12 week ultrasound? 👶🏼💕

At my 8 week ultrasound I apparently forgot to set up the 12 week ultrasound and blood test for genetic stuff they said it tested for autism and other things that can be wrong with baby but I don’t remember declining and when I went for my 12 week checkup they said I wouldn’t get a finger stick or ultrasound and I was apparently 12 weeks 2 days on Wednesday and they said if I want to reschedule it’ll have to be before I’m 13 weeks an set it for Monday I’m just curious why it’s got to be so precise and what it’s checking for and stuff because they really didn’t explain it🤷🏼‍♀️