In terms of scans, I think that your age also plays a part. The blood test which they run concurrently with the nucal scan privately will give a more accurate assessment of the risk of downs, etc for the fetus particularly if one is older, as this will automatically put you in a higher risk category without the blood tests.
I'm going privately for the nuchal scan because they will also do things like measure the angle of the genital tubernacle to give a pretty good idea of the sex of the baby (just really want to know, and it makes shopping more efficient).
According to the scale, I've only gained 2 pounds, but to look at me, you would think I've gained 10 pounds! Didn't realize that my stomach muscles were so ravaged by my first birth!