Blood work?

When did you have to take blood work for Down syndrome and etc....? 
What weeks? And how many times? 
My ultrasound looked good. Did one blood work at 11 weeks and another at 16weeks. Midwife said they couldn't see the 1st results until I do the second blood work cause they like to compare it first. She says it normal but is it really? Freaking out just a tad.