ultrasounds after 20 weeks?

My sister-in-law with both of her pregnancies, had 3 ultrasounds. the first around 9-12 weeks, the second around 18-20 weeks, and the third around 32- 37 weeks. The first time, we had the same dr. at the same medical center, however, she had her membranes scraped to induce bevause her BF was leaving for marine boot camp. I delivered at 38 weeks exactly, and only had 2 ultrasounds the entire pregnancy. For baby#2, she was in a different state in a lot bigger city. 3 ultrasounds that time too. Im currently only 23 weeks, so Ive only had 2 ultrasounds so far. I asked the dr. And he said that they usually only do 2. Has this been true for anyone else? Family and friends of mine had said they got 3 as well. Im in New Mexico if that helps anyone. Thanks!