genetic screening

my second trimester screening results for my "risk" for Downs syndrome were great!
I got a 1 in 9500 chance of one of the babies having Downs and the very best you can do is 1 in 10.000 so this made me very happy! In my first trimester screening it was 1 in 8500, so we've improved I guess! Both numbers are great anyway.
The spina bifida results made me worry just a bit. I got a 1 in 1600 chance of one of them having SB. The geneticist said the very best is 1 in 6000. I told her I thought my number was a bit low/ my risk a bit high but she said this is actually a really good number for SB and that they don't even start to get concerned until your risk gets to 1 in 200. She said the numbers just work differently and that my level 2 US more than likely would have seen something if they had it and everything looked perfect on my US AND it was reviewed by the peri which is reassuring.
This did make me feel better but I still have that tiny hint of worry in the back of my mind, although it's very tiny.
Trisomy 18 risk can't be tested for in twin pregnancies. Something about the way the numbers work?
But we probably would have seen a defect of some sort if one of them had this.