticed it.  

Do baby's toes normally have a large separation at 18weeks such that something normal may look abnormal?  

If the blood tests come back normal (I am assuming they are going to look at the alphafetoprotein levels), will that indicate for sure whether or not the baby has down syndrome?  

How risky is an amniocentesis and is it necessary if the blood levels are ok or is alphafetoprotein levels not a clear indicator of the health of the baby.  

~we are all very concerned.  I greatly appreciate any advice or insight you could offer! Thank you very much ~ Lisa

Answer
All of the sonographic signs you mentioned are "soft" signs of chormosomal abnormalities and are not always accurate. A single umbilical artery does NOT indicate trisomy-21.It is an indicator of structural defects in 30% of cases where it is seen. It is seen in placental vascular occlusion or thrombosis. Trisomy 21 is indicated with a "nuchal fold transluscency" and must be investigated further. The space between the toes does not indicate Down's syndrome, but the crease on the hand may. Alphafetoprotein can indicate Down's if it is lower than normal , but it is not definitive, by far. Amniocentesis is the only definitive test we have adn we recommend it to all women over 35 years of age.