As a vegan, I had to face a lot of questions and some opposition for my choices when I was pregnant. I even had one doctor storm out of the room when she found out I was vegan. She asked that I make my next appt with another doctor.
If there's one thing I learned during my vegan pregnancy it's how to approach people with my "different" ideas.
The trick, I found, is to make sure you dont' sound like you're calling their expertise into question. You may be right, but if the doctor thinks you are insulting them it only engenders harsh feelings.
Since I also declined many of their tests and had to explain all that too, here's what I said, "I know most people get this test, so it may come as sort of a shock to hear that I'm going to skip it this time around. I've given it a lot of thought and done some research and I just don't think it's for me."
If they try to coerce me I simply say, "I understand, but I've made the decision not to."
If they keep blabbering on about it I say, "Sure, I understand. However, I'm not going to take the test. I'm sure it must seem odd to you, but don't worry about me. I'm certain of my decision."
Or something to that effect. First I gave them acknowledgement that I heard them, then I imformed them that I made a different decision and that I was very comfortable with that.
might work in this case too. Good luck!