Not totally unheard of, but my docs ordered ultrasounds for dating and viability for both of my pregnancies at 10 and 11 weeks, respectively. I also think there's a window for the Nucal Translucency scan if you wish to go that route that is before 18 weeks. A lot can also be determined by your insurance. 18 weeks is usually the minimum gestational age for the anatomy scan.
At my doctors office, your FIRST and ONLY ultrasound is between week 18-20 for the fetal anatomy scan UNLESS there is some kind of problem or complication that occurs.
That is typical at our office too, however we chose to do the NT Scan (an additional cost) and my dr ordered a dating ultrasound so we ended up not having to wait.
i'm with @munchkin421 I was to only get 1 around then unless something was wrong, which I had an issue last week at 32 weeks and had to get another one. They're expensive, so you can have more, but IMO I could use the money to buy diapers and wipes and things for the nursery instead of dropping a few hundred on an ultrasound.
At my office I would get one in the first trimester and then the anatomy scan between weeks 18-20. There is also the NT scan as others have mentioned. It is between weeks 12-14, but I elected to do the cell-free DNA testing instead. It gives more definitive results, but you don't get an u/s.
TTC Since January 2012
Me:37 DH:34 DX July 2013: Unexplained Infertility New DX Dec 2013: DOR
Re: First ultrasound at 18 weeks?!
4 rounds of clomid, 2 with IUI = BFN