Baymic,
I wanted to go for Amnio test because I am not very happy with my BT ratio although the overall risk is Low but.. borderline kind.
The Genetic counsellor told me that why they don't suggest Low risk (no matter borderline or very low) as long as low risk they dont recommend is because..
Suppose your risk of having is Down Syndrome baby is higher, that means the risk of you doing an Amnio is lesser. On the other hand, if your risk of having a Down syndrome baby is lower, risk of you doing Amnio is higher.
Actually most of them would recommend doing a level 2 ultrasound (sometimes with the help of doppler) to check for soft markers if you are in high risk but not those very high kind like says 1:250. This is what I heard..
Anyway, I'll be seeing a Prof at Camden Medical who is an expert in fetal assessment. He even published a few medical article on First trimester screening for Down syndrome. He specialised in Genetic scan where if you research online, is known to detect up to 93% of Down syndrome babies.