Re: Advice needed on 21 week u/s (could only get one kidney)
We also were told that they were not able to see everything they needed to see when we went in for our 19 weeks ultrasound (I think the baby was sleeping, lol).
So we had a follow-up u/s at 23 weeks and she was very active and they were able to see all the things that they couldn't get a "visual on" before, and that is when they were able to tell us she was a girl! She was born healthy! I think, and this is just a guess, that if they actually thought that your baby was missing a kidney, that they would expediate the the follow-up u/s.
I hope everything continues to go well in your pregnancy!