My healthcare provider does an official one at around 18-20 weeks, but sometimes they use their portable, in-office machines for a quick one with the first prenatal visit, if there are any issues with dates, etc.
For me, I had a m/c due to a blighted ovum last time and didn't find out till 10 weeks, so I am extremely nervous this time and my doc offered to do one early to *hopefully* put my mind at ease.
With my first pregnancy (10 years ago!) my ob at the time did them at every visit and captured it on a video that we got to keep. That was nice, though I know there is some controversy around doing a lot of u/s (or even any, without cause).
Declutter - 789/2010 (counting the stuff on my porch waiting for a Freecycler to pick it up! )