Going in for my 20 week US... what to look for?

Lots of sites will tell me what they will do and such but not any seem to mention what they will look for or what I should worry about if I already had my 13 week one?

Are they looking for new stuff or just seeing that all is well?
What should I be asking?
Should I be less worried since we sailed though our 13 week?

Any advice would be great!
(We are NOT finding out the gender since my DH will be traveling and can't come to the appointment. We are having them put the answer in an envelope and seal it and I will give it to my friend to hold until my DH and I chose to... or possibly not... open it!)

Thanks, Ladies!
Sara H

PS- Did you know Hallmark makes a "PG after infertility" card? Seriously? What DON'T they have a card for???
(It was very nice, a little sappy, and did make us smile!)