I just had mine a few weeks ago, and they told me the ultrasound measurement is no different than a singleton, except that it can take longer to get two babies to cooperate for the right positioning to get the images they need. I also had bloodwork at the same time, which they said can be less specific for twins but is still useful. My measurements and results were well below the threshholds for increased risk, so I can't say anything about the interpretation of any other kinds of results. But the ultrasound itself was, to my understanding, no different. Hope that helps.
2.5 years TTC with MFI, 3 failed IUIs
IVF w/ICSI October 2014: 17R, 13M, 12F 4 Frosties
ET of two blasts 11/2/14 BFP!!! It's TWINS! EDD 7/21/15
Re: Nuchal translucency US