To go for an amnecentesis is a tough choice. So you better do triple blood and ultra sound first.

triple blood gives an estimate of odds of Down
based upon mother's age. Then it is said Ultra
sound can deteck half of Down syndrom.

So you can modify triple-blood-based odds based upon
ultra sound results. Using Bayest estimtor,
I can roughly say if ultrasound is positive,
then you should increase your odds by ten times.
If ulstra sound is negative (most likley), then
your further cut odds by half.

Then based upon this new odds, you decide whether to
go for amniocentesis or not