Hettie.
Stop spinning. Deep breath.
I am going to be very matter of fact. Apologies if softer language would suit you more.
What you have is a number. You have 150 future possibilities. One of those possibilities is a baby with Down syndrome. 149 future possibilities is a "normal" baby.
Those 149 future possibilities could contain any manner of other genetic conditions, or birth trauma, that is not tested for or forseeable.
Cut offs for labels such as high low medium risk are not really that helpful. Because if you had a 1 in 3000 risk, and you were that one, it is of no consolation that you had a label low risk.
Your choices are these.
Firstly, you continue without further testing. Either none at all, or the usual USS. The 20 week scan may show evidence of any problems, but the pick up of cardiac defects is nowhere near 100% - it is serious cardiac defects that cause most deaths in young babies and children with Down syndrome. So it may be falsely reasurring. It may also show that the foetus is profoundly abnormal. Which will either be helpful to you, or lead you to more difficult decisions.
Secondly, you go on to have amniocentesis. This is tricky. There is a 1% quoted risk of miscarrying from having the test. Bear in mind that your current risk is a 1 in 149 chance of your foetus not having down. So should you be very unfortunate to miscarry as a result of having the procedure
you are most likely to miscarry a normal foetus.
Thirdly, you can abort now.
None of this helps you out with the emotions that go along with these decisions. I really feel for you, it is going to be a very difficult time the next few weeks.