I think the only other thing an ultrasound might show, other than twins, would be a BIG fibroid.
I had the same situation in pg#2 - at about 18 weeks I measured 22 weeks; midwife was not too concerned, I was not, but DH was nervous about twins, so I had an ultrasound. No twins, no fibroid or anything else unusual, and babe only measured a couple of days bigger than my dates. I still measured big at subsequent visits, but less of a difference each time. I also typically put on a whole lot of weight (like 10 pounds in a month) around month 4-5 in each of my pregnancies, and then it tapered off so overall my weight gain was average. I think it's ok for there to be a "growth spurt" in your fundal height and/or wieght gain, as long as over the course of time it evens out.
Looking back, I would delay the ultrasound and see if growth evened out; if not, an ultrasound can always be done later if there's a quesiton of twins.