For my first pregnancy I did this, but since then I have never had an ultrasound for the sole purpose of finding out the gender. I'm not against them or anything, I just don't like going in for ultrasounds.
With my second, I had an ultrasound to determine dates because I was pretty sure of my conception date but my doctor didn't agree with me. It turned out he was right. By my LMP date my EDD was March 3 but after the ultrasound it was reset at January 28. My baby was born on January 19 and looked like she was full-term. I'm glad I had that ultrasound, otherwise I would have been in a panic when my water broke over a month before my EDD...it was shocking enough that it was two weeks "early."I had a second ultrasound with her because the radiologist thought her legs looked a little short and they wanted to make sure they were growing properly.
With my third I went in for cramping at about 12w but it was too early to see the gender even if I had wanted to. That was my only u/s for that pregnancy.
I hadn't planned on getting one for my fourth pregnancy but my midwife was starting to suspect possible twins, so I had one. I didn't want to find out the gender but he made it pretty obvious!
In my last pregnancy I had zero ultrasounds except for the one I had at the hospital when I went into premature labor. I don't know if an ultrasound would have revealed any problems that may have given me an opportunity to have steroid shots or get on bedrest and prevent all the time my baby was hospitalized after birth, but we have decided to have ultrasounds during this pregnancy in order to keep an eye on things. I am planning an HBAC and if there are problems, I want to know about it before I go through with a birth at home. I have had one ultrasound already to determine dates because there were two possible conception dates a month apart, and I plan to have at least one more in a few weeks to look for problems and probably find out the gender. I have been surprised twice and I've found out three times. Both ways are good...and I have a lot to knit, so I'd like to know what color to use.