For your 20w scan, it will depend on whether its at your regular doctor or a mat/fet specialist. At your regular office they may do the vitals simply because you're there (bp, weigh, pee in a cup) at the specialist they don't do that stuff. Then you just have a really long ultrasound. They go from the top of the head to the tips of the toes. They measure how big baby is, check his/her heart and brain, check out the bits if you want to know the gender, check the spine, the arm and leg bones, hands and feet (super cute), that kind of stuff. We went to a specialist for my level 2 because there is a history a various birth defects on my husbands side, so they did the scan then the very nice doctor and resident came in to scan again for cleft lip and pallet (my husband has both) told us Alex is fine, scan looked great, I asked how much Alex weighed (10 oz at 18w5d -lucky me) and that was that.
I was nervous too, for some reason I was stressing that he would have something wrong with his heart (IDK why) but the scan itself is just a scan. Try and enjoy the peek in to your womb and the amazing pictures of your amazing baby. GL.