My OB sends me to the Maternal Fetal Medicine department at the hospital to be done. And I am sooooo glad she does. The hospital has the latest equipment, and my u/s was amazing. You could see everything so clearly, and the tech really took the time to explain everything to us at our A/S. Compared to my friend's u/s pics, which were done in the office, mine were gorgeous and amazingly clear!
On top of that, my OB was able to order the u/s along with a consult with the physician at the MFM center so that I could have my results right away. I felt extremely confident in the process.
As far as only getting one u/s, that is the standard really. 20 week A/S. Dating u/s's are typically done for high-risk patients only, and for IF patients (and for IF patients, they're really done for statistical purposes - once there's a heart beat, the RE can count that as a successful cycle; certain IF treatments also put a woman at higher risk for ectopic). Later in the 3rd tri, growth scans might be done if something warrants it. I am having a growth scan b/c my blood sugars have been borderline and I am measuring ahead. Most people do not have a growth scan either.