Screening tests can be performed differently in different countries. It sounds like the blood tests might be what we call 2nd trimester screen. In Australia, women can have 1st trimester screening (NT measurement plus blood test) OR 2nd trimester screening which is a blood test only.

The second trimester screen gives you a risk figure for Trisomy 21, Trisomy 13/18 and also neural tube defects (like spina bifida).

In Australia, they like to do the NT with blood tests and give one risk figure for the pregnancy. So, you don't have both 1st & 2nd trimester screens, just one.

I'm not sure what they were measuring with the ultrasound. It's possible that it's another marker for chromosomal issues. Will try find out for you.

This might be silly question, but you weren't looking at baby upside down were you, and it could have been at the top of the spine not the bottom?