Twins and High Odds for DS

I'm havin twins and at 13 wks had an Ultrasounds and blood work (finger ***** on 4 round circles) and I will be 38 when I deliver.  These will be my 3rd and 4th children and my previous pregancies were 100% normal no issues whatsoever.

Anyway...one baby came back with an increased Nuchal fold of 3.5MM which has increased the risk of DS to 1:34 for that baby.  The other baby is about 1:143 or something.  My OB said all the other test results were normal.  However I'm not sure what else they looked at and have heard that with twins these blood test are not as accurate as one baby may be showing low/high numbers and the other have the opposite, hence skewing the results.  I'm nervous and can't deterine how "bad" this situation.

Any thoughts?

Thanks